research on caffeine consumption

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The Impact of Caffeine and Coffee on Human Health

Coffee is one of the most widely consumed beverages in the world and is also a major source of caffeine for most populations [ 1 ]. This special issue of Nutrients , “The Impact of Caffeine and Coffee on Human Health” contains nine reviews and 10 original publications of timely human research investigating coffee and caffeine habits and the impact of coffee and caffeine intake on various diseases, conditions, and performance traits.

With increasing interest in the role of coffee in health, general knowledge of population consumption patterns and within the context of the full diet is important for both research and public health. Reyes and Cornelis [ 1 ] used 2017 country-level volume sales (proxy for consumption) of caffeine-containing beverages (CCBs) to demonstrate that coffee and tea remain the leading CCBs consumed around the world. In a large coordinated effort spanning 10 European countries, Landais et al. [ 2 ] quantified self-reported coffee and tea intakes and assessed their contribution to the intakes of selected nutrients in adults where variation in consumption was mostly driven by geographical region. Overall, coffee and tea contributed to less than 10% of the energy intake. However, the greatest contribution to total sugar intake was observed in Southern Europe (up to ~20%). These works not only emphasize the wide prevalence of coffee and tea drinking, but also the need for data on coffee and tea additives in epidemiological studies of these beverages in certain countries as they may offset any potential benefits these beverages have on health.

Doepker et al. [ 3 ] provided a user-friendly synopsis of their systematic review [ 4 ] of caffeine safety, which concluded that caffeine doses (400 mg/day for healthy adults, for example) previously determined in 2003 [ 5 ] as not to be associated with adverse effects, remained generally appropriate despite new research conducted since then. Further concerning caffeine safety is the systematic review of caffeine-related deaths by Capelletti et al. [ 6 ]. Suicide, accidental, and intentional poisoning were the most common causes of death and most cases involved infants, psychiatric patients, and athletes. Both Doepker et al. [ 3 ] and Capelletti et al. [ 6 ] alluded to the increasing interest in the area of between-person sensitivity resulting from environmental and genetic factors, of which the latter is a topic of additional papers in this special issue and thus reiterates this interest.

Advancements in high-throughput analyses of the human genome, transcriptome, proteome, and metabolome have presented coffee researchers with an unprecedented opportunity to optimize their research approach while acquiring mechanistic and causal insight to their observed associations [ 7 ]. Three timely reviews [ 8 , 9 , 10 ] and an original report [ 11 ] addressed the topic of human genetics and coffee and caffeine consumption. Interest in this area received a boost by the success of genome-wide association studies (GWAS), which identified multiple genetic variants associated with habitual coffee and caffeine consumption as discussed by Cornelis and Munafo [ 8 ] in their review of Mendelian randomization (MR) studies on coffee and caffeine consumption. MR is a technique that uses genetic variants as instrumental variables to assess whether an observational association between a risk factor (i.e., coffee) and an outcome aligns with a causal effect. The application of this approach to coffee and health is growing, but has important statistical and conceptual challenges that warrant consideration in the interpretation of the results. Southward et al. [ 9 ] and Fulton et al. [ 10 ] reviewed the impact of genetics on physiological responses to caffeine. Both emphasized a current clinical interest limited to CYP1A2 and ADORA2A variations, suggesting opportunities to expand this research to more recent loci identified by GWAS. Despite the advancements in integrating genetics into clinical trials of caffeine, such designs remain susceptible to limitations [ 9 , 10 , 12 , 13 ]. Some of these limitations were further highlighted by Shabir et al. [ 14 ] in their critical review on the impact of caffeine expectancies on sport, exercise, and cognitive performance. Interestingly, the original findings from a randomized controlled trial of regular coffee, decaffeinated coffee, and placebo suggested the stimulant activity of coffee beyond its caffeine content, raising issues with the use of decaffeinated coffee as a placebo [ 15 ].

The impact of coffee intake on gene expression and the lipidome were investigated by Barnung et al. [ 16 ] and Kuang et al. [ 17 ], respectively. Barnung et al. [ 16 ] reported on the results from a population-based whole-blood gene expression analysis of coffee consumption that pointed to metabolic, immune, and inflammation pathways. Using samples from a controlled trial of coffee intake, Kuang et al. [ 17 ] reported that coffee intake led to lower levels of specific lysophosphatidylcholines. These two reports provide both novel and confirmatory insight into mechanisms by which coffee might be impacting health and further demonstrate the power of high-throughput omic technologies in the nutrition field.

Heavy coffee and caffeine intake continue to be seen as potentially harmful on pregnancy outcomes [ 18 ]. Leviton [ 19 ] discussed the biases inherent in studies of coffee consumption during pregnancy and argued that all of the reports of detrimental effects of coffee could be explained by one or more of these biases. The impact of dietary caffeine intake on assisted reproduction technique (ART) outcomes has also garnered interest. An original report by Ricci et al. [ 20 ] in this special issue found no relationship between the caffeine intake of subfertile couples and negative ART outcomes.

Van Dijk et al. [ 21 ] reviewed the effects of caffeine on myocardial blood flow, which support a significant and clinically relevant influence of recent caffeine intake on cardiac perfusion measurements during adenosine and dipyridamole induced hyperemia. Original observational reports on the association between habitual coffee consumption and liver fibrosis [ 22 ], depression [ 23 ], hearing [ 24 ], and cognition indices [ 25 ] have extended the research in these areas to new populations.

Finally, given the widespread availability of caffeine in the diet and the increasing public and scientific interest in the potential health consequences of habitual caffeine intake, Reyes and Cornelis [ 1 ] assessed how current caffeine knowledge and concern has been translated into food-based dietary guidelines (FBDG) from around the world; focusing on CCBs. Several themes emerged, but in general, FBDG provided an unfavorable view of CCBs, which was rarely balanced with recent data supporting the potential benefits of specific beverage types.

This collection of original and review papers provides a useful summary of the progress on the topic of caffeine, coffee, and human health. It also points to the research needs and limitations of the study design, which should be considered going forward and when critically evaluating the research findings.

Conflicts of Interest

The author declares no conflict of interest.

• Open access
• Published: 14 April 2022

Prevalence of caffeine consumers, daily caffeine consumption, and factors associated with caffeine use among active duty United States military personnel

• Joseph J. Knapik   ORCID: orcid.org/0000-0002-1568-1860 1 ,
• Ryan A. Steelman 2 ,
• Daniel W. Trone 3 ,
• Emily K. Farina 1 &
• Harris R. Lieberman 1  

Nutrition Journal volume  21 , Article number:  22 ( 2022 ) Cite this article

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Although representative data on caffeine intake in Americans are available, these data do not include US service members (SMs). The few previous investigations in military personnel largely involve convenience samples. This cross-sectional study examined prevalence of caffeine consumers, daily caffeine consumption, and factors associated with caffeine use among United States active duty military service members (SMs).

A stratified random sample of SMs were asked to complete an on-line questionnaire on their personal characteristics and consumption of caffeinated products (exclusive of dietary supplements). Eighteen percent ( n  = 26,680) of successfully contacted SMs ( n  = 146,365) completed the questionnaire.

Overall, 87% reported consuming caffeinated products ≥1 time/week. Mean ± standard error per-capita consumption (all participants) was 218 ± 2 and 167 ± 3 mg/day for men and women, respectively. Caffeine consumers ingested 243 ± 2 mg/day (251 ± 2 mg/day men, 195 ± 3 mg/day women). On a body-weight basis, men and women consumed respectively similar caffeine amounts (2.93 vs 2.85 mg/day/kg; p  = 0.12). Among individual caffeinated products, coffee had the highest use (68%), followed by sodas (42%), teas (29%), energy drinks (29%) and gums/candy/medications (4%). In multivariable logistic regression, characteristics independently associated with caffeine use (≥1 time/week) included female gender, older age, white race/ethnicity, higher body mass index, tobacco use or former use, greater alcohol intake, and higher enlisted or officer rank.

Compared to National Health and Nutrition Examination Survey data, daily caffeine consumption (mg/day) by SMs was higher, perhaps reflecting higher mental and physical occupational demands on SMs.

Peer Review reports

Caffeine is a widely consumed psychoactive stimulant ingested in various beverages including coffees, teas, sodas, and energy drinks. About 90% of United States (US) adults consume caffeinated products with little difference between men and women in how frequently the products are ingested [ 1 , 2 ]. Among caffeine consumers, the average caffeine intake is about 211 mg/day [ 1 ]. Although there are cases where consumption of very high dosages of caffeine has led to seizures, transient cardiovascular problems, and even deaths [ 3 , 4 ], comprehensive reviews have concluded that consumption of < 400 mg/day is generally safe, enhances certain aspects of mental, physical, and occupational performance, and may confer other health benefits [ 5 , 6 , 7 ]. Among healthy adults, moderate coffee consumption has been reported to be associated with reduced risk of certain health conditions including chronic liver disease, gout, Parkinson’s disease, Alzheimer’s disease, Type 2 diabetes, certain types of cancers, all-cause mortality, and cause-specific mortality [ 5 , 8 , 9 ]. However, concerns about caffeine use by pregnant women and increased consumption of energy drinks by young adults has been expressed [ 5 , 10 ]. For pregnant women, caffeine dosages in the range of < 200 to 300 mg/day are recommended [ 7 ] because of increased risk for pregnancy-related adverse effects (low birth weight, pregnancy loss, childhood leukemia) [ 8 ]. Data from the nationally-representative National Health and Nutrition Examination Survey (NHANES) indicate caffeine consumption from energy drinks increased from 2001 to 2016 in 19- to 39-year-olds, but was relatively low [ 10 , 11 ]. Furthermore, total caffeine consumption did not significantly increase, as there was a concurrent reduction in caffeine intake from sodas in 2001–2010 [ 11 ].

Although representative data on caffeine intake in Americans are available [ 1 , 2 , 11 , 12 , 13 , 14 , 15 ], these data do not include US service members (SMs). Among civilians, caffeine use varies depending on occupation; working in “legal” and “management” occupations was associated with greater caffeine intake than the average of all other occupations investigated [ 15 ]. Military personnel must engage in a number of physically- and cognitively-demanding tasks such as intelligence gathering, tactical planning, guard duty, lengthy marches with heavy backpacks, lifting and carrying substantial loads, movement over and under obstacles, and other operational tasks that can require lengthy and intense activity. Furthermore, military work hours are not regulated (restricted) by the labor laws governing the civilian population in the US, and active duty SMs are employed full-time. The extensive physical demands placed on SMs include early morning physical training and limited sleep during training, operations, and deployments. Studies of US SMs have found that approximately 70% sleep < 6 h per night and only ~ 30% are getting the recommended 7–8 h of sleep per night [ 16 ]. This contrasts with studies of the US civilian population where 72% of civilians sleep ≥7 h per night and only 28% sleep < 6 h/night [ 16 , 17 ]. Demanding tasks and lack of adequate sleep may lead SMs to consume more caffeinated substances than the general population. In particularly demanding circumstances, such as combat environments, and in populations such as aviators, caffeine use is particularly high [ 18 , 19 ].

Caffeine consumption in Army [ 20 ], Navy/Marine Corps [ 21 ] and Air Force [ 22 ] personnel has been investigated in separate surveys by our group, usually in convenience samples, and was higher than the civilian population [ 1 , 2 , 15 , 20 , 21 , 22 ]. The purpose of the current investigation was to examine the more recent prevalence of caffeine consumers, amount of caffeine consumption, and factors associated with use in a single, large, stratified random sample of US military personnel from all services.

Materials and methods

This investigation involved a cross-sectional survey completed by US military SMs and was part of a larger study involving the health effects of dietary supplements [ 23 ]. The Naval Health Research Center’s institutional review board approved the study protocol, and SMs consented to participate by signing an informed consent document. Investigators adhered to policies and procedures for protection of human subjects as prescribed by Department of Defense Instruction 3216.01, and the research was conducted in adherence with provisions of Title 32, Code of Federal Regulations, Part 219.

Sampling frame and solicitation procedures

Details of the sampling frame, solicitation of SMs, participant flow through the study, and response bias were described previously [ 23 ]. Briefly, investigators requested a random sample of 200,000 SMs, stratified by sex (88% male and 12% female) and branch of service (Army 36%, Air Force 24%, Marine Corps 15%, and Navy 25%), from the Defense Manpower Data Center. Recruitment of SMs into the study from this random sample involved a maximum of 12 sequential contacts. Investigators sent the prospective participant an introductory postal letter with a $1 pre-incentive designed to increase the response rate [ 24 , 25 ]. The letter described the survey, included a link to a secure website, and a unique login that could be used to access the web-based survey and electronically sign the consent form. SMs who did not initially complete the survey were sent a follow-up email message after 10 days, and a postcard after 3 weeks as a reminder. If the SM did not respond after having received the postcard, he/she received up to seven additional email reminders and three postcards evenly distributed during the time the survey was open. Reminders were sent only to those who had not responded. All postal and on-line contacts stated the SM could decline participation at any time and be removed from the contact list. Recruitment began in December 2018; after August 2019, no further recruitment was conducted, and no surveys were accepted.

Survey description

The questionnaire was administered on-line and was designed to characterize participants and quantify the frequency and amount of caffeinated products they consumed in the last 6 months. To characterize participants, there were questions on demographics (gender, age, education, race/ethnicity, height, weight), lifestyle factors (exercise, tobacco use, alcohol consumption, sleep), and military characteristics (rank, occupation assignment, service branch). Participants were asked to select serving sizes for coffee, tea, and soft drinks and how often they were consumed. Sizes available for selection included 8, 12, 16, 20, and 24+ ounces; frequencies available for selection included per day, week, month, or year; and number of times consumed available for selection ranged from 0 to 8 for each frequency option. For energy shots and energy beverages, participants provided the number of bottles or packets (0 to 16) and the frequency (per day, week, month, or year). For caffeinated candy and gum, participants provided the number of candies or sticks of gum (0 to 16) and the frequency (per day, week, month, or year). For caffeinated medications, participants provided the number of pills consumed (0 to 16) and the frequency (per day, week, month, or year). Examples were provided for energy shots (e.g. 5-Hour Energy), energy beverages (e.g., Red Bull, Monster, Full Throttle), caffeinated candy/gum (e.g., Military Energy, GoFastGum), and caffeinated medications (e.g., Bayer, Excedrin, No Doz).

Statistical analysis

Statistical analyses were conducted using the Statistical Package for the Social Sciences (Version 21.0.0.0, 2019, IBM Corporation). Caffeine products were grouped into five types: 1) coffee; 2) tea; 3) sodas; 4) energy drinks (energy shots and energy beverages combined); 5) caffeinated gums/medications (candies, gum, and medications combined). All types were then combined to determine an aggregated caffeine intake (i.e., any caffeine product).

An individual who used any caffeinated product ≥1 time/week was considered a caffeine consumer. This frequency was selected to be relatively consistent with consumption frequencies used in other studies [ 2 , 12 , 20 , 21 , 22 , 26 , 27 , 28 ]. Caffeine consumption (mg/day) was calculated based on the serving size and the frequency of consumption using publicly available databases and estimates of caffeine content in various products [ 27 , 29 ]. One ounce of coffee, tea, and soda was considered to contain 12, 6, and 3 mg caffeine per ounce, respectively. Energy drinks and energy shots were considered to contain 160 and 200 mg caffeine, respectively. Energy gum/candy and medication were considered to contain 50 and 100 mg caffeine, respectively.

