Home Ingredients Caffeine
EnergyAthletic PerformanceCognitiveMood & Mental Health

Caffeine

Evidence Level
Very Strong
9 Clinical Trials
5 Documented Benefits
5/5 Evidence Score

Caffeine is a stimulant found naturally in coffee, tea, cocoa, guarana and kola nut, added to cola and energy drinks, and sold in supplements and some medicines. It works mainly by blocking adenosine receptors in the brain. In placebo-controlled trials, about 40 to 300 mg improves alertness, vigilance and reaction time, while effects on memory are less reliable. For exercise, 3 to 6 mg per kg of body weight, usually about an hour before, improves performance in many but not all studies, most clearly in endurance tests, partly because effort feels easier. Responses vary, partly with genes that affect caffeine breakdown. Low doses can lift mood briefly, high doses raise anxiety and jitteriness, and 400 mg taken 6 hours before bed can still shorten sleep. The FDA cites 400 mg a day as not generally linked to negative effects in most adults; pregnancy limits are lower, and pure caffeine powders can be dangerous.

Studied Dose Alertness: about 40 to 300 mg. Exercise: 3 to 6 mg per kg of body weight, usually about 60 minutes before. Most adults: up to 400 mg a day from all sources.
Active Compound Caffeine (1,3,7-trimethylxanthine)

Benefits

Alertness and reduced mental fatigue

Caffeine blocks adenosine receptors, and low to moderate doses (about 40 to 300 mg) improve alertness, vigilance, attention and reaction time. In a double-blind crossover trial of 48 adults, 75 or 150 mg reduced self-rated mental fatigue and raised alertness in both regular and non-regular users. In soldiers kept awake on three nights, repeated 200 mg doses kept reaction speed and vigilance up.

Supported by Trial 5, Trial 6

Endurance and exercise performance

A meta-analysis of 40 double-blind studies found caffeine improved exercise test results by 12.3% versus placebo, most for endurance and time-to-exhaustion tests. In eight trained runners, 3 and 6 mg per kg each extended running time by about 22%, while 9 mg per kg gave no significant gain. Gains in strength and sprinting are small to moderate and not seen in every study.

Supported by Trial 1, Trial 2

Attention and reaction time

Reviews find caffeine speeds reaction time and improves attention, but it does not usually help learning or long-term memory and can hinder tasks that lean heavily on working memory. In a double-blind trial of 102 light users, it improved vigilance and reaction time but impaired a memory task. Its thinking benefits come largely from effects on arousal and are larger when people are tired.

Supported by Trial 7, Trial 5

Exercise feels easier

A meta-analysis of 21 studies found caffeine lowered ratings of perceived exertion during exercise by 5.6% compared with placebo, and this lower sense of effort explained about 29% of the improvement in performance.

Supported by Trial 2

Short-term mood lift

At low doses caffeine improves hedonic tone, a measure of pleasant mood, and reduces anxiety, while high doses raise tension, anxiety and jitteriness. In placebo-controlled trials, single doses raised positive mood and alertness ratings, and regular users tended to get more mood benefit than non-users. These are short-term effects in healthy people; whether regular use improves mood over time is uncertain.

Supported by Trial 5, Trial 7

Mechanism of action

1

Adenosine receptor blockade

Caffeine blocks adenosine receptors, which is the main way it raises alertness, attention and reaction time and affects physical performance.

2

Lower perceived effort

During exercise, caffeine makes the same workload feel easier, which accounts for part of its performance benefit.

3

Arousal rather than pure brain boosting

Caffeine acts mainly as a mild stimulant. Its effects on arousal, mood and concentration drive most of its cognitive benefits, which is why gains are larger in tired people and smaller for memory.

4

Why people respond differently

In 101 male athletes, caffeine sped a 10 km cycling time trial only in carriers of the AA version of the CYP1A2 gene, had no effect in AC carriers and slowed CC carriers at 4 mg per kg. Genes linked to caffeine metabolism and response, and habitual intake, also help explain why effects on sleep and anxiety vary.

Clinical trials

1
Exercise testing meta-analysis
PubMed

Meta-analysis of 40 double-blind studies with 76 effect sizes (Doherty et al. 2004, Int J Sport Nutr Exerc Metab)

Participants in endurance, graded and short-term exercise tests

Caffeine improved exercise test outcomes by 12.3% (95% CI 9.1 to 15.4) versus placebo, an effect size of 0.41. Endurance tests improved more than graded or short-term tests, and time-to-exhaustion protocols showed the largest effect.