Body mass index (BMI) was calculated as self-reported weight/height 2 (kg/m 2 ). Weekly duration of aerobic training or resistance training (minutes/week) was calculated by multiplying weekly exercise frequency (sessions/week) by the duration of training (minutes/session). Tobacco users were defined as individuals who reported using any tobacco products in the past week; former tobacco users were defined as those who reported having used tobacco products in the past but had quit within the last year or earlier.

“Caffeine prevalence” refers to the proportion (%) of SMs using a caffeinated product ≥1 day/week; “caffeine consumption” refers the total amount of caffeine consumed each day (mg/day). Prevalence of caffeine consumers (%) with standard errors (SE) was calculated for each caffeine product type individually and for all caffeine products in aggregate (i.e., any caffeine). Chi-square statistics were used to examine differences in the prevalence of caffeine consumers across various strata of demographic, lifestyle, and military characteristics. A one-way analysis of variance (ANOVA) examined differences in caffeine consumption (mg/day) across strata of these characteristics. For ordinal variables (i.e., age, education, BMI, aerobic training duration, resistance training duration, alcohol intake, sleep), tests for linear trend, Mantel-Haenszel statistics, and ANOVA linear contrasts were also performed. Since some participants did not complete all of the questions, tables present the number of participants for each variable. Multivariable logistic regression was used to examine associations between use and non-use of caffeine products (≥1 time/week) and independent variables that included all the demographic, lifestyle, and military characteristics. Six separate regression models were developed for each caffeine product type: “any caffeine,” coffee, tea, soda, energy drinks, and caffeinated gum/medications. A one-way ANOVA for linear trend compared caffeine consumption across age groups in men and women separately. Self-reported sleep duration was not included in the multivariable analyses because only 78% of SMs responded to this question. Since multivariable analysis requires complete data on all variables, including sleep duration would have removed a large number of SMs from the multivariable analyses.

A separate analysis compared high versus lower caffeine consumers, defined as ≥400 mg/day and < 400 mg/day, respectively. Prevalence of caffeine consumers (%) with SEs were calculated for high and lower consumers on each of the demographic, lifestyle, and military characteristic. Univariable logistic regression compared unadjusted differences between high and low consumers across the characteristics; multivariable logistic regression compared differences adjusted for all demographic, lifestyle and military characteristics.

From the initial list of 200,000 potential volunteers, 146,365 (73%) were successfully contacted (i.e., no returned postal mail). Of these, 26,680 (18.2%) signed the informed consent and completed the questionnaire online.

Caffeine use prevalence

Table  1 provides the prevalence of caffeine consumers by demographic, lifestyle, and military characteristics. Overall, 87% of participants reported using products containing caffeine ≥1 time per week, with coffee and soda being the most frequently employed. For energy drinks and energy shots considered individually, use (prevalence±SE) was 28.4 ± 0.3% and 2.4 ± 0.1%, respectively; for gums/candies and medications individually, use was 0.9 ± 0.1% and 3.5 ± 0.1%, respectively.

Table 1 indicates there was little difference between men and women in aggregate use of any caffeinated product or coffee; however, women were much more likely to use tea and gums/medications while men were much more likely to use soda and energy drinks. The proportion of SMs using any caffeinated product increased with age, especially for coffee, tea, soda, and gums/medications, but use of energy drinks decreased with age. The proportion of SMs using any caffeinated product increased with formal educational level, especially for coffee and tea, while use of soda and energy drinks decreased as formal education increased; use of gums/medications were highest among those with some college. Among race/ethnicities, white SMs had the largest proportion using caffeinated products, especially for coffee, soda, and energy drinks, while black SMs had the lowest proportion using these same products. As BMI increased, so did use of most caffeinated products, except tea, as prevalence was highest among the lowest BMI category.

For most caffeinated products, aerobic exercise duration was not related to use in any systematic way, although energy drink use increased modestly as exercise duration increased. SMs performing the most resistance training had the lowest use of any caffeinated products, especially for coffee and tea. As resistance training increased, use of soda decreased, and use of energy drinks increased. Among those who reported any weekly resistance training ( n  = 22,872), use of any caffeinated product was 87.9 ± 0.2%, compared to 80.5 ± 0.6% among those who did not report any weekly resistance training ( n  = 3808) ( p  < 0.01). Smokers and former smokers had the highest use of caffeinated products among all product types except tea, where there were no significant differences among groups. Smokeless tobacco users and former users also had the highest use of caffeine for all product types except tea, where those who had never used smokeless tobacco had the highest caffeine use. Use of caffeinated products among all types increased as alcohol consumption increased. Those reporting ≥5 h/night of sleep had the highest aggregated caffeine and coffee use, but those reporting < 6 h/night had the highest use of tea, soda, energy beverages, and gums/medications.

Among both enlisted SMs and officers, as rank increased, so did aggregated use of caffeinated products, especially coffee, tea, soda, and gum/medication. For energy drinks, the trend was the opposite: as rank increased, energy drink use decreased. Enlisted soldiers were more likely to use energy drinks than officers, and the lowest use of energy drinks was among senior officers. SMs in combat arms occupations were more likely to use any caffeinated product, especially coffee and energy drinks, while combat service support personnel had the highest use of tea and gums/medications. Navy personnel had the highest use of caffeinated products of all types, except energy drinks, where Marine Corps personnel had the highest use.

Caffeine consumption

Table  2 provides the estimated daily caffeine consumption (mg/day) among caffeine consumers by their demographic, lifestyle, and military characteristics. The average daily consumption of caffeine was 243 mg/day. Coffee, tea, soda, energy drinks, and gums/medications accounted for 69, 8, 6, 17, and < 1% of caffeine consumption, respectively. The per-capita consumption (all participants including non-consumers) was 211 ± 1 mg/day, with men ingesting 218 ± 2 mg/day and women 167 ± 3 mg/day.

Men consumed more total caffeine than women due to a greater intake from coffee, soda, and energy drinks; women consumed more caffeine from tea. When total caffeine consumption was determined on a body weight basis, consumption was similar among male and female consumers (2.93 vs 2.85 mg/day/kg, men and women, respectively, p  = 0.12). Caffeine consumption increased with age, largely accounted for by the increase from coffee, while caffeine consumption from energy drinks decreased with age. Caffeine consumed from tea and soda was greatest in the youngest and oldest age groups. Total caffeine consumption differed little by formal educational level, but caffeine from coffee increased with more formal education, while caffeine from soda, energy drinks, and gums/medications decreased with more formal education. White and Hispanic SMs consumed the most total caffeine, accounted for largely by coffee, soda, and energy beverages, while black SMs consumed the least total caffeine and had the least caffeine consumption from coffee, soda, and energy beverages. As BMI increased, so did total caffeine consumption, especially from coffee, soda, and energy drinks; caffeine from gums/medications was highest among the lowest BMI group.

As the amount of aerobic exercise increased, so did total caffeine consumption, accounted for largely by caffeine from coffee and energy drinks. Caffeine from soda decreased as aerobic exercise increased; caffeine from tea and gums/medications was highest in the group performing the most aerobic exercise. As the amount of resistance training increased, caffeine from coffee, tea, and soda tended to decrease, while caffeine from energy drinks increased. Among smokers and smokeless tobacco users, the pattern of caffeine consumption was similar: current and former users had the highest total caffeine consumption, accounted for largely by consumption from coffee, soda, and energy drinks. As alcohol intake increased, so did the total consumption of caffeine, accounted for by caffeine from coffee. Caffeine from tea, soda, energy drinks, and gums/medications was highest among non-alcohol users and those in the highest alcohol level. Consumption of caffeine from all sources increased as the amount of sleep decreased. The average ± standard deviation for self-reported sleep was 6.3 ± 1.4 h.

Consumption of total caffeine and caffeine from coffee increased with rank among enlisted personnel and officers, while consumption from energy drinks decreased with rank among enlisted and officers. Caffeine from soda decreased with rank among enlisted SMs but increased with rank among officers. SMs employed in combat arms occupations had the highest total consumption of caffeine and consumption from coffee and energy drinks, while those in combat service support occupations had the highest consumption from tea and soda. Marine Corps and Navy personnel had the highest total consumption of caffeine. Caffeine from coffee and tea was highest among Navy personnel, while caffeine from energy drinks was highest among Marine Corps personnel. Air Force personnel had the lowest total caffeine consumption and the lowest consumption from coffee and energy drinks.

Characteristics independently associated with prevalence of caffeine consumers

Table  3 provides results of the multivariable logistic regression examining factors associated with the use of caffeinated products ≥1 time per week. The results are for six full models with all characteristics entered. About 91% ( n  = 24,324) of SMs had complete data on all variables and were included in each model.

Characteristics associated with higher overall caffeine use included female gender, older age, white race/ethnicity, higher BMI, less resistance training, current or former tobacco use, higher alcohol intake, and higher enlisted or officer rank. Higher coffee use was associated with female gender, older age, higher formal education, white race/ethnicity, higher BMI, former or current tobacco use, higher alcohol intake, higher enlisted or officer rank, and service in the Navy (compared to the Air Force). Higher use of tea was associated with female gender, older age, more formal education, other race/ethnicity (compared to whites), white race/ethnicity (compared to Hispanics), more aerobic exercise, less resistance training, current smoking, never using smokeless tobacco, higher alcohol intake, and service in the Navy (compared to the Air Force) or Air Force (compared to the Marine Corps). Higher use of soda was associated with male gender, less formal education, white race/ethnicity, higher BMI, less resistance training, current or former smoking, higher alcohol consumption, junior enlisted status (compared to junior officer status), and service in the Air Force (compared to all other services). Higher use of energy drinks was associated with male gender, younger age, less formal education, white race/ethnicity, higher BMI, more resistance training, current or former tobacco use, higher alcohol consumption, lower enlisted rank (compared to officers), and service in the Army or Marine Corps (compared to the Air Force) or in the Air Force (compared to the Navy). Higher use of caffeinated gums/medication was independently associated with female gender, older age, white and black race/ethnicity, higher BMI, higher alcohol intake, and service in the Army (compared to the Air Force).

Prevalence and characteristics of high caffeine consumers

The proportion of high caffeine consumers (≥400 mg/day) was 15.9% (17.1% of men and 8.9% of women), and the proportion with an overall consumption ≥300 mg/day was 27.3% (28.8% of men and 17.5% of women). The types of products ingested by the high caffeine consumers were similar to those of the entire cohort: coffee, teas, sodas, energy drinks, and gums/medications accounted for 68, 7, 5, 19, and 1% of caffeine consumption, respectively.

Table  4 compares high caffeine consumers (≥400 mg/day) to lower consumers (≤400 mg/day) on their demographic, lifestyle and military characteristics. In univariable analyses, higher caffeine use was associated with male gender, older age, less formal education, white race/ethnicity, higher BMI, more aerobic exercise, less resistance training, tobacco use or former use, higher alcohol intake, less sleep, higher enlisted or officer rank, combat arms occupations, and service in the Army, Marine Corps, or Navy (compared to the Air Force). About 91% of caffeine consumers ( n  = 21,443) had complete data for the multivariate model. In the multivariable analyses with all characteristics included, most of the relationships found in the univariate analyses were retained, although somewhat attenuated; rank and occupational assignment group were no longer significant.

Caffeine consumption by age and sex

Figure  1 presents daily caffeine consumption (mg/day) from all types of caffeinated products by age and sex. As age increased, there was a significant linear trend for increasing consumption of any caffeine and caffeine from coffee among both men and women ( p  < 0.01, both sexes). In contrast, there was a significant linear trend for less consumption of energy drinks as age increased for both men and women ( p  < 0.01, both sexes). While there was a significant linear trend of increased caffeine consumption from tea over age among men ( p  = 0.02), there was no such trend among women ( p  = 0.42). There were no significant linear trends over age for soda (men p  = 0.07, women p  = 0.48) or for gums/medications (men p  = 0.13, women p  = 0.82).

Daily average consumption of caffeinated substances by gender and age

This very large ( n  = 26,680), comprehensive assessment of SM caffeine consumption found 87% of SMs consumed caffeinated products, with an average estimated consumption of 243 mg/day for consumers. Men consumed more caffeine than women, but when adjusted for body weight, consumption was similar by gender. Coffee was the most frequently consumed beverage, followed in descending order of prevalence by soda, tea, energy drinks, and gums/candies/medications. By total caffeine consumption (mg/day) and in desending order coffee, energy drinks, tea, soda, and gums/medications were the most often used. Consuming any caffeinated product was independently associated with female gender, older age, white race/ethnicity, higher BMI, less resistance training, current or former tobacco use, higher alcohol intake, and higher enlisted or officer rank. Higher energy drink prevalence was associated with male gender, younger age, less formal education, white race/ethnicity, higher BMI, more resistance training, current or former tobacco use, higher alcohol consumption, lower enlisted rank (compared to officers), and service in the Army or Marine Corps (compared to the Air Force) or in the Air Force (compared to the Navy).

It is well documented that the civilian and military populations are generally aware of the effects of caffeine on human cognitive and physical performance. Surveys of SMs and college students found they consume caffeine-containing products for several reasons related to the performance benefits of caffeine [ 19 , 30 , 31 ]. Furthermore, SMs assigned to units in Afghanistan and likely to be engaged in combat consumed higher levels of caffeine than SMs at their home bases. Caffeine use by these SMs was higher among those reporting difficulty remaining awake during guard duty, poor sleeping conditions, and sleep disruptions during nighttime operations [ 18 ]. In addition, a survey of active duty Army aviators found they consumed more caffeine than their peers in non-aviation units, especially to enhance performance degraded due to insufficient sleep and very disruptive work schedules [ 19 ].

US Department of Defense laboratories and their international collaborators have conducted multiple studies designed to simulate military operations demonstrating the cognitive and physical benefits of caffeine consumption by military personnel [ 32 , 33 , 34 ]. The Department of Defense recognizes the ability of caffeine to enhance cognitive performance and provides it in rations, when necessary, with appropriate labeling to inform SMs of the presence and effects of caffeine [ 35 ].

Prevalence of caffeine consumers and daily caffeine consumption

Previous studies have been conducted on the prevalence of caffeine consumers and daily consumption among Air Force [ 22 ], Army [ 20 ], and Navy/Marine Corps [ 21 ] personnel. All of these studies [ 20 , 21 , 22 ] used a slightly different questionnaire but the same definitions for caffeine sources. The Air Force [ 22 ] and Army [ 20 ] studies used a convenience sampling technique involving volunteers in face-to-face administrations at installations across the US and overseas, and the Navy and Marine Corps study [ 21 ] identified a random sample and asked for volunteers by postal letter and e-mail. The present study was quite similar to the Navy/Marine Corps study [ 21 ] in that a random sample of SMs were studied, but the questionnaire differed from that of previous studies [ 20 , 21 , 22 ]. Those studies listed not only generic sources of caffeine (e.g., coffee, tea, soft drinks), as in the present study, but specific products (e.g., Dr. Pepper soda, Monster energy drink, No Doz Gum) as well. Given these differences in study design, Table  5 compares caffeine use prevalence and daily consumption among the military services in the current and past studies. Estimates of the prevalence of caffeine consumers for any caffeinated product (≥ 1/week) were similar across all studies. With regard to individual caffeinated products, the previous Army and Navy/Marine Corps studies [ 20 , 21 ] found the highest prevalence of consumers for coffee, but Air Force personnel were unique in that cola was the most ingested product, with coffee ranking second [ 22 ]. The current study found that in all services, coffee was the product consumed most often. Daily caffeine consumption estimates were similar for Air Force personnel in the current and past [ 22 ] investigations, but estimates for Army, Navy, and Marine Corps personnel were 38% lower, 21% higher, and 16% higher, respectively [ 20 , 21 ]. Differences in estimation of caffeine consumption from individual products in past studies [ 20 , 21 , 22 ] versus estimates from generic types (coffee, tea, soda) in the current study likely accounted for these differences. Most past studies [ 20 , 21 ] and the current one agree in that SMs in all services consumed the most total caffeine (mg/day) from coffee, with energy drinks ranking second.

There have also been several studies of the prevalence of caffeine consumers and consumption in the military of other countries, although all studies used convenience samples and many were conducted over a decade ago. Among British soldiers deployed to Iraq in 2009, and Afghanistan in 2010, 89 and 92%, respectively, reported consuming a caffeinated product [ 36 ]. In 2010–2011, 42% of United Kingdom-based British soldiers reported using energy drinks and 8% caffeine tablets [ 37 ]. Among Australian soldiers, 71% reported using caffeinated products and 28% reporting energy drink consumption [ 38 ]. New 19-yr old Danish conscripts surveyed in 2001–2006 reported consuming an average of 199 mg/day from coffee, tea, soda, and foods [ 39 ]. These data indicate that British soldiers and US SMs have a similar prevalence of caffeine use, but Australian soldiers appear to have a lower use prevalence. Nonetheless, Australian soldiers have a very similar prevalence of energy drink use compared to the US SMs, but British soldiers report much higher use. Overall caffeine consumption in comparably aged US military personnel appears similar to that of Danish conscripts.

Several population-based estimates of caffeine consumption in adult Americans based on very large population samples using state-of-the-art dietary intake procedures are available. NHANES caffeine intake [ 1 , 13 , 15 ] was calculated based on 24-h dietary recalls in 2001–2012. Estimated caffeine use prevalence in adults (> 19 years) was 89% for men and 89% for women [ 1 ]. Caffeine consumption estimates for consumers of caffeine varied from 189 to 211 mg/day for men and 149 to 161 mg/day for women [ 1 , 13 , 15 ]. The Kantar Worldpanel Beverage Consumption Panel obtained data on US consumers from an online, 7-day beverage consumption record and found ~ 90% of individuals ≥18 years of age consumed caffeinated beverages, with average caffeine consumption equal to about 200 mg/day among caffeine consumers [ 2 ]. The prevalence of caffeine consumers in these population-based studies were similar to the 87% observed in SMs (≥1 week), while the average consumption in SMs of 251 and 195 mg/day for males and females, respectively, was somewhat higher than in the civilian population.

At least three other surveys [ 20 , 21 , 22 ] of the individual branches of service have observed caffeine-intake levels similar to those reported here and higher than those in the civilian population. The extensive and unique demands of military service may be a factor that explains the difference in caffeine intake in military versus civilian personnel. Differences in the methods and the demographic characteristics of the samples used in civilian studies and the current investigation must also be considered when interpreting these differences. For example, active duty SMs are younger, fully employed, and sleep somewhat less than the general population [ 16 ].

Energy drink prevalence (≥ 1 time/week) was 29% in the present study and varied from 21 to 39% in the previous military studies [ 20 , 21 , 22 , 40 , 41 , 42 ]. Various studies of the prevalence of energy drink use among US college students found that 39% reported consuming in the past week [ 43 ], 36% within the past 2 weeks [ 44 ] and 36% within the past year [ 30 ]. Data from several NHANES cycles indicated that prevalence of daily consumption of energy drinks among adults has increased from 2003 to 2016 [ 10 ]. With regard to caffeine consumption, the current study found that 17% of the total caffeine was consumed from energy drinks. Data from NHANES suggested only 1–2% of total caffeine consumed by Americans was from energy drinks [ 1 , 13 ], but a study of a convenience sample of geographically dispersed college students in the US found 22% of their total caffeine consumption was from energy drinks [ 30 ]. In summary, the prevalence of energy drink consumption by SMs, and the proportion of total caffeine consumption from energy drinks by SMs, are similar to those of college students— despite the generally older age of SMs—and much higher than those of the general US population.

Characteristics associated with caffeine use

In the univariate analysis, there was little gender difference in the prevalence of use for any caffeinated products and for coffee. In the multivariate analysis, however, women had greater odds of use than men. This was primarily due to the confounding influence of alcohol consumption in the statistical models, although smoking and smokeless tobacco also had minor effects. Caffeine consumption increased as alcohol intake increased, or if individuals were tobacco users; men were more likely to be higher alcohol consumers or tobacco users. The strength of the association between caffeine use and alcohol and tobacco use was stronger in men than women and this reduced the effect of male gender alone. This reduced effect of male gender allowed female gender to become highly significant. In statistical terms, alcohol or tobacco use accounted for a larger proportion of the odds ratio for the effect of sex on caffeine use in men than in women. Because of this, the odds of using caffeine became lower in men than in women. Dividing the higher odds of caffeine use in women by the lower odds of caffeine use in men resulted in the larger odds ratio for women for any caffeinated product and coffee. If alcohol consumption, smoking, and smokeless tobacco use were not included in models 1 and 2 (Table 3 ) the odds ratios (95% confidence intervals) for women (compared to men) were 1.02 (0.92–1.14) and 1.02 (0.94–1.11), respectively.

In agreement with the current study, others [ 1 , 12 , 13 , 20 , 21 ] have reported that men consumed more caffeine than women. Nonetheless, this study and others [ 12 , 21 , 22 ] found that when caffeine consumption was determined on a per kg body weight basis, men and women consumed similar amounts. Although coffee was the major source of caffeine for both men and women, female SMs consumed more caffeine from tea while male SMs consumed more caffeine from soda and energy drinks. Acute caffeine consumption modestly affects moods such as vigor and fatigue as well as hemodynamic measures (e.g. blood pressure, cardiac output) in men and women [ 45 , 46 , 47 ], although cardiovascular effects are more likely to be observed at higher doses. Both men and women report consuming caffeinated products to provide behavioral benefits such as increased alertness [ 19 , 30 , 31 ].

Investigations involving representative civilian [ 1 , 2 , 12 , 13 , 27 ] and military [ 21 , 22 ] samples reported that overall use and/or amount of caffeine consumption increased with age, although prevalence of use and/or caffeine amounts decline at the highest age groups in civilian studies (generally > 65 years) [ 1 , 2 , 12 , 13 , 27 ]. Also in general agreement with past military studies [ 20 , 21 , 22 ], the current study found that coffee consumption accounted for most of the caffeine ingested in all age groups, but younger (< 40 years) individuals consumed over twice as much caffeine from energy drinks as older (≥40 years) individuals (46 vs 22 mg/day, p  < 0.01) and were almost twice as likely to consume energy drinks (33 vs 17%, p  < 0.01). Energy drinks were introduced into the American market in 1997 [ 48 ], and their advertising was targeted to teenagers and individuals in 18- to 34-year-olds [ 49 ]. This advertising may have influenced energy drink consumption in the younger age groups in the current study.

Other civilian [ 13 , 15 ] and military [ 20 , 21 , 22 ] studies have reported that compared to whites, blacks have a lower prevalence of caffeine use and a lower total caffeine consumption, accounted for largely by less coffee consumption [ 20 , 21 , 22 , 26 ]. There are race/ethnic differences in dietary intake [ 50 , 51 ], and some of these differences appear to be partly accounted for by educational level and income [ 51 , 52 ]. In the current study, differences between black and white SMs in caffeine and coffee use prevalence remained after controlling for formal educational level, rank (a surrogate for income), and other factors, in agreement with past military studies [ 21 , 22 ]. The reasons for the race/ethnic differences are likely complex and may be different in the military compared to the general population.

In agreement with other investigations [ 20 , 21 , 30 ], the current study found no systematic association between weekly aerobic exercise duration and caffeine use prevalence. One study of Air Force personnel [ 22 ] found that the prevalence of caffeine consumers decreased with increased aerobic activity duration; in the current study, when Air Force personnel were considered separately, this relationship was not duplicated (data not shown). For resistance training, both univariate and multivariable analysis showed the lowest caffeine use prevalence in the group exercising the most with little difference among the other groups, in general agreement with most other military studies [ 21 , 22 ]. One study which separated Army personnel into those who performed weight training and those who did not found that trainers had higher overall use prevalence [ 20 ], also in agreement with the current study. Previous military studies have shown that dietary supplement use was strongly associated with increasing resistance training duration [ 21 , 23 ]. Many dietary supplements contain caffeine, and the caffeine content of some of these can be very high [ 53 ]. Accurately determining the caffeine content of dietary supplements is difficult because manufactures are not required to list the amount of caffeine on their supplement facts labels, amounts are usually not available on company websites, and if the ingredients are proprietary, the manufacturer is not required to list caffeine at all [ 53 ]. It is possible that SMs involved in large amounts of resistance training consumed less caffeine from beverages to avoid adverse effects resulting from high dosages of caffeine in their dietary supplements. Overall, the current data and previous investigations suggest little relationship between aerobic exercise duration and caffeine use prevalence, but for resistance training there appears to be a bimodal relationship such that those performing the least or the most training have lower use prevalence than those performing moderate amounts of training.

Current or former tobacco use (smoking or smokeless tobacco) was associated with a higher use and higher intake of caffeine, especially for coffee and energy drinks, in both univariate and multivariable analyses. Although associations with smokeless tobacco have not been previously investigated, associations between caffeine use prevalence and smoking have repeatedly been reported in both military [ 20 , 22 ] and civilian populations [ 14 , 15 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ]. Smoking accelerates caffeine metabolism and reduces its half-life [ 61 , 62 ] suggesting that smokers consume more caffeine to achieve stimulatory effects. In addition, both caffeine and smoking increase dopaminergic activity in different brain regions, and the two substances may be used concurrently to potentiate stimulation [ 63 ].

Another lifestyle factor strongly associated with prevalence of caffeine consumers and caffeine consumption was alcohol intake. In both univariate and multivariable analyses, use of caffeinated products of all types increased in a dose-response manner as alcohol consumption increased. The amount of caffeine consumed from coffee and energy drinks increased as alcohol intake increased. Similar relationships have been found in other studies for coffee [ 14 , 21 , 58 , 64 ], energy drinks [ 21 , 28 , 65 , 66 ], and overall caffeine use [ 15 , 21 , 28 ]. Studies of monozygotic and dizygotic twins suggested that there was a common genetic factor underlying this association, but environmental influences still seemed to contribute to the variance in caffeine consumption [ 67 , 68 , 69 ]. A recent study based on variations in single nucleotide polymorphisms support that the genes underlying the use of both coffee and alcohol were heritable [ 70 ]; however, two-sample Mendelian randomization suggested there was no causal association between coffee consumption and alcohol consumption [ 70 , 71 ]. Psychosocial factors may play a role in this association since studies have consistently shown that higher levels of sensation–seeking behaviors are associated with both higher caffeine and alcohol use [ 72 , 73 , 74 ].

In the current study, SMs who reported less daily sleep consumed more caffeine for all sources, in agreement with past military [ 21 , 22 , 75 ] and civilian [ 76 ] studies. Military personnel sleep less than civilian populations [ 16 , 17 ] and averaged 6.3 h in current study, less than the recommended ≥7 h/night [ 77 ]. Military training and operations can occur at any time of the day, can extend continuously for many days, and can involve substantial loss of sleep. Caffeine can increase alertness due to its ability to block central adenosine receptors [ 78 ]; when ingested in sufficient dosages, it can reduce sleep duration [ 79 ], and it improves cognitive performance, especially vigilance [ 80 , 81 , 82 ].

High caffeine consumers

The estimated average daily caffeine consumption of military personnel who are regular caffeine consumers was well below the levels that are widely recognized as safe: 400 mg/day for men and 300 mg/day for women of reproductive age [ 5 , 6 , 7 ]. Nonetheless, the present study found that caffeine consumption of 17% of men and 9% of women exceeded 400 mg/day, and that of 18% of women exceeded 300 mg/day. These proportions are similar to those found in past military studies [ 21 , 22 ]. Some individuals may be able to consume higher amounts of caffeine without adverse effects, although this cannot be determined from the current data. A genetic polymorphism allows some individuals to metabolize (N 3 -demethylation) caffeine in the liver more rapidly than others, and another polymorphism may be associated with higher caffeine tolerance and consumption [ 83 , 84 , 85 ].

Interestingly, the proportions of caffeine consumed from various dietary sources were very similar for the entire cohort and high caffeine consumers, suggesting high consumers just ingested a larger volume of these products. High caffeine consumers also shared many of the demographic and lifestyle characteristics of the entire cohort, except that they were almost twice as likely to be men, had less formal education, and were less likely to serve in the Air Force. Women have greater health awareness in that they are more likely to seek medical care [ 86 , 87 , 88 ] and make behavioral changes to improve health [ 89 , 90 , 91 ] that could moderate caffeine consumption. Individuals who have achieved higher education levels are generally more proactive, health conscious, prone to engage in health promoting behaviors, and likely to explore multiple channels of information related to their health [ 92 , 93 , 94 , 95 ], that could also be associated with management of caffeine consumption.

Strengths and limitations

A major strength of this study was recruitment of a very large, stratified random sample of SMs who answered a standard set of questions on their consumption of specific caffeinated products. With a few exceptions, the data largely confirm results of past investigations of caffeine prevalence and consumption involving smaller studies of separate military services, using largely convenience samples [ 20 , 21 , 22 ]. Nonetheless, there are several limitations to the current analyses, most of which relate to difficulty in estimating daily caffeine consumption. First, all data were self-reported, and the usual shortcomings associated with this method, including recall bias, social desirability, errors in self-observation, and inadequate recall, apply [ 96 , 97 ]. These biases could account for errors in reporting serving sizes and how many times per week SMs used caffeinated products and, as a consequence, errors in estimating caffeine consumption. Second, caffeine data for this study were obtained from beverages and gums/medications; we purposely did not assess caffeine intake from food sources as beverages account for 98% of caffeine consumption [ 1 ]. Third, caffeine contents of products were estimated based on standardized values of each type of caffeinated product. Specific products can differ in caffeine content [ 29 , 98 , 99 , 100 ]. Fourth, the questionnaire used in this study was not validated against other measures of caffeine consumption such as plasma caffeine levels or beverage records. Fifth, caffeine from dietary supplements was not assessed and it is known that SMs use a larger number of dietary supplements [ 23 ]. This likely resulted in an underestimate of total caffeine consumption. Thus, this study is focused on caffeine consumption from commonly consumed caffeine sources exclusive of dietary supplements. Finally, a large number of statistical tests examining relationships between caffeine prevalence and consumption and the demographic, lifestyle, and military characteristics were conducted, thus increasing the probability of Type 1 errors.