2
Perceived exertion meta-analysis
PubMed

Meta-analysis of 21 studies with 109 effect sizes (Doherty et al. 2005, Scand J Med Sci Sports)

Participants in constant-load and exhausting exercise tests

Caffeine lowered ratings of perceived exertion during exercise by 5.6% (effect size -0.47) but not at the end of exhausting exercise, and improved performance by 11.2%. Lower exertion explained about 29% of the performance gain.

3
Three caffeine doses and running endurance
PubMed

Placebo-controlled trial of capsules with placebo or 3, 6 or 9 mg per kg caffeine taken 1 hour before running at 85% of maximal oxygen uptake to exhaustion, after 48 hours without caffeine (Graham et al. 1995, J Appl Physiol (1985))

8 well-trained endurance athletes

Running time to exhaustion rose by about 22% with both 3 and 6 mg per kg (placebo time 49.4 minutes), while 9 mg per kg had no significant effect. Only the higher doses raised epinephrine, so the authors concluded the endurance gain was not explained by catecholamines.

4
Caffeine, CYP1A2 gene type and cycling time trial
PubMed

Split-plot randomized, double-blind, placebo-controlled trial of 0, 2 or 4 mg per kg caffeine before a 10 km cycling time trial, with genotyping of the CYP1A2 gene (rs762551) (Guest et al. 2018, Med Sci Sports Exerc)

101 competitive male athletes, mean age 25

Overall, 4 mg per kg cut cycling time by 3% versus placebo (17.6 vs 18.1 minutes). The effect depended on genotype: AA carriers were 4.8% faster at 2 mg per kg and 6.8% faster at 4 mg per kg, AC carriers saw no effect, and CC carriers were 13.7% slower at 4 mg per kg.

5
Habitual users and non-users
PubMed

Placebo-controlled double-blind balanced crossover trial of 75 mg, 150 mg caffeine or placebo after overnight abstinence (Haskell et al. 2005, Psychopharmacology (Berl))

24 habitual caffeine consumers and 24 non-consumers

Caffeine improved simple reaction time, vigilance reaction time, working memory reaction time and sentence verification accuracy, reduced self-rated mental fatigue and raised alertness in both groups. Consumers tended to get more mood benefit and non-consumers more benefit on the tasks.

6
Repeated doses during overnight wakefulness
PubMed

Randomized placebo-controlled trial of four 200 mg doses (800 mg a day) or placebo given in the late evening and early morning on three successive nights, each followed by a 4 hour afternoon sleep (Kamimori et al. 2015, Psychopharmacology (Berl))

20 Special Forces soldiers, 10 per group

Caffeine maintained psychomotor vigilance speed, improved event detection in field vigilance testing, increased correct responses on a vigilance monitor and sped logical reasoning across all three nights. Live-fire marksmanship was not changed.

7
Four doses in light caffeine users
PubMed

Randomized double-blind within-subject trial of 0, 50, 150 and 450 mg caffeine capsules in four sessions, with tasks 40 minutes after dosing (Childs et al. 2006, Psychopharmacology (Berl))

102 light, nondependent caffeine users

Caffeine raised feelings of arousal, positive mood and high, increased hits and shortened reaction times on a vigilance task, but impaired scores on a memory task and raised blood pressure.

8
Caffeine taken 0, 3 or 6 hours before bed
PubMed

Randomized double-blind, placebo-controlled crossover trial of 400 mg caffeine at bedtime or 3 or 6 hours before, with sleep measured at home by a portable monitor and a diary; funded by a grant from the monitor's maker (Zeo Inc), and one author was listed with that company (Drake et al. 2013, J Clin Sleep Med)

12 healthy normal sleepers aged 19 to 48

All three timings disturbed sleep compared with placebo. Even at 6 hours before bed, the monitor showed more than 1 hour less sleep, while diary-reported sleep fell by 41 minutes, which was not significant (p = 0.08).