Conclusions

Among all military personnel surveyed, 87% reported using caffeinated products ≥1 time/week, with male and female consumers ingesting (mean ± SE) 251 ± 2 and 195 ± 3 mg/day, respectively. The prevalence of caffeine consumption by military personnel was similar to that reported in NHANES data, but total caffeine consumption was higher. Compared to civilians, SMs may consume more caffeine to enhance their cognitive and physical performance due to the intense occupational demands of their profession. The most commonly consumed caffeinated products (% users) were coffee (68%), soda (42%), tea (29%), and energy drinks (29%). Coffee, tea, soda, energy drinks, and gums/medications accounted for 69, 8, 6, 17, and > 1% of total caffeine consumption, respectively. The prevalence of energy drinks consumption and amount of caffeine ingested from energy drinks was about twice as high among those < 40 years of age compared to those ≥40 years of age. Characteristics associated with caffeine use in SMs were generally similar to those observed in investigations of civilians.

Availability of data and materials

The datasets generated and/or analyzed during the current study are not publicly available due to US government restrictions, but are available from the corresponding author on reasonable request.

Abbreviations

Analysis of variance

Body mass index

National Health and Nutrition Examination Survey

Service member

Standard error

United States

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Acknowledgements

Thanks to Dr. Kathryn Taylor for assistance in interpreting the data, to Ms. Maureen Humphrey-Shelton for assistance in obtaining references, and to Ms. Lauren Thompson for editorial comments.

This work was funded by Department of Defense Center Alliance for Nutrition and Dietary Supplement Research of the Defense Medical Research and Development Program, the US Army Medical Research and Development Command (USAMRDC). The Bureau of Medicine and Surgery also supported this work under Work Unit No. N1335.

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Knapik, J.J., Steelman, R.A., Trone, D.W. et al. Prevalence of caffeine consumers, daily caffeine consumption, and factors associated with caffeine use among active duty United States military personnel. Nutr J 21 , 22 (2022). https://doi.org/10.1186/s12937-022-00774-0

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• Published: 25 February 2021

The impact of daily caffeine intake on nighttime sleep in young adult men

• Janine Weibel 1 , 2 ,
• Yu-Shiuan Lin 1 , 2 , 3 ,
• Hans-Peter Landolt 4 , 5 ,
• Joshua Kistler 1 , 2 ,
• Sophia Rehm 6 ,
• Katharina M. Rentsch 6 ,
• Helen Slawik 7 ,
• Stefan Borgwardt 3 ,
• Christian Cajochen 1 , 2   na1 &
• Carolin F. Reichert 1 , 2   na1  

Scientific Reports volume  11 , Article number:  4668 ( 2021 ) Cite this article

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• Slow-wave sleep

Acute caffeine intake can delay sleep initiation and reduce sleep intensity, particularly when consumed in the evening. However, it is not clear whether these sleep disturbances disappear when caffeine is continuously consumed during daytime, which is common for most coffee drinkers. To address this question, we investigated the sleep of twenty male young habitual caffeine consumers during a double-blind, randomized, crossover study including three 10-day conditions: caffeine (3 × 150 mg caffeine daily), withdrawal (3 × 150 mg caffeine for 8 days, then switch to placebo), and placebo (3 × placebo daily). After 9 days of continuous treatment, electroencephalographically (EEG)-derived sleep structure and intensity were recorded during a scheduled 8-h nighttime sleep episode starting 8 (caffeine condition) and 15 h (withdrawal condition) after the last caffeine intake. Upon scheduled wake-up time, subjective sleep quality and caffeine withdrawal symptoms were assessed. Unexpectedly, neither polysomnography-derived total sleep time, sleep latency, sleep architecture nor subjective sleep quality differed among placebo, caffeine, and withdrawal conditions. Nevertheless, EEG power density in the sigma frequencies (12–16 Hz) during non-rapid eye movement sleep was reduced in both caffeine and withdrawal conditions when compared to placebo. These results indicate that daily caffeine intake in the morning and afternoon hours does not strongly impair nighttime sleep structure nor subjective sleep quality in healthy good sleepers who regularly consume caffeine. The reduced EEG power density in the sigma range might represent early signs of overnight withdrawal from the continuous presence of the stimulant during the day.

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Introduction

Caffeine is the most popular psychoactive substance in the world 1 , consumed daily by around 80% of the population 2 . While caffeine is frequently used to counteract sleepiness and boost performance 3 , its consumption is commonly avoided in the evening 4 , 5 to prevent adverse consequences on nocturnal sleep 3 . The sleep disrupting effects of caffeine are mainly attributed to its influence on the homeostatic component of sleep-wake regulation. Sleep homeostasis describes the increase in sleep pressure during time awake and its dissipation during the following sleep episode 6 , which has been suggested to be related to rising and decreasing concentrations of adenosine 7 . Caffeine is an adenosine receptor antagonist, which blocks the A 1 and A 2A adenosine receptors in the central nervous system 1 . It may, thus, attenuate the increase in sleep pressure during wakefulness 8 and lead to delayed sleep initiation and more superficial sleep 9 .

The effects of caffeine intake on the quality and quantity of sleep depend on the timing of its consumption. More specifically, caffeine consumed in the evening hours prolongs sleep latency 10 , 11 , 12 , 13 , 14 , reduces total sleep time (TST) 10 , 11 , 12 , 14 , 15 , shortens deep sleep 10 , 12 , 13 , 14 , 15 , and decreases electroencephalographically (EEG)-derived slow-wave activity (SWA) 10 , while activity in the sigma range is increased 10 . However, evening caffeine intake only accounts for approximately 10–20% of the total daily caffeine intake in regular consumers 4 , 5 . It needs to be elucidated whether habitual caffeine intake restricted to the morning and afternoon hours similarly affects nighttime sleep.

Furthermore, not only the timing but also the frequency of preceding caffeine intake prior to sleep may be an important factor for the repercussions on sleep. The majority of the worldwide population consumes caffeine on a daily basis 2 , which can lead to tolerance development due to the recurrent supply of the psychostimulant 1 . In line with these results, the sleep-disrupting effects of continuous high-dose caffeine in the morning, afternoon, and evening (3 × 400 mg) intake vanished and only stage 4 sleep remained reduced after 1 week of caffeine intake 12 . However, whether more sensitive markers for sleep intensity such as spectral sleep EEG measures, adapt to the long-term exposure to the stimulant has to our best knowledge not yet been investigated.

Importantly, not only caffeine per se, but also the state of acute abstinence to which regular consumers expose themselves every night, might affect sleep. This so-called overnight abstinence represents the start of a caffeine withdrawal phase 16 . Withdrawal symptoms such as increased tiredness 17 , longer sleep duration, and better sleep quality 18 can be observed at a subjective level starting roughly 12 h after last caffeine intake 17 . However, the influence of caffeine withdrawal on objective EEG-derived sleep variables were not systematically reported up to date and remain to be compared against a placebo-baseline.

Here we aimed at determining whether daily caffeine intake during morning and afternoon hours impairs nighttime sleep structure and sleep intensity after continuous daytime caffeine intake over 9 days. We hypothesized a reduced depth of sleep after caffeine intake, indexed in shortened slow-wave sleep (SWS) duration and a decrease in SWA compared to placebo. Moreover, we hypothesized that the abrupt cessation from the daily intake generates acute subjective withdrawal symptoms, and changes sleep structure and intensity compared to both the daily caffeine intake and the placebo-baseline.

Salivary caffeine levels

Caffeine levels significantly differed between each of the three conditions (main effect of condition: F 2,90.7  = 46.12, p  < 0.001) with the highest levels in the caffeine condition and the lowest in the placebo condition (post-hoc comparisons: p all  < 0.01). In addition, a significant interaction of the factors condition and time ( F 2,89.6  = 10.65, p  < 0.001) confirmed that caffeine levels were modulated by time with levels decreasing during nighttime sleep in the caffeine condition only (post-hoc comparison: p  < 0.001), see Fig.  1 .

Average caffeine levels collected prior to and after nighttime sleep (grey bar) in the placebo (black open circles), caffeine (blue filled circles), and withdrawal (red semi-filled circles) condition (mean values ± standard errors). The x-axis indicates the mean time of day of sample collection and color-coded asterisks represent significant ( p  < 0.05) post-hoc comparisons of the interaction effect condition × time.

Table 1 summarizes the statistical analyses of subjective sleep quality and objective sleep structure assessed during nighttime sleep. Analyses of subjective sleep quality assessed with the Leeds Sleep Evaluation Questionnaire (LSEQ) did not reveal significant differences among the three conditions in any of the four domains of sleep quality ( p all  > 0.05).

In line with these results, the analyses of the polysomnography (PSG) did not reveal significant differences in total sleep time (TST), sleep efficiency (SE), sleep latencies, or the relative amount of sleep stages among the three conditions ( p all  > 0.05).

In a next step, we analyzed all-night EEG power density in the range of 0.75–32 Hz over the central derivations recorded during non-rapid eye movement (NREM) sleep. In contrast to our assumptions, we did not find any significant differences among the three conditions in the lower frequency bins (0.75–13.25 Hz; p all  > 0.05). However, power density was significantly reduced compared to placebo in the sigma range during both withdrawal (frequency bins 13.5–17.25 Hz and 18–18.5 Hz; p all  < 0.05) and caffeine (frequency bins 13.5–16 Hz; p all  < 0.05).

In a second step, we were interested in the temporal dynamics of both SWA and sigma activity across the night assessed during NREM sleep. As depicted in Fig.  2 (top panel), SWA showed a typical temporal pattern with increased activity during the first NREM cycle followed by a steady decline across the night (main effect of time: F 39,613  = 26.28, p  < 0.001). However, differences among the three conditions did not reach significance (main effect of condition: F 2,178  = 1.33, p  = 0.27). Also, the interaction of condition and time was not significant ( F 78,1060  = 0.89, p  = 0.74).

Temporal dynamics of SWA (top) and sigma activity (bottom) during the first four sleep cycles in the placebo (black open circles), caffeine (blue filled circles), and the withdrawal (red semi-filled circles) condition (mean values). The x-axis indicates the mean time of day. While SWA (0.75–4.5 Hz) was not significantly affected by the treatment, sigma activity (12–16 Hz) showed reduced activity during both caffeine and withdrawal compared to the placebo condition ( p all  < 0.05). The inset in each right upper corner represents the mean values ± standard errors of the all-night SWA and sigma activity respectively during NREM sleep in the placebo, caffeine, and withdrawal condition. While all-night SWA (0.75–4.5 Hz) did not differ among the conditions, sigma activity (12–16 Hz) was lower in the caffeine and withdrawal condition compared to placebo ( p  < 0.05). All analyses are based on log-transformed data.

As illustrated in Fig.  2 (bottom panel), sigma activity was significantly reduced in both the caffeine and withdrawal conditions compared to placebo intake (main effect of condition: F 2,209  = 19.96, p  < 0.001; post-hoc comparisons: p  < 0.001) and the interaction of condition and time tended to be significant ( F 78,1049  = 1.25, p  = 0.08).

Taken together, we could not confirm our assumption of a caffeine-induced reduction of sleep depth, neither in terms of shorter SWS nor in terms of reduced SWA in the caffeine compared to the placebo condition. Based on the discrepancies between the present results and a previous study about the effects of chronic caffeine intake on sleep 12 , we thus explored whether differences in the individual levels of caffeine before sleep could explain the variance within SWS and SWA. However, no significant effects were observed when controlling for dependent observations within subjects ( p  > 0.05).

Subjective caffeine withdrawal symptoms

Analyses of the relative withdrawal symptoms yielded a significant main effect of condition ( F 2,20.2  = 11.30, p  < 0.01) indicating more withdrawal symptoms during the withdrawal compared to the caffeine condition (post-hoc comparison: p  < 0.01), depicted in Fig.  3 . This effect was modulated by time (interaction of condition × time: F 2,37.2  = 3.43, p  = 0.04), such that the increase in symptoms during the withdrawal compared to caffeine condition was particularly present during the last measurement ( p  < 0.01), i.e. 31 h after the last caffeine intake in the withdrawal condition.

Relative withdrawal symptoms in the caffeine and withdrawal condition (i.e. withdrawal score of the caffeine and withdrawal condition respectively minus the score of the placebo condition) assessed 35 min, 4 h, and 8 h after wake-up on day ten of treatment. Depicted are mean values and standard errors of the relative values (i.e. difference to placebo). Overall, volunteers reported more withdrawal symptoms in the withdrawal condition compared to the caffeine condition ( p  < 0.05). This difference was particularly present 8 h after wake-up during withdrawal compared to caffeine ( p  < 0.001).

The aim of the present study was to investigate the influence of daily daytime caffeine intake and its cessation on nighttime sleep in habitual caffeine consumers under strictly controlled laboratory conditions. Strikingly, caffeine consumption did not lead to clear-cut changes in nighttime sleep structure nor in subjective sleep quality when assessed 8 and 15 h after the last intake in the caffeine and withdrawal condition, respectively. The evolution of subjective withdrawal symptoms indicates that withdrawal becomes perceivable at earliest between 27–31 h after intake. However, compared to placebo, EEG power density was reduced in the sigma range during both caffeine and withdrawal conditions. We conclude that daily daytime intake of caffeine does not strongly influence nighttime sleep structure nor subjective sleep quality in healthy men when consumed in the morning, midday, and in the afternoon. In contrast to the reported increases in sigma activity after acute caffeine intake 10 , the observed changes in the sigma frequencies might point to early signs of caffeine withdrawal which occur due to overnight abstinence and presumably derive from preceding caffeine-induced changes in adenosine signaling.

To quantify the influence of caffeine on sleep, the stimulant is commonly administered close to the onset of a sleep episode 10 , 11 , 12 , 13 , 14 , for instance within 1 h prior to bedtime 10 , 11 , 13 , 14 . Taking into account that caffeine plasma levels peak within 30–75 min following caffeine ingestion 19 , consumption within 1 h prior to sleep allows the stimulant to exert its maximum effects at sleep commencement. Indeed, the sleep disrupting effects of caffeine are frequently reported to affect sleep initiation or the first half of the sleep episode 10 , 11 , 12 , 13 , 14 . Moreover, sleep intensity, which is usually strongest at the beginning of the night 20 , was particularly disrupted during the first sleep cycle, as indexed in reduced SWS and SWA 10 . However, caffeine intake in the evening, particularly after 9 pm is rare 5 , presumably to avoid impairment of subsequent sleep 3 . Up to date it remained fairly unclear whether caffeine intake in the morning and afternoon still bears the potential to disrupt nighttime sleep. While we observed a delay of 25 min in sleep episodes during caffeine intake prior to the laboratory part, PSG-derived data after 9 days of regular caffeine intake did not yield a significant change in sleep architecture. Thus, our data provide first evidence that daily daytime caffeine intake does not necessarily alter subsequent sleep structure and SWA when consumed > 8 h prior to sleep. Importantly, our findings do not preclude potential impairments of nighttime sleep after morning caffeine intake, if preceded by several days of abstinence from the stimulant 21 . It rather appears likely that the duration of preceding caffeine consumption drives the discrepancies between acute and chronic effects of caffeine on sleep.