9
Caffeine before a regadenoson heart scan
PubMed

Multicenter double-blind randomized trial of 200 mg or 400 mg caffeine or placebo 90 minutes before a repeat regadenoson stress scan (Tejani et al. 2014, Int J Cardiovasc Imaging)

207 adults with a high likelihood of coronary artery disease

Both caffeine doses reduced the number of heart segments where the scan detected reduced blood flow, while placebo did not (P < 0.001), meaning caffeine can hide findings on this test.

Side effects and drug interactions

Common Potential side effects

Too much caffeine can cause a fast heart rate, palpitations, high blood pressure, insomnia, anxiety, jitters, upset stomach, nausea and headache, according to the FDA.
High doses increase tension, nervousness and jitteriness. Very high doses (around 9 mg per kg) bring a high rate of side effects and are not needed for a performance benefit.
Effects on sleep and anxiety vary widely between people, partly because of genes linked to how caffeine is broken down and how people respond to it, and partly with habitual intake.
Pregnancy and breastfeeding: EFSA considers caffeine from all sources up to 200 mg a day not a safety concern for the fetus, and a Health Canada review suggested 300 mg a day or less for women of reproductive age. The FDA advises people who are pregnant, trying to become pregnant or breastfeeding to talk to their provider about limits.
Children: a Health Canada review suggested no more than 2.5 mg per kg a day, and EFSA proposes 3 mg per kg a day for habitual intake in children and adolescents. The FDA notes that medical experts advise against energy drinks for children and teens.
Pure powders and liquids: the FDA warns that pure and highly concentrated caffeine products can cause serious harm, including death. Toxic effects such as seizures can occur with rapid intake of about 1,200 mg, less than half a teaspoon of pure caffeine. Bulk products require measuring a safe serving from what can be a toxic amount, so the FDA urges caution.
Sleep: in a crossover trial in 12 healthy sleepers, 400 mg taken as early as 6 hours before bed cut sleep measured by a home monitor by more than 1 hour. EFSA notes that single doses of 100 mg may affect sleep in some adults, particularly close to bedtime.

Important Drug interactions

Heart stress tests: in a randomized trial, 200 or 400 mg of caffeine taken before a regadenoson stress scan reduced the problems the scan could detect. Follow your care team's instructions to avoid caffeine before such tests.
Clozapine: cola drinks, which contain caffeine, have been reported to raise clozapine blood levels, which can be toxic. Keep caffeine intake steady and tell your prescriber about changes.
Lithium: stopping caffeine can raise lithium blood levels, and cola drinks have been reported to lower them. Avoid sudden changes in caffeine intake and tell your prescriber.
Warfarin and some other drugs: cola drinks have been reported to lower warfarin levels and to raise levels of methotrexate, carbamazepine and phenytoin. Ask your pharmacist if you take these.
Medications and conditions in general: the FDA notes that some medications and certain conditions make people more sensitive to caffeine's effects, so ask your pharmacist or prescriber if you take regular medication.

Featured in these supplement guides

Energy & Fatigue→Weight Loss→

More Energy Ingredients

Energy
Iron
Iron is an essential mineral needed to make hemoglobin, the protein in red blood cells tha…
Energy
Ferrous Sulfate
Ferrous sulfate is the reference oral iron salt, the form that most other iron supplements…
Energy
Vitamin B12
Vitamin B12 (cobalamin) is a water-soluble vitamin needed for red blood cell formation, DN…
Energy
Ferrous Fumarate
Ferrous fumarate is an iron salt that is 33 percent elemental iron by weight, compared wit…

Related articles from our blog

📝 Article
Paraxanthine vs Caffeine: What the Evidence Really Says
Paraxanthine is having a moment. It is turning up in pre-workouts, canned energy drinks, and standalone capsules, sold a…
August 9, 2026
📝 Article
Supplements for Energy That Aren't Caffeine
Here is the uncomfortable truth about caffeine: it does not give you energy. It blocks the brain signal that tells you t…
June 21, 2026
📝 Article
Natural Caffeine vs Caffeine Anhydrous – What’s Best for Focus, Energy & Performance?
When it comes to boosting energy, sharpening focus, and improving physical performance, caffeine is king. But not all ca…
July 02, 2025

Frequently asked questions about Caffeine

How do the FDA, EFSA and Health Canada caffeine limits compare?

For most healthy adults, the FDA cites 400 mg a day, about two to three 12-ounce cups of coffee, as an amount not generally associated with negative effects, and EFSA gives the same 400 mg figure, adding that single doses up to 200 mg raise no concern. For pregnancy, EFSA considers up to 200 mg a day from all sources not a concern for the fetus, while a Health Canada review suggested 300 mg a day or less for women of reproductive age. The FDA advises talking to your provider if you are pregnant, trying to become pregnant or breastfeeding.

When should caffeine be taken before exercise?

Exercise studies most often give 3 to 6 mg per kilogram of body weight about 60 minutes before exercise, and doses as low as 2 mg per kg may work. Caffeine gum may need less lead time than capsules. Very high doses, around 9 mg per kg, bring many side effects, and in one trial of trained runners 9 mg per kg did not improve endurance significantly while 3 and 6 mg per kg did.

How long before bed should I stop caffeine?

In a home sleep trial, 400 mg taken 6 hours before bed still cut sleep measured by a monitor by more than 1 hour, and the authors suggest avoiding substantial caffeine for at least 6 hours before bedtime. EFSA notes that even 100 mg may affect sleep in some adults when taken close to bedtime. Sensitivity varies between people, partly because of genetics.

Why are pure caffeine powders riskier than coffee?

The FDA warns that pure and highly concentrated caffeine products can have serious health consequences, including death. Toxic effects such as seizures can occur with rapid intake of about 1,200 mg, which is less than half a teaspoon of pure caffeine, and bulk products require measuring a safe serving from what can be a toxic amount.

Does caffeine work the same for regular coffee drinkers?

In a crossover trial of 24 habitual users and 24 non-users tested after overnight abstinence, 75 and 150 mg improved reaction times and alertness in both groups; regular users tended to get more mood benefit and non-users more performance benefit. In a separate trial of 102 light users, caffeine still raised alertness and positive mood, so its effects are not only reversal of withdrawal.

What is Caffeine?

Caffeine is a stimulant found naturally in coffee, tea, cocoa, guarana and kola nut, added to cola and energy drinks, and sold in supplements and some medicines. It works mainly by blocking adenosine receptors in the brain.

What is Caffeine used for?

Caffeine is researched primarily for Energy, Athletic Performance, and Cognitive. Caffeine blocks adenosine receptors, and low to moderate doses (about 40 to 300 mg) improve alertness, vigilance, attention and reaction time.

What is the recommended dosage of Caffeine?

The clinically studied dose is Alertness: about 40 to 300 mg. Exercise: 3 to 6 mg per kg of body weight, usually about 60 minutes before. Most adults: up to 400 mg a day from all sources. Always follow the product label and check with a healthcare provider for personal advice.

Is Caffeine safe, and does it have side effects?

Side effects and cautions reported for Caffeine include: Too much caffeine can cause a fast heart rate, palpitations, high blood pressure, insomnia, anxiety, jitters, upset stomach, nausea and headache, according to the FDA. High doses increase tension, nervousness and jitteriness. It may also interact with some medications. Caffeine is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Caffeine interact with any medications?

Possible interactions include: Heart stress tests: in a randomized trial, 200 or 400 mg of caffeine taken before a regadenoson stress scan reduced the problems the scan could detect. Follow your care team's instructions to avoid caffeine before such tests. If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Caffeine?