Chronic caffeine intake induces some tolerance development in both physiological measures such as cortisol 22 , blood pressure 23 , heart rate 24 , and also subjective measures such as alertness 18 . Over time, the stimulatory effects of the substance vanish potentially due to changes in adenosine levels 25 and/or adenosine receptors 26 , 27 , 28 . Accordingly, a 1-week treatment of caffeine reduced the sleep disrupting effects, even under conditions of high evening dosages 12 . Thus, the available evidence and the absence of clear-cut changes in the present study point to adaptive processes in sleep initiation, sleep structure, and subjective sleep quality due to the long-term exposure to the stimulant.

However, chronic caffeine consumption bears the risk of withdrawal symptoms when abruptly ceased. These symptoms have been reported to occur as early as 6 h but with peak intensity being reached within 20–51 h after last caffeine intake 17 . While 25 h of caffeine abstinence might not affect nighttime sleep structure 12 , 32 h of abstinence improved subjective sleep quality 18 . Thus, scheduling the start of the sleep episode to 15 h after the last caffeine intake, as in our withdrawal condition, was probably too early to detect changes in sleep structure or subjective sleep quality. In line with this assumption, volunteers subjectively indicated withdrawal symptoms 31 h after caffeine abstinence in the withdrawal condition compared to caffeine. Thus, our findings support the notion that the alterations in sleep structure and subjective sleep quality induced by caffeine abstinence potentially develop at a later stage (> 27 h) of caffeine withdrawal.

Most strikingly and unexpectedly, a reduction in NREM sigma activity during both the withdrawal and caffeine conditions was observed, a phenomenon which is commonly reported under conditions of enhanced sleep pressure 29 , 30 , 31 , 32 . Thus, it seems at first glance in contrast to the reported increases in this frequency range 10 , 21 and the well-known alerting effects after acute caffeine intake 18 . However, during conditions of chronic caffeine intake, mice showed a deeper sleep compared to placebo 33 . Moreover, repeated caffeine intake enhances the sensitivity of adenosine binding 34 presumably due to upregulated adenosine receptors 26 , 27 , 28 or changes in the functions of adenosine receptor heteromers 35 . These neuronal alterations in the adenosinergic system might drive the commonly observed changes in the homeostatic sleep-wake regulation such as increased sleepiness when caffeine intake is suddenly ceased 17 . As reported previously, we also observed in the present study higher subjective sleepiness following caffeine withdrawal when compared to the placebo and caffeine conditions 36 . Thus, the reduction in sigma activity might reflect adenosinergic changes which already emerge 8 and 15 h after the last caffeine intake in the caffeine and withdrawal condition, respectively. This reduction might reflect withdrawal symptoms which chronic consumers reverse daily by the first caffeine dose. Given the high prevalence of daily caffeine consumers in the society, these findings stress the importance to carefully control for prior caffeine intake when assessing sleep in order to exclude potential confounding by induced withdrawal symptoms which are only detectable in the microstructure of sleep.

Our study has some limitations which must be taken into careful consideration when interpreting the present findings. First, age moderates the effects of caffeine on sleep 11 , 14 . Thus, the present results cannot be generalized to other age groups such as to middle-aged consumers which are more vulnerable to the caffeine-induced effects on sleep 11 , 14 . Second, only a limited number of participants were studied. However, a well-controlled study design was employed and power calculation on the basis of an earlier study 12 indicated a sufficient sample size. Third, we do not have any information about the participants’ genetic polymorphisms which have been shown to modulate the metabolism of caffeine 37 . In addition, a genetic variation of the ADORA2A genotype has been linked with caffeine sensitivity to the effects on sleep 38 . Thus, carriers of this genetic variance are more likely to curtail caffeine consumption and are consequently excluded from the present study leading to a selection bias. However, the focus of the present study was to investigate habitual caffeine consumers as they represent the majority of the worldwide population 2 . Fourth, to reduce variance in the data incurred by the influence of the menstrual cycle on sleep 39 and the interaction between caffeine metabolism and the use of oral contraceptives 40 , 41 , only male volunteers were included which clearly reduces the generalizability of the findings.

In conclusion, we report evidence that daily daytime intake of caffeine and its cessation has no strong effect on sleep structure or subjective sleep quality. However, the quantitative EEG analyses revealed reduced activity in the sigma range during both caffeine and withdrawal. These subtle alterations point to early signs of caffeine withdrawal in the homeostatic aspect of sleep-wake regulation which are already present as early as 8 h after the last caffeine intake. Thus, habitual caffeine consumers constantly expose themselves to a continuous change between presence and absence of the stimulant. Around the clock, their organisms dynamically adapt and react to daily presence and nightly abstinence.

Participants

Twenty male volunteers were recruited into the present study through online advertisements and flyers distributed in public areas. Interested individuals aged between 18 and 35 years old (mean age ± SD: 26.4 ± 4 years) and reporting a daily caffeine consumption between 300 and 600 mg (mean intake ± SD: 478.1 ± 102.8 mg) were included. The self-rating assessment for the daily amount of caffeine intake was structured based on Bühler et al. 42 , and the amount of caffeine content was defined according to Snel and Lorist 3 . To ensure good health, volunteers were screened by self-report questionnaires and a medical examination conducted by a physician. Additionally, all volunteers reported good sleep quality assessed with the Pittsburgh Sleep Quality Index (PSQI; score ≤ 5) 43 and showed no signs of sleep disturbances (SE > 70%, periodic leg movements < 15/h, apnea index < 10) in a PSG recorded during an adaptation night in the laboratory scheduled prior to the start of the study. To control for circadian misalignment, volunteers who reported shiftwork within 3 months and transmeridian travels (crossing > 2 time zones) within 1 month prior to study admission were excluded. Further exclusion criteria comprised body mass index (BMI) < 18 or > 26, smoking, drug use, and extreme chronotype assessed by the Morningness-Eveningness Questionnaire (MEQ; score ≤ 30 and ≥ 70) 44 . To reduce variance in the data incurred by the effect of menstrual cycle on sleep 39 and the interaction between caffeine metabolism and the use of oral contraceptives 40 , 41 , only male volunteers were studied. A detailed description of the study sample can be found in Weibel et al. 36 .

All volunteers signed a written informed consent and received financial compensation for study participation. The study was approved by the local Ethics Committee (EKNZ) and conducted according to the Declaration of Helsinki.

Design and protocol

We employed a double-blind, randomized, crossover study including a caffeine, a withdrawal, and a placebo condition. Volunteers were allocated to the order of the three conditions based on pseudo-randomization, for more details see Weibel et al. 36 . As illustrated in Fig.  4 , each condition started with an ambulatory part of 9 days, followed by a laboratory part of 43 h. In each condition, participants took either caffeine (150 mg) or placebo (mannitol) in identical appearing gelatin capsules (Hänseler AG, Herisau, Switzerland) three times daily, scheduled at 45 min, 255 min, and 475 min after awakening, for a duration of 10 days. This regimen was applied based on a previous study investigating tolerance to the effects of caffeine and caffeine cessation 18 . To enhance caffeine withdrawal in the withdrawal condition, treatment was abruptly switched from caffeine to placebo on day nine of the protocol (255 min after wake-up, 15 h before sleep recording).

Illustration of the study design. Twenty volunteers participated in a placebo, a caffeine, and a withdrawal condition during which they ingested either caffeine or placebo capsules three times daily (wake-up + 45 min, + 255 min, and + 475 min). Each condition started with an ambulatory part of 9 days and was followed by a laboratory part of 43 h. After 9 days of continuous treatment, we recorded 8 h of polysomnography (PSG), indicated as arrows, during nighttime sleep under controlled laboratory conditions. The sleep episode was scheduled to volunteers’ habitual bedtime.

During the 9 days of the ambulatory part, volunteers were asked to maintain a regular sleep-wake rhythm (± 30 min of self-selected bedtime/wake-up time, 8 h in bed, no daytime napping), verified by wrist actimetry (Actiwatch, Cambridge Neurotechnology Ltd., Cambridge, United Kingdom), and to keep subjective sleep logs. While the participants were compliant, they scheduled sleep episodes differently within the accepted range of ± 30 min. During intake of caffeine (i.e. caffeine and withdrawal condition), the ambulatory sleep episodes were on average around 25 min later as compared to placebo (results see supplements). The duration of the ambulatory part was set for 9 days based on the maximum duration of withdrawal symptoms 17 and thus, to avoid carry-over effects from the previous condition. Furthermore, volunteers were requested to refrain from caffeinated beverages and food (e.g. coffee, tea, soda drinks, and chocolate), alcohol, nicotine, and medications. Caffeine abstinence and compliance to the treatment requirements were checked by caffeine levels from the daily collection of fingertip sweat of which results are reported in the supplemental material of Weibel et al. 36 and which indicate very good adherence to the treatments.

On day nine, volunteers admitted to the laboratory at 5.5 h prior to habitual sleep time. Upon arrival, a urinary drug screen (AccuBioTech Co., Ltd., Beijing, China) was performed to ensure drug abstinence. Electrodes for the PSG were fitted and salivary caffeine levels collected. An 8-h nighttime sleep episode was scheduled at volunteers’ habitual bedtime starting 8 and 15 h after the last caffeine intake in the caffeine and withdrawal condition, respectively. The next day, volunteers rated their subjective sleep quality by the LSEQ 45 and potential withdrawal symptoms by the Caffeine Withdrawal Symptom Questionnaire (CWSQ) 46 .

To reduce potential masking effects on our outcome variables, we standardized food intake, light exposure, and posture changes throughout the laboratory part. Accordingly, volunteers were housed in single apartments under dim-light (< 8 lx) during scheduled wakefulness and 0 lx during sleep. Volunteers were asked to maintain a semi-recumbent position during wakefulness, except for restroom breaks. In addition, volunteers received standardized meals in regular intervals. Social interactions were restricted to team members and no time-of-day cues were provided throughout the in-lab protocol.

Salivary caffeine

To characterize individual caffeine levels during nighttime sleep, we report salivary caffeine levels assessed 3 h prior to the scheduled sleep episode and 5 min after wake-up. Samples were stored at 5 °C following collection, later centrifuged (3000 rpm for 10 min) and subsequently kept at − 28 °C until analyses. Liquid chromatography coupled to tandem mass spectrometry was used to analyze the levels of caffeine. One dataset in the withdrawal condition was lost.

Subjective sleep quality

Subjective sleep quality was assessed 10 min upon scheduled wake-up time with a paper and pencil version of the LSEQ 45 . Volunteers were asked to rate 10 items on visual analogue scales which are grouped into four domains (getting to sleep (GTS), quality of sleep (QOS), awake following sleep (AFS), and behavior following wakening (BFW)).

Polysomnographic recordings

PSG was continuously recorded during 8 h of nighttime sleep using the portable V-Amp device (Brain Products GmbH, Gilching, Germany). Grass gold cup electrodes were applied according to the international 10–20 system including two electrooculographic, two electromyographic, two electrocardiographic, and six electroencephalographic derivations (F3, F4, C3, C4, O1, O2). Channels were referenced online against the linked mastoids (A1, A2). Signals were recorded with a sampling rate of 500 Hz and a notch filter was online applied at 50 Hz.

Each epoch of 30 s of the recorded PSG data was visually scored according to standard criteria 47 by three trained team members blind to the condition. SWS was additionally classified into stage 3 and 4 based on Rechtschaffen and Kales 48 . The scoring agreement between the three scorers was regularly confirmed to reach > 85%.

TST was defined as the sum of the time spent in sleep stages 1–4 and rapid eye movement (REM) sleep. Sleep latency to stage 1 and 2 was calculated as minutes to the first occurrence of the corresponding sleep stage following lights off. REM sleep latency was calculated as minutes to the first occurrence of REM sleep following sleep onset. NREM sleep was calculated as sum of sleep stages 2, 3 and 4. All sleep stages are expressed as relative values (%) of TST.

Spectral analysis was performed by applying fast Fourier transformation (FFT; hamming, 0% overlapped, 0.25 Hz bins) on 4-s time windows. Artifacts were manually removed based on visual inspection, and data were log-transformed prior to spectral analyses. All-night EEG power density during NREM sleep was analyzed for each 0.25 Hz frequency bin in the range of 0.75–32 Hz recorded over the central derivations (C3, C4). SWA was defined as EEG power density between 0.75–4.5 Hz and sigma activity between 12–16 Hz. Sleep cycles were defined based on adapted rules developed by Feinberg and Floyd 49 and divided into 10 NREM and four REM sleep intervals within each cycle. Ten nights were excluded from sleep analyses due to technical problems (placebo: n  = 3; caffeine: n  = 4; withdrawal: n  = 3).

Caffeine withdrawal symptoms

Withdrawal symptoms were first assessed 35 min after wake-up and subsequently prior to each treatment administration with the self-rating CWSQ 46 . Twenty-three items are grouped into seven factors (fatigue/drowsiness, low alertness/difficulty concentrating, mood disturbances, low sociability/motivation to work, nausea/upset stomach, flu-like feelings, headache) and were rated on a 5 point scale by choosing between 1 (not at all) and 5 (extremely). Prior to analyses, eight items have been reversed scored as they were positively worded (e.g. alert or talkative) in the questionnaire. To assess caffeine withdrawal, we first calculated a sum score comprising all 23 items of the caffeine withdrawal questionnaire. Missing responses to single items were replaced by the median response of each condition over all volunteers in the respective time of assessment. In a next step, we calculated relative withdrawal symptoms in the caffeine and withdrawal condition (i.e. the difference of the withdrawal score in the caffeine and withdrawal condition respectively minus the score of the placebo condition). The data of one volunteer was lost due to technical difficulties.

Statistical analyses

Analyses were performed with the statistical package SAS (version 9.4, SAS Institute, Cary, NC, USA) by applying mixed model analyses of variance for repeated measures (PROC MIXED) with the repeated factors ‘condition’ (placebo, caffeine, withdrawal) and/or ‘time’ (levels differ per variable) and the random factor ‘subject’. The LSMEANS statement was used to calculate contrasts and degrees of freedom were based on the approximation by Kenward and Roger 50 . Post-hoc comparisons were adjusted for multiple comparisons by applying the Tukey-Kramer method. A statistical significance was defined as p  < 0.05. One dataset has been excluded from all the analyses due to non-compliance with the treatment requirements (caffeine: n  = 1).

Data availability

The present data are available upon request from the corresponding author.

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Acknowledgements

The present work was performed within the framework of a project granted by the Swiss National Science Foundation (320030_163058) and was additionally funded by the Nikolaus und Bertha Burckhardt-Bürgin-Stiftung and the Janggen-Pöhn-Stiftung. Further, we thank our interns Andrea Schumacher, Laura Tincknell, Sven Leach, and all our study helpers for their help in data acquisition and all our volunteers for participating in the study. Moreover, we gratefully acknowledge the help in study organization provided by Dr. Ruta Lasauskaite and the medical screenings conducted by Dr. med. Martin Meyer and Dr. med. Corrado Garbazza.

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These authors contributed equally: Christian Cajochen and Carolin F. Reichert.

Authors and Affiliations

Centre for Chronobiology, Psychiatric Hospital of the University of Basel, Basel, Switzerland

Janine Weibel, Yu-Shiuan Lin, Joshua Kistler, Christian Cajochen & Carolin F. Reichert

Transfaculty Research Platform Molecular and Cognitive Neurosciences, University of Basel, Basel, Switzerland

Neuropsychiatry and Brain Imaging, Psychiatric Hospital of the University of Basel, Basel, Switzerland

Yu-Shiuan Lin & Stefan Borgwardt

Institute of Pharmacology and Toxicology, University of Zürich, Zürich, Switzerland

Hans-Peter Landolt

Sleep & Health Zürich, University Center of Competence, University of Zürich, Zürich, Switzerland

Laboratory Medicine, University Hospital Basel, Basel, Switzerland

Sophia Rehm & Katharina M. Rentsch

Clinical Sleep Laboratory, Psychiatric Hospital of the University of Basel, Basel, Switzerland

Helen Slawik

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Contributions

C.R., C.C. and S.B. designed the study; J.W., Y.S.L. and HS collected the data; J.W., C.R. and C.C. analyzed and interpreted the data; J.W. and C.R. drafted the manuscript; C.C., Y.S.L. and H.P.L. critically revised the manuscript regarding its intellectual content; J.K., S.R. and K.R. provided the resources for the caffeine measurements and performed its analyses; all authors reviewed the present article.

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Correspondence to Christian Cajochen .

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Weibel, J., Lin, YS., Landolt, HP. et al. The impact of daily caffeine intake on nighttime sleep in young adult men. Sci Rep 11 , 4668 (2021). https://doi.org/10.1038/s41598-021-84088-x

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Many of us can’t imagine starting the day without a cup of coffee. One reason may be that it supplies us with a jolt of caffeine, a mild stimulant to the central nervous system that quickly boosts our alertness and energy levels. [1] Of course, coffee is not the only caffeine-containing beverage. Read on to learn more about sources of caffeine, and a review of the research on this stimulant and health.

Absorption and Metabolism of Caffeine

The chemical name for the bitter white powder known as caffeine is 1,3,7 trimethylxanthine. Caffeine is absorbed within about 45 minutes after consuming, and peaks in the blood anywhere from 15 minutes to 2 hours. [2] Caffeine in beverages such as coffee, tea, and soda is quickly absorbed in the gut and dissolves in both the body’s water and fat molecules. It is able to cross into the brain. Food or food components, such as fibers, in the gut can delay how quickly caffeine in the blood peaks. Therefore, drinking your morning coffee on an empty stomach might give you a quicker energy boost than if you drank it while eating breakfast.

Caffeine is broken down mainly in the liver. It can remain in the blood anywhere from 1.5 to 9.5 hours, depending on various factors. [2] Smoking speeds up the breakdown of caffeine, whereas pregnancy and oral contraceptives can slow the breakdown. During the third trimester of pregnancy, caffeine can remain in the body for up to 15 hours. [3]

People often develop a “caffeine tolerance” when taken regularly, which can reduce its stimulant effects unless a higher amount is consumed. When suddenly stopping all caffeine, withdrawal symptoms often follow such as irritability, headache, agitation, depressed mood, and fatigue. The symptoms are strongest within a few days after stopping caffeine, but tend to subside after about one week. [3] Tapering  the amount gradually may help to reduce side effects.

Sources of Caffeine

Caffeine is naturally found in the fruit, leaves, and beans of coffee , cacao, and guarana plants. It is also added to beverages and supplements. There is a risk of drinking excess amounts of caffeinated beverages like soda and energy drinks because they are taken chilled and are easy to digest quickly in large quantities.

• Coffee. 1 cup or 8 ounces of brewed coffee contains about 95 mg caffeine. The same amount of instant coffee contains about 60 mg caffeine. Decaffeinated coffee contains about 4 mg of caffeine. Learn more about coffee .
• Espresso. 1 shot or 1.5 ounces contains about 65 mg caffeine.
• Tea. 1 cup of black tea contains about 47 mg caffeine. Green tea contains about 28 mg. Decaffeinated tea contains 2 mg, and herbal tea contains none. Learn more about tea .
• Soda. A 12-ounce can of regular or diet dark cola contains about 40 mg caffeine. The same amount of Mountain Dew contains 55 mg caffeine.
• Chocolate (cacao) . 1 ounce of dark chocolate contains about 24 mg caffeine, whereas milk chocolate contains one-quarter of that amount.
• Guarana. This is a seed from a South American plant that is processed as an extract in foods, energy drinks, and energy supplements. Guarana seeds contain about four times the amount of caffeine as that found in coffee beans. [4] Some drinks containing extracts of these seeds can contain up to 125 mg caffeine per serving.
• Energy drinks. 1 cup or 8 ounces of an energy drink contains about 85 mg caffeine. However the standard energy drink serving is 16 ounces, which doubles the caffeine to 170 mg. Energy shots are much more concentrated than the drinks; a small 2 ounce shot contains about 200 mg caffeine. Learn more about energy drinks .
• Supplements. Caffeine supplements contain about 200 mg per tablet, or the amount in 2 cups of brewed coffee.

Recommended Amounts

In the U.S., adults consume an average of 135 mg of caffeine daily, or the amount in 1.5 cups of coffee (1 cup = 8 ounces). [5] The U.S. Food and Drug Administration considers 400 milligrams (about 4 cups brewed coffee) a safe amount of caffeine for healthy adults to consume daily. However, pregnant women should limit their caffeine intake to 200 mg a day (about 2 cups brewed coffee), according to the American College of Obstetricians and Gynecologists.

The American Academy of Pediatrics suggests that children under age 12 should not consume any food or beverages with caffeine. For adolescents 12 and older, caffeine intake should be limited to no more than 100 mg daily. This is the amount in two or three 12-ounce cans of cola soda.

Caffeine and Health

Caffeine is associated with several health conditions. People have different tolerances and responses to caffeine, partly due to genetic differences. Consuming caffeine regularly, such as drinking a cup of coffee every day, can promote caffeine tolerance in some people so that the side effects from caffeine may decrease over time. Although we tend to associate caffeine most often with coffee or tea, the research below focuses mainly on the health effects of caffeine itself. Visit our features on coffee , tea , and energy drinks for more health information related to those beverages.

Caffeine can block the effects of the hormone adenosine, which is responsible for deep sleep . Caffeine binds to adenosine receptors in the brain, which not only lowers adenosine levels but also increases or decreases other hormones that affect sleep, including dopamine, serotonin, norepinephrine, and GABA. [2] Levels of melatonin, another hormone promoting sleep, can drop in the presence of caffeine as both are metabolized in the liver. Caffeine intake later in the day close to bedtime can interfere with good sleep quality. Although developing a caffeine tolerance by taking caffeine regularly over time may lower its disruptive effects, [1] those who have trouble sleeping may consider minimizing caffeine intake later in the day and before going to bed.

In sensitive individuals, caffeine can increase anxiety at doses of 400 mg or more a day (about 4 cups of brewed coffee). High amounts of caffeine may cause nervousness and speed up heart rate, symptoms that are also felt during an anxiety attack. Those who have an underlying anxiety or panic disorder are especially at risk of overstimulation when overloading on caffeine.

Caffeine stimulates the heart, increases blood flow, and increases blood pressure temporarily, particularly in people who do not usually consume caffeine. However, strong negative effects of caffeine on blood pressure have not been found in clinical trials, even in people with hypertension, and cohort studies have not found that coffee drinking is associated with a higher risk of hypertension. Studies also do not show an association of caffeine intake and atrial fibrillation (abnormal heart beat), heart disease , or stroke. [3]

Caffeine is often added to weight loss supplements to help “burn calories.” There is no evidence that caffeine causes significant weight loss. It may help to boost energy if one is feeling fatigued from restricting caloric intake, and may reduce appetite temporarily. Caffeine stimulates the sympathetic nervous system, which plays a role in suppressing hunger, enhancing satiety, and increasing the breakdown of fat cells to be used for energy. [6] Cohort studies following large groups of people suggest that a higher caffeine intake is associated with slightly lower rates of weight gain in the long term. [3] However, a fairly large amount of caffeine (equivalent to 6 cups of coffee a day) may be needed to achieve a modest increase in calorie “burn.” Additional calories obtained from cream, milk, or sweetener added to a caffeinated beverage like coffee or tea can easily negate any calorie deficit caused by caffeine.

Caffeine can cross the placenta, and both mother and fetus metabolize caffeine slowly. A high intake of caffeine by the mother can lead to prolonged high caffeine blood levels in the fetus. Reduced blood flow and oxygen levels may result, increasing the risk of miscarriage and low birth weight. [3] However, lower intakes of caffeine have not been found harmful during pregnancy when limiting intakes to no more than 200 mg a day. A review of controlled clinical studies found that caffeine intake, whether low, medium, or high doses, did not appear to increase the risk of infertility. [7]

Most studies on liver disease and caffeine have specifically examined coffee intake. Caffeinated coffee intake is associated with a lower risk of liver cancer, fibrosis, and cirrhosis. Caffeine may prevent the fibrosis (scarring) of liver tissue by blocking adenosine, which is responsible for the production of collagen that is used to build scar tissue. [3]

Studies have shown that higher coffee consumption is associated with a lower risk of gallstones. [8] Decaffeinated coffee does not show as strong a connection as caffeinated coffee. Therefore, it is likely that caffeine contributes significantly to this protective effect. The gallbladder is an organ that produces bile to help break down fats; consuming a very high fat diet requires more bile, which can strain the gallbladder and increase the risk of gallstones. It is believed that caffeine may help to stimulate contractions in the gallbladder and increase the secretion of cholecystokinin, a hormone that speeds the digestion of fats.

Caffeine may protect against Parkinson’s disease. Animal studies show a protective effect of caffeine from deterioration in the brain. [3] Prospective cohort studies show a strong association of people with higher caffeine intakes and a lower risk of developing Parkinson’s disease. [9]

Caffeine has a similar action to the medication theophylline, which is sometimes prescribed to treat asthma. They both relax the smooth muscles of the lungs and open up bronchial tubes, which can improve breathing. The optimal amount of caffeine needs more study, but the trials reviewed revealed that even a lower caffeine dose of 5 mg/kg of body weight showed benefit over a placebo. [10] Caffeine has also been used to treat breathing difficulties in premature infants. [3]

Caffeine stimulates the release of a stress hormone called epinephrine, which causes liver and muscle tissue to release its stored glucose into the bloodstream, temporarily raising blood glucose levels. However, regular caffeine intake is not associated with an increased risk of diabetes . In fact, cohort studies show that regular coffee intake is associated with a lower risk of type 2 diabetes , though the effect may be from the coffee plant compounds rather than caffeine itself, as decaffeinated coffee shows a similar protective effect. [3] Other observational studies suggest that caffeine may protect and preserve the function of beta cells in the pancreas, which are responsible for secreting insulin. [11]

Signs of Toxicity

Caffeine toxicity has been observed with intakes of 1.2 grams or more in one dose. Consuming 10-14 grams at one time is believed to be fatal. Caffeine intake up to 10 grams has caused convulsions and vomiting, but recovery is possible in about 6 hours. Side effects at lower doses of 1 gram include restlessness, irritability, nervousness, vomiting, rapid heart rate, and tremors.

Toxicity is generally not seen when drinking caffeinated beverages because a very large amount would need to be taken within a few hours to reach a toxic level (10 gm of caffeine is equal to about 100 cups of brewed coffee). Dangerous blood levels are more often seen with overuse of caffeine pills or tablets. [3]

Did You Know?

• Caffeine is not just found in food and beverages but in various medications. It is often added to analgesics (pain relievers) to provide faster and more effective relief from pain and headaches. Headache or migraine pain is accompanied by enlarged inflamed blood vessels; caffeine has the opposite effect of reducing inflammation and narrowing blood vessels, which may relieve the pain.
• Caffeine can interact with various medications. It can cause your body to break down a medication too quickly so that it loses its effectiveness. It can cause a dangerously fast heart beat and high blood pressure if taken with other stimulant medications. Sometimes a medication can slow the metabolism of caffeine in the body, which may increase the risk of jitteriness and irritability, especially if one tends to drink several caffeinated drinks throughout the day. If you drink caffeinated beverages daily, talk with your doctor about potential interactions when starting a new medication.

Energy Drinks

• Clark I, Landolt HP. Coffee, caffeine, and sleep: A systematic review of epidemiological studies and randomized controlled trials. Sleep medicine reviews . 2017 Feb 1;31:70-8. *Disclosure: some of HPL’s research has been supported by Novartis Foundation for Medical-Biological Research.
• Institute of Medicine (US) Committee on Military Nutrition Research. Caffeine for the Sustainment of Mental Task Performance: Formulations for Military Operations. Washington (DC): National Academies Press (US); 2001. 2, Pharmacology of Caffeine. Available from: https://www.ncbi.nlm.nih.gov/books/NBK223808/
• van Dam RM, Hu FB, Willett WC. Coffee, Caffeine, and Health.  NEJM .  2020 Jul 23; 383:369-378
• Moustakas D, Mezzio M, Rodriguez BR, Constable MA, Mulligan ME, Voura EB. Guarana provides additional stimulation over caffeine alone in the planarian model. PLoS One . 2015 Apr 16;10(4):e0123310.
• Drewnowski A, Rehm CD. Sources of caffeine in diets of US children and adults: trends by beverage type and purchase location. Nutrients . 2016 Mar;8(3):154.
• Harpaz E, Tamir S, Weinstein A, Weinstein Y. The effect of caffeine on energy balance. Journal of basic and clinical physiology and pharmacolog y. 2017 Jan 1;28(1):1-0.
• Bu FL, Feng X, Yang XY, Ren J, Cao HJ. Relationship between caffeine intake and infertility: a systematic review of controlled clinical studies.  BMC Womens Health . 2020;20(1):125.
• Zhang YP, Li WQ, Sun YL, Zhu RT, Wang WJ. Systematic review with meta‐analysis: coffee consumption and the risk of gallstone disease. Alimentary pharmacology & therapeutics . 2015 Sep;42(6):637-48.
• Hong CT, Chan L, Bai CH. The Effect of Caffeine on the Risk and Progression of Parkinson’s Disease: A Meta-Analysis. Nutrients . 2020 Jun;12(6):1860.
• Welsh EJ, Bara A, Barley E, Cates CJ. Caffeine for asthma.  Cochrane Database Syst Rev . 2010;2010(1):CD001112.
• Lee S, Min JY, Min KB. Caffeine and Caffeine Metabolites in Relation to Insulin Resistance and Beta Cell Function in US Adults. Nutrients . 2020 Jun;12(6):1783.

Last reviewed July 2020

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Caffeine: How much is too much?

Caffeine has its perks, but it can pose problems too. Find out how much is too much and if you need to curb your consumption.

If you rely on caffeine to wake you up and keep you going, you aren't alone. Millions of people rely on caffeine every day to stay alert and improve concentration.

How much is too much?

Up to 400 milligrams (mg) of caffeine a day appears to be safe for most healthy adults. That's roughly the amount of caffeine in four cups of brewed coffee, 10 cans of cola or two "energy shot" drinks. Keep in mind that the actual caffeine content in beverages varies widely, especially among energy drinks.

Caffeine in powder or liquid form can provide toxic levels of caffeine, the U.S. Food and Drug Administration has cautioned. Just one teaspoon of powdered caffeine is equivalent to about 28 cups of coffee. Such high levels of caffeine can cause serious health problems and possibly death.

Although caffeine use may be safe for adults, it's not a good idea for children. Adolescents and young adults need to be cautioned about excessive caffeine intake and mixing caffeine with alcohol and other drugs.

Women who are pregnant or who are trying to become pregnant and those who are breast-feeding should talk with their doctors about limiting caffeine use to less than 200 mg daily.

Even among adults, heavy caffeine use can cause unpleasant side effects. And caffeine may not be a good choice for people who are highly sensitive to its effects or who take certain medications.

Read on to see if you may need to curb your caffeine routine.

You drink more than 4 cups of coffee a day

You may want to cut back if you're drinking more than 4 cups of caffeinated coffee a day (or the equivalent) and you have side effects such as:

• Nervousness
• Irritability
• Frequent urination or inability to control urination
• Fast heartbeat
• Muscle tremors

Even a little makes you jittery

Some people are more sensitive to caffeine than are others. If you're susceptible to the effects of caffeine, even small amounts may prompt unwanted effects, such as restlessness and sleep problems.

How you react to caffeine may be determined in part by how much caffeine you're used to drinking. People who don't regularly drink caffeine tend to be more sensitive to its effects.

You're not getting enough sleep

Caffeine, even in the afternoon, can interfere with your sleep. Even small amounts of sleep loss can add up and disturb your daytime alertness and performance.

Using caffeine to mask sleep deprivation can create an unwelcome cycle. For example, you may drink caffeinated beverages because you have trouble staying awake during the day. But the caffeine keeps you from falling asleep at night, shortening the length of time you sleep.

You're taking medications or supplements

Some medications and herbal supplements may interact with caffeine. Examples include:

• Ephedrine. Mixing caffeine with this medication — which is used in decongestants — might increase your risk of high blood pressure, heart attack, stroke or seizure.
• Theophylline. This medication, used to open up bronchial airways, tends to have some caffeine-like effects. So taking it with caffeine might increase the adverse effects of caffeine, such as nausea and heart palpitations.
• Echinacea. This herbal supplement, which is sometimes used to prevent colds or other infections, may increase the concentration of caffeine in your blood and may increase caffeine's unpleasant effects.

Talk to your doctor or pharmacist about whether caffeine might affect your medications.

Curbing your caffeine habit

Whether it's for one of the reasons above or because you want to trim your spending on coffee drinks, cutting back on caffeine can be challenging. An abrupt decrease in caffeine may cause withdrawal symptoms, such as headaches, fatigue, irritability and difficulty focusing on tasks. Fortunately, these symptoms are usually mild and get better after a few days.

To change your caffeine habit, try these tips:

• Keep tabs. Start paying attention to how much caffeine you're getting from foods and beverages, including energy drinks. Read labels carefully. But remember that your estimate may be a little low because some foods or drinks that contain caffeine don't list it.
• Cut back gradually. For example, drink one fewer can of soda or drink a smaller cup of coffee each day. Or avoid drinking caffeinated beverages late in the day. This will help your body get used to the lower levels of caffeine and lessen potential withdrawal effects.
• Go decaf. Most decaffeinated beverages look and taste much the same as their caffeinated counterparts.
• Shorten the brew time or go herbal. When making tea, brew it for less time. This cuts down on its caffeine content. Or choose herbal teas that don't have caffeine.
• Check the bottle. Some over-the-counter pain relievers contain caffeine. Look for caffeine-free pain relievers instead.

The bottom line

If you're like most adults, caffeine is a part of your daily routine. Usually, it won't pose a health problem. But be mindful of caffeine's possible side effects and be ready to cut back if necessary.

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• Lieberman HR, et al. Daily patterns of caffeine intake and the association of intake with multiple sociodemographic and lifestyle factors in U.S. adults based on the NHANES 2007-2012 surveys. Journal of the American Academy of Nutrition and Dietetics. 2019; doi:10.1016/j.jand.2018.08.152.
• 2015-2020 Dietary Guidelines for Americans. U.S. Department of Health and Human Services and U.S. Department of Agriculture. https://health.gov/our-work/food-nutrition/2015-2020-dietary-guidelines/guidelines. Accessed Feb. 1, 2020.
• Spilling the beans: How much caffeine is too much. U.S. Food and Drug Administration. https://www.fda.gov/consumers/consumer-updates/spilling-beans-how-much-caffeine-too-much. Accessed Sept. 20, 2019.
• Duyff RL. Think your drinks. In: Academy of Nutrition and Dietetics Complete Food and Nutrition Guide. 5th ed. Houghton Mifflin Harcourt; 2017.
• Bordeaux B. Benefits and risks of caffeine and caffeinated beverages. https://www.uptodate.com/contents/search. Accessed Sept. 20, 2019.
• Pure and highly concentrated caffeine. U.S. Food and Drug Administration. https://www.fda.gov/food/dietary-supplement-products-ingredients/pure-and-highly-concentrated-caffeine. Accessed Sept. 20, 2019.
• Caffeine. Natural Medicines. https://naturalmedicines.therapeuticresearch.com. Accessed Feb. 7, 2020.

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‘I quit caffeine for a week and this is what happened’

Bye, bye coffee

Yet while coffee has been linked to a myriad of health benefits – numerous studies suggest that coffee drinkers live longer and have lower risks of Type 2 diabetes, Parkinson’s disease, cardiovascular conditions and some cancers – I do worry that my caffeine intake might not be a wholly positive thing .

My coffee-drinking habit isn’t excessive – I tend to have one, ok two, coffees in the morning, and a tea or a caffeinated soft drink in the afternoon when the 3pm slump hits. But I’ve recently noticed that I’ve been more stressed and anxious than usual, and my sleep quality isn’t great either. And with celebrities like Adele giving up caffeine completely and friends who have done the same raving about the health benefits (less anxiety, better sleep), I wondered if cutting it out of my life would help me deal with some of these issues.

To put it to the test, I decided to quit caffeine for one week – here’s how I got on....

How much coffee should you drink in a day? According to the Mayo Clinic , up to 400mg of caffeine a day seems to be safe for most adults. That's roughly the amount of caffeine in four cups of brewed coffee.

Why I quit caffeine

There are a few reasons why I thought caffeine might be to blame for my increased stress and bad sleep. Caffeine increases the production of stress hormones such as cortisol, and the NHS advises people who deal with general anxiety disorders to reduce their caffeine intake to help manage their symptoms. In the past, I’ve dealt with severe anxiety and although my mental health has improved a lot over the years, I’ve noticed myself feeling more anxious recently, so I was hoping going cold turkey on coffee might help me deal with this.

Studies have also shown that a daily cup of coffee can alter your sleep cycle, causing restless sleep. According to a recently published review of 24 studies, caffeine consumption can reduce nighttime sleep by an average of 45 minutes – so experts tend to advise limiting caffeine intake to before 5pm, to mitigate its disruptive effects on sleep. Again, since I’ve recently been finding it difficult to fall asleep and I’ve also found myself waking up in the middle of the night, I wondered if caffeine could be to blame?

Quitting caffeine: The experiment

In the UK, we drink a total of approximately 98 million cups of coffee per day, but according to Mintel , 39% of coffee drinkers are trying to reduce their caffeine intake – so I’m definitely not alone in my curiosities about how caffeine is affecting my health.

I wake up most mornings looking forward to my first coffee of the day and it took me until I’d showered and put my shoes on to remember that it was going to have to be decaf on the first morning of my experiment. I tend to feel lethargic on Monday mornings – even with my regular coffees – and although I could still enjoy the ritual of a flat white (albeit, decaf) I definitely noticed that my energy levels were lower than usual. The mental challenge of not giving in to caffeine was part of this struggle, and it was easy to let myself mope about the fact that there was nothing I could do about my tiredness.

Aside from the physical effects of going without coffee, the awareness that I was going without caffeine could have actually been what was making me feel so much worse, according to Sophie Medlin, director and specialist dietitian at CityDietitians . ‘There's a very strong psychosomatic (mind and body) response to coming off coffee,’ Medlin explains. ‘You're going to be aware that you're coming off it and that can create more problems.’

.css-1cugboc{margin:0rem;font-size:2.125rem;line-height:1.2;font-family:Domaine,Domaine-roboto,Domaine-local,Georgia,Times,Serif;color:#f7623b;font-weight:bold;}.css-1cugboc em,.css-1cugboc i{font-style:italic;font-family:inherit;}.css-1cugboc b,.css-1cugboc strong{font-family:inherit;font-weight:bold;} ‘A lack of coffee gave me headaches’

However, some physical conditions did soon start to kick in. On Tuesday morning, I woke up with a headache, something I’d been expecting but not looking forward to. ‘Caffeine is a vasoconstrictor which means it narrows blood vessels. So when you're having coffee regularly and you stop, the blood vessels in your brain start to dilate again and increase in blood flow, which can cause headaches,’ Medlin explains. ‘It's also linked to neurotransmitters such as dopamine and serotonin, which are affected by caffeine so cutting out caffeine can disrupt that balance and contribute to headaches.’

Because of this, it isn’t always the best idea to go cold turkey on caffeine, particularly if you’re drinking at least four cups a day, according to Medlin. ‘The main goal should be to eliminate caffeine after 3pm for sleep quality,’ she explains. ‘Then reducing your overall intake from there is a good idea, particularly if you feel you're quite dependent.’

‘Quitting caffeine affected my mood’

One of the other things I noticed during the week I quit caffeine is that my mood wasn’t the best, and Medlin says that low mood and irritability are two common symptoms of caffeine withdrawal. Part of this was changing my daily habits, particularly when caffeinated drinks like an afternoon Pepsi are part of the ways I relax while working. To try and make this part of giving up caffeine easier, I swapped out my usual soft drink for a Trip CBD drink, which I felt definitely helped me to relax, and I felt less stressed than I usually would after having a caffeinated and sugary soft drink, so this is definitely a habit I’ll be sticking to.

Common caffeine withdrawal symptoms

1. headache.

A critical review of 66 studies on caffeine withdrawal symptoms found that the incidence of a headache is around 50%, so it's a pretty common withdrawal symptom to caffeine.

‘You're likely to feel low in energy if you cut out caffeine and more tired than you normally would when you've had that caffeine,’ Medlin says, explaining a lot of people take caffeine to boost their alertness, particularly in the morning, so it’s expected that you’ll feel more tired without the stimulant.

3. Low mood

Dopamine is a type of neurotransmitter and hormone that activates pleasure in the brain and helps to regulate our emotions. Caffeine increases the number of available dopamine receptors in the brain, according to a 2015 study . This means that cutting out caffeine might reduce your brain’s ability to produce dopamine, leading to low mood and irritability.

4. Constipation

Caffeine is a laxative. ‘People may also experience a change in bowel habit when cutting out caffeine,’ Medlin says. ‘You might notice that your bowel is not quite as active as it was before and some people might get a bit constipated.’

This isn’t something you need to worry about unless your symptoms continue, or you’re experiencing other symptoms such as severe abdominal pain, in which case you should speak to your doctor.

How I feel after giving up coffee for one week

There were plenty of ups and downs during my week without coffee. I’d have to continue to cut it out for longer to see how going without it really affects my health – for example, would the withdrawal symptoms (headaches, fatigue, difficulty concentrating) subside after a few weeks?

One of the positives I did notice is that I had less energy slumps throughout the day. Often, a couple of hours after my first coffee, I start to feel tired, which is usually when I opt for a second, and this cycle tends to continue throughout the working day. Although it was difficult to say no to caffeine in the morning, and my energy levels were lower at the start of the day, I definitely felt better in the afternoon and more relaxed in the evenings too, which I think helped me fall asleep quicker.

On top of this, cutting out coffee helped me feel more in touch with my body. Generally, I’m used to consuming caffeine almost as soon as I wake up, so it's difficult to tell where my natural energy levels are at. This week, I noticed that some days I was feeling better than others and it was useful to think about why that was, reflect on that and make changes to improve my energy levels in the future. But I’d be lying if I said I didn’t miss the way caffeine – and my AM ritual – ordered my morning. So I don’t think I’ll be saying goodbye to coffee for good.

After all, there are some days when it does provide me with an energy boost and help me concentrate, and I have to admit that I do prefer the taste of my flat white when I don't have to order it decaf. However, I’ll certainly be trying to avoid drinking coffee every day and thinking about whether some days a herbal tea or a CBD drink might be a better option to maintain my energy levels and reduce anxiety.

The bottom line: Listen to your body – if caffeine is affecting your sleep, or making you feel anxious or jittery, cut back. But, generally, two to three cups a day seems to be the sweet spot for reaping coffee’s benefits without experiencing its drawbacks.

A guide to coffee

• 'I halved my coffee intake and can't believe the impact it had on my anxiety '
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These foods don't deserve their bad reputations, dietitians and doctors say

Eggs, potatoes, coffee : These kitchen staples, among others, have gained bad reputations, nutrition experts say, but don’t necessarily deserve it. In fact, registered dietitians, doctors and nutrition professors are increasingly advising people to eat them.

Nutrition advice is ever changing, which can leave consumers uncertain about which foods are actually healthy. NBC News asked nine health experts about the foods they think have been wrongly villainized. Here are some of the items they listed and the benefits people may miss out on if they forgo them entirely.

Eggs are packed with protein

Eggs have been demonized for being high in dietary cholesterol, which health experts once believed could contribute to heart disease, said Dr. Maya Vadiveloo, an associate professor of nutrition at the University of Rhode Island.

But updated science has debunked that notion, showing that dietary cholesterol and blood cholesterol affect heart health differently.

Eating foods high in saturated fat, such as red meat, fried foods and fatty dairy can increase the type of blood cholesterol that raises one’s risk of heart problems, according to the Centers for Disease Control and Prevention . But consuming things with cholesterol, like eggs and shellfish, has little correlation to high cholesterol in the blood or a risk of heart disease.

Despite this shift, many people still view eggs poorly. Vadiveloo said that could also be because of other breakfast foods they’re often paired with.

“When people think of eggs, they also think of bacon and home fries,” she said — items that are high in salt and saturated fat. But on their own, eggs are nutritious, she added.

The American Heart Association says people can enjoy one or two eggs every day as a high-quality source of protein; each egg contains about 6 grams.

Eggs are also a source of vitamin D and choline , a nutrient that plays a role in metabolism, memory and muscle control.

Eggs even have a stamp of approval from the WeightWatchers program, which uses a point system to assign values to each type of food and drink based on its nutritional profile. Fruits, vegetables and lean meats are worth zero points, meaning followers of the diet don’t need to measure their portions. Eggs have been on the "ZeroPoint" list since 2017.

Just don’t fry your potatoes

It’s no secret that how you cook and season food influences how healthy it is. Caroline Susie, a registered nutritionist and spokesperson for the Academy of Nutrition and Dietetics, said potatoes have been demonized because of the unhealthy ways they’re prepared.

“Potatoes are just fantastic. What happens is, unfortunately, we tend to screw them up by not eating the skin or frying or mixing them with everything under the sun, like sour cream and butter and bacon,” Susie said. Such toppings add saturated fat, which should be limited to 13 grams or less per day, according to the American Heart Association .

A 2021 study found that consuming higher quantities of french fries was associated with an increased risk for chronic diseases such as Type 2 diabetes and high blood pressure. But boiled, baked and mashed potatoes weren’t linked to a higher risk of hypertension in that study and were only slightly associated with an increased risk of Type 2 diabetes.

Potato skins are high in fiber, which aids digestion. Potatoes also contain vitamin C and potassium, Susie added.

She advised roasting, baking, mashing or boiling potatoes and seasoning with olive oil, salt, pepper and herbs.

Frozen doesn’t always mean less healthy

Seven nutrition experts bemoaned the common perception that frozen fruits and vegetables are less healthy than their fresh counterparts.

“Frozen vegetables and frozen foods are picked at their pinnacle of nutrient density and then flash-frozen. So in many cases, they retain higher nutrient content than their fresh counterparts,” Vadiveloo said, “particularly when you live in a place that has more seasonal variation and availability.”

Susie said that in addition to retaining their nutrients , frozen vegetables are sometimes cheaper than fresh ones and can help people prevent food waste.

“Sometimes when I buy fresh produce, it essentially just goes to, I joke, the veggie bin graveyard. It just goes there to die,” she said. “But canned and frozen lasts longer.”

A few cups of coffee are not cause for concern

Coffee’s poor reputation comes from its caffeine, which is addictive and can cause jitters or anxiety for some people when overconsumed.

However, the Food and Drug Administration says people can drink up to four or five cups per day. Research shows coffee can contribute to a decreased risk of cancer , heart failure , Type 2 diabetes and even death .

Vadiveloo said she drinks three to five cups of coffee with milk every day. Studies suggest it can improve cognitive function, she said, so she believes coffee’s benefits outweigh the potential drawbacks of caffeine consumption.

“That’s a myth that I regularly debunk. Because a lot of people will say, ‘Oh, I’m trying to reduce coffee or caffeine.’ And the research just doesn’t support that coffee, particularly if you’re not adding a ton of added sugar or creamer and things like that, has any health risks within a reasonable consumption amount,” Vadiveloo said.

Alicia Henson, the education specialist for the Master of Nutritional Sciences and Dietetics program at the University of California, Berkeley, said the health value of coffee — much like potatoes — depends on what’s added.

“If you’re going to Starbucks and you’re drinking frappuccinos or you’re drinking coffee that has a ton of added sugar and cream to it, then that’s not necessarily a healthy addition,” Henson said.

The type of carbohydrate makes all the difference

Experts said carbohydrates as a whole are often assumed to be unhealthy, in part because of the popularity of low-carb and ketogenic diets. But it’s incorrect to think that all carbs are the same.

“It has to do with the quality of the carbohydrates — so refined versus whole grains,” said Dr. Linda Shiue, an internist and the director of culinary and lifestyle medication at Kaiser Permanente.

Refined grains, such as those used to make processed food like white bread, crackers and pastries, lack the fiber and nutrients that make whole grains healthy, Shiue said. That includes iron and B vitamins. But quinoa, farro and brown rice, for instance, offer protein, magnesium, iron and fiber, which keeps you feeling full.

Dr. Melina Jampolis, a physician nutrition specialist with a private practice in Los Angeles, said she often recommends one particular whole grain to patients as a snack, much to their surprise: popcorn.

Many people associate popcorn with the movie theater version, which is full of salt, butter and sometimes sugar — and often sold alongside a large soda. But when you prepare popcorn at home with just olive oil and spices, Jampolis said, the snack is fibrous and can be part of a balanced diet. Research also shows popcorn contains phenolic acids , a type of antioxidant.

Jampolis added that people shouldn’t fixate on avoiding any one food; instead, it’s best to cultivate a healthy eating pattern that prioritizes whole foods over ultraprocessed items with added sugar.

“That’s what the real experts do,” she said. “We don’t look at single foods necessarily in isolation.”

Katie Mogg is an intern at NBC News.

Our advice is expert-vetted and based on independent research, analysis and hands-on testing from our team of Certified Sleep Coaches. If you buy through our links, we may get a commission. Reviews ethics statement

Top 10 Common Reasons You're Oversleeping and How to Stop

Most people know that not sleeping enough is bad for you. But did you know that sleeping too much can also backfire? Here's why it's happening to you.

We often hear a lot about how lack of sleep affects our physical and mental health , but oversleeping can also lead to health issues. According to recent statistics from SingleCare , more than 50 million adults in the US have a sleep disorder -- whether undersleeping or oversleeping. The average adult needs anywhere from 7 to 9 hours of sleep each night, so consistently getting anything under or over that can put you at risk of developing chronic health problems . 

Let's dig into the common culprits of oversleeping and what you should do to get your sleeping habits back on track. 

For more on getting better sleep, learn how to use breathing exercises to reduce stress , which yoga poses promote the best sleep and our top picks for natural sleep aids . 

10 common causes of excessive sleep and fatigue

If you've asked yourself, "Why am I sleeping so much?" know there are several causes for excessive sleeping. It could be due to stress , diet , jet lag  or another reason entirely. Here we'll discuss why you might sleep all day and how to combat it.

1. Sleep disorder 

Sleep disorders like insomnia , sleep apnea  and restless leg syndrome are common reasons for sleepless nights . When you don't get enough rest during the night, you might want to take a nap or try to make up for it with more hours of sleep during the day. With insomnia, you'll experience bouts of an inability to sleep properly, which can sometimes be treated with things like prescriptions or cognitive behavioral therapy . 

Sleep apnea is a breathing condition that can interrupt sleep, often treated by various breathing apparatuses like a CPAP machine. Restless leg syndrome is exactly what it sounds like and can make it hard to sleep soundly because you need to move your legs. This, too, can be treated with prescription medications from your doctor.

2. Jet lag 

Jet lag throws your circadian rhythm out of whack. This happens when you travel across time zones or have a daily routine that doesn't coincide with your natural sleep-wake cycle . If you've ever flown from the US to Europe, you probably had to take a few days to return to your normal sleep schedule. 

During this period of jet lag , you might find it hard to fall asleep and experience other periods of sleeping in the daytime. Ideally, if you can plan your travel for a day or two before you have to return to work, it can help you iron out your schedule, but your best bet is to force yourself to stay awake through the day and go to bed at night.

3. Anxiety or stress

According to Harvard Health, stress and anxiety have been linked to poor sleep . Often, people who are anxious or stressed out will have difficulty falling asleep and staying asleep. Because of this disruption to a steady sleep schedule, these people will also occasionally find themselves sleeping too much as their body tries to make up for the lost sleep. 

There are a few ways to improve sleep if anxiety and stress are the issues , mainly by improving your sleep hygiene. That means setting yourself up for success at bedtime by creating  ideal sleep conditions -- a dark bedroom with a comfortable temperature and no screens. It may also help to exercise earlier in the day to wear you out more and avoid stimulants like alcohol, caffeine and certain foods.

4. Nutrition

While you may have been told that eating turkey on Thanksgiving makes you tired, thanks to the tryptophan, it's probably in your head. While tryptophan can make you sleepy, the tryptophan in turkey doesn't work that way because of the amino acids involved. 

That doesn't mean there isn't still a link between food and sleep, however. It's possible to experience excess sleepiness after eating large amounts of carbohydrates or protein, as those take a while for the body to digest, and that work makes your system tired. You may also feel extra tired after you eat a large meal for a similar reason. Instead, eat smaller meals ( not too close to bedtime ) and don't overeat problematic foods like sugar or pasta.

5. Medical conditions 

Several medical conditions can affect sleep, including depression, heart disease and some cancers. Research has linked some illnesses to sleep because they affect your brain. A disruption in brain function can manifest in either a lack of sleep or too much sleep, depending on how your medical condition affects you. While it can be difficult to pinpoint why you're sleeping too much, if it persists and you can't figure out a cause, it could be one of these more serious concerns. If that's the case, seeing your doctor to discuss what may be happening is important.

6. Medications 

Plenty of medications can actually make you tired (just like some can cause insomnia). Those that can tire you include antihistamines, antidepressants, muscle relaxers, proton-pump inhibitors and beta blockers. While some of these medications are helpful in their ability to induce sleep -- like a muscle relaxer or antidepressant -- others can disrupt your sleep schedule to the point of becoming a larger problem. If you're currently taking a medication that's interfering with your sleep and making you sleep too much, discuss it with your doctor to see if there might be a different medication to take.

7. Injuries 

In general, when you hurt yourself -- if you break a bone or pull a muscle, for example -- you might feel extra tired. This is a good thing, though. Your body has to do a lot of hard work to heal, which can make you tired. This may also be exacerbated by painkillers you might be taking, which often will also induce sleep. On the flip side, there are times when an injury disrupts your sleep because of the pain. There aren't many ways to work through that other than making sure your bed is set up comfortably and having your doctor outline a pain regimen that can help you rest at night.

8. The wrong mattress or pillows 

Your mattress has a lot to do with how you sleep at night, and it's important to find a mattress suited to your sleeper type. People who sleep on their backs need a different mattress and pillow setup from those who sleep on their stomachs or sides. 

Pillows and mattresses have different firmness levels, which you should choose based on your preferred sleep positions. If you're a stomach sleeper and have a firm pillow, you may not sleep well because of the pain. The first step toward getting your setup right is knowing which type of sleeper you are and setting your bed up accordingly.

9. Drinking excessive alcohol or caffeine

You already know caffeine can wreak havoc on your sleep because it's a stimulant. If you have caffeine too late in the day, you may not be able to sleep well. That means you might wake up groggy and have more caffeine, putting yourself on an endless cycle of tiredness that can lead to a crash of oversleeping. Alcohol, on the other hand, might make you fall asleep easily, but you most likely won't sleep well (and probably won't wake up feeling great). This disrupted sleep can also mean you'll sleep too much later to make up for it. To avoid either of these issues, limit your caffeine and alcohol intake, especially late in the day.

10. Sleep environment

Even if you're someone who thinks they can sleep anywhere, most likely, it won't be quality sleep. If you're sleeping in a poor sleep environment, you might get bad quality sleep, which will mean making up for it later and feeling fatigued until you do. A good sleep environment is a relaxing dark room with a comfortable temperature and no screens. You might sleep soundly if you use a diffuser with lavender essential oil or a white noise machine.

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Can caffeine be harmful to a christian’s spiritual life john piper answers.

Notable Bible teacher John Piper recently addressed the question of whether caffeinated drinks like energy drinks can have a negative influence on a Christian’s spiritual life.

In Monday's episode of the “ Ask Pastor John ” podcast, a listener identified as José told Piper that caffeinated drinks were “controversial in our youth group.”

“As someone who likes them, I was wondering if there are any negative effects or reason to not drink them,” José asked. “They help me focus and have energy during my work shift. I only drink one every two or three days, but I would like to have some spiritual insight in order that I might run this race without being slowed down.”

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As part of his response, Piper focused on 1 Corinthians 6:12–13, which reads: "'I have the right to do anything,’ you say — but not everything is beneficial. ‘I have the right to do anything’ — but I will not be mastered by anything. You say, ‘Food for the stomach and the stomach for food, and God will destroy them both.’ The body, however, is not meant for sexual immorality but for the Lord, and the Lord for the body.”

“So, the body matters to God morally. And, in particular, foods matter and sex matters. And so, the guidelines he gives matter,” Piper said.

Piper added that the question was “part of a much bigger issue,” specifically a question about “the proper use of not just caffeine but other stimulants, medications.”

“Are energy drinks, or whatever I’m taking, are they masking deeper problems that I’m not dealing with, because I’m masking them, or are they helping me really address and be freed from the deeper problems that I may have?” he said.

“If José or any of us is masking deeper problems with stimulants, then they’re not being used as a gift from God for our good; they’re being used as a flight from truth and from the good that God wants to do deeper down.”

Piper went on to offer “three summary guidelines” regarding the consumption of caffeinated drinks: “Are they truly helpful?” “Are they dominating me, mastering me, and obscuring that Jesus is my real master?” And, “Am I using them in love? Am I building others up? Am I seeking to build my own faith and the faith of others?”

“I have a box of energy drinks in my office,” Piper acknowledged. “If I’ve got a pressing task and I cannot stay awake, yes, I’ll go there.”

“If my real problem is that John Piper doesn’t have the discipline to go to bed at night and therefore get six hours instead of eight hours of sleep, and therefore he’s always falling asleep at his tasks, and thus he resorts to an artificial stimulant, that’s masking, that’s hiding, that’s running away from God.”

Last September, Piper garnered controversy when he wrote a post on X  about whether congregations should "reassess whether Sunday coffee-sipping in the sanctuary fits."

"Considering the New Testament church primarily met in folks' homes, and often shared a common meal together, sipping coffee in the sanctuary should not only be practiced, but encouraged,"  replied  Evangelical podcaster Jimmy Humphrey.

In a January podcast episode, Piper elaborated on his sentiments, believing that "sipping coffee in the holiest hour of congregational worship does not fit with the reverence and awe that Hebrews 12:28 calls for.”

"Sipping coffee is not the heart of the matter. The heart of the matter is that people and leaders don't have a heart that resonates with what I mean by 'reverence and awe' and the holiness, the sacredness of that hour of congregational worship on Sunday morning (usually)," Piper stated back in January.

"Those realities are not prominent in their mind and heart, those reverent realities. They know those words: reverence, awe. They know the words, but the words don't have compelling existential content, with the kind of serious joy that makes people eager for reverence and awe. They're just words."

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The Nespresso Coffee Pod That Has The Most Caffeine

N espresso pods are great for making coffee at home, especially since they're compatible with some of the best coffee pod machines available, but sometimes, you prefer a pure caffeine kick over a smooth taste. While Nespresso pods' levels of intensity are reflected on the brand's website, the precise caffeine content is not, begging the question, which pod provides the biggest caffeine boost?

This appears to be a fairly common question, appearing as a top-five FAQ on Nespresso's website . The FAQ explains that, while most coffee pods have a similar caffeine level, those meant for higher volume beverages consequently have a higher level of caffeine. The FAQ specifically mentions the Carafe Pour-Over Style pod, which contains "over 200mg of caffeine per complete capsule serving." A double shot of espresso will usually have between 58 and 185 milligrams of caffeine, so caffeine content exceeding 200 milligrams is pretty impressive.

Of course, this is based on a slight technicality. Because the pour-over pods are intended to be served in an 18-ounce carafe or 12-ounce cup, a significantly larger measurement than the 1.35-ounce cup recommended for an espresso pod, the caffeine is distributed throughout a much larger volume of liquid. The FAQ states that an espresso pod normally contains between 50 and 100 milligrams of caffeine, so if served correctly, the pour-over pod won't feel quite like the adrenaline shot you'd expect.

Read more: How To Get More Flavor From Your Coffee Pods & Other Keurig Hacks

Which Single-Serving Nespresso Pods Have The Most Caffeine?

While there's no reason you can't use the pour-over pod in a single serving, it won't do your taste buds or your wallet any favors. Espresso pods cost between 85 and 90 cents per capsule, whereas pour-over pods come in at $1.65, almost double the price. The pour-over pods are optimized for higher volumes of liquid, so the taste may seem overly strong and bitter if used for a short drink. In theory, you could reuse the pod , but the quality of the drink takes something of a nosedive.

Luckily, there are some single-serving exceptions to the 50 to 100-milligram rule (although they don't surpass 200 milligrams like pour-over capsules). According to Nespresso's FAQ, Vertuo coffees contain a caffeine content ranging between 170 and 200 milligrams. They're best served as 2.7-ounce double shots, but if you use Nespresso pods for the energy boost rather than to sample the best flavors , you're likely well acquainted with double shots already. These pods are naturally more expensive than single-shot capsules and will set you back between $1.15 and $1.25, depending on which pod you choose.

Read the original article on Mashed .