NutraSmarts rates the evidence for Caffeine as Very Strong (5 out of 5). It is backed by 9 study summaries and 17 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(17 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Doherty M, Smith PM. Effects of caffeine ingestion on exercise testing: a meta-analysis. Int J Sport Nutr Exerc Metab. 2004;14(6):626-46. doi: 10.1123/ijsnem.14.6.626.PubMedUsed to support: Meta-analysis of 40 double-blind studies. Caffeine improved exercise test outcomes by 12.3% versus placebo (effect size 0.41), most for endurance and time-to-exhaustion tests.
  2. Doherty M, Smith PM. Effects of caffeine ingestion on rating of perceived exertion during and after exercise: a meta-analysis. Scand J Med Sci Sports. 2005;15(2):69-78. doi: 10.1111/j.1600-0838.2005.00445.x.PubMedUsed to support: Meta-analysis of 21 studies. Caffeine reduced perceived exertion during exercise by 5.6% and improved performance by 11.2%; lower exertion explained about 29% of the performance gain.
  3. Guest NS, VanDusseldorp TA, Nelson MT, Grgic J, Schoenfeld BJ, Jenkins NDM, Arent SM, Antonio J, Stout JR, Trexler ET, Smith-Ryan AE, Goldstein ER, Kalman DS, Campbell BI. International society of sports nutrition position stand: caffeine and exercise performance. J Int Soc Sports Nutr. 2021;18(1):1. doi: 10.1186/s12970-020-00383-4.PubMedUsed to support: ISSN position stand: caffeine at 3 to 6 mg per kg improves many aspects of exercise performance in many but not all studies, with aerobic endurance showing the most reliable moderate to large benefits; doses as low as 2 mg per kg may work; 60 minutes before exercise is the most common timing; very high doses (9 mg per kg) cause many side effects; it improves attention and vigilance; responses vary with genetics and habitual intake. Several authors report ties to supplement, coffee or genetic testing companies, and the ISSN has received grants from companies selling caffeine products.
  4. McLellan TM, Caldwell JA, Lieberman HR. A review of caffeine's effects on cognitive, physical and occupational performance. Neurosci Biobehav Rev. 2016;71:294-312. doi: 10.1016/j.neubiorev.2016.09.001.PubMedUsed to support: Review: low to moderate caffeine doses improve alertness, vigilance, attention and reaction time, with less consistent effects on memory and executive function, act by blocking adenosine receptors, and improve physical performance above about 200 mg.
  5. Nehlig A. Is caffeine a cognitive enhancer? J Alzheimers Dis. 2010;20 Suppl 1:S85-94. doi: 10.3233/JAD-2010-091315.PubMedUsed to support: Review: caffeine is not a pure cognitive enhancer; it speeds reaction time, improves hedonic tone and reduces anxiety at low doses, raises anxiety and jitteriness at high doses, and helps more when people are fatigued.
  6. Nawrot P, Jordan S, Eastwood J, Rotstein J, Hugenholtz A, Feeley M. Effects of caffeine on human health. Food Addit Contam. 2003;20(1):1-30. doi: 10.1080/0265203021000007840.PubMedUsed to support: Health Canada review: up to 400 mg a day is not associated with adverse effects in healthy adults; suggests 300 mg a day or less for women of reproductive age and 2.5 mg per kg or less for children.
  7. Haskell CF, Kennedy DO, Wesnes KA, Scholey AB. Cognitive and mood improvements of caffeine in habitual consumers and habitual non-consumers of caffeine. Psychopharmacology (Berl). 2005;179(4):813-25. doi: 10.1007/s00213-004-2104-3.PubMedUsed to support: Double-blind crossover trial in 24 habitual caffeine users and 24 non-users. 75 or 150 mg improved reaction times, accuracy and alertness and reduced mental fatigue; users tended to get more mood benefit.
  8. Tejani FH, Thompson RC, Kristy R, Bukofzer S. Effect of caffeine on SPECT myocardial perfusion imaging during regadenoson pharmacologic stress: a prospective, randomized, multicenter study. Int J Cardiovasc Imaging. 2014;30(5):979-89. doi: 10.1007/s10554-014-0419-7.PubMedUsed to support: Randomized double-blind trial in 207 adults. 200 or 400 mg caffeine before a regadenoson stress scan reduced the ischemia the scan detected compared with placebo.
  9. Mester R, Toren P, Mizrachi I, Wolmer L, Karni N, Weizman A. Caffeine withdrawal increases lithium blood levels. Biol Psychiatry. 1995;37(5):348-50. doi: 10.1016/0006-3223(94)00243-V.PubMedUsed to support: Brief report with no PubMed abstract; its title reports that caffeine withdrawal increases lithium blood levels.
  10. Nomani H, Moghadam AT, Emami SA, Mohammadpour AH, Johnston TP, Sahebkar A. Drug interactions of cola-containing drinks. Clin Nutr. 2019;38(6):2545-2551. doi: 10.1016/j.clnu.2019.01.029.PubMedUsed to support: Review of drug interactions with cola drinks, which are acidic and contain caffeine; the authors suggest both properties drive the interactions. Reports raised blood levels of clozapine, methotrexate, carbamazepine, phenytoin and ibuprofen, and lowered levels and efficacy of lithium and warfarin.
  11. U.S. Food and Drug Administration. Spilling the Beans: How Much Caffeine is Too Much? FDA Consumer Updates. 2024;Online consumer update, content current as of August 28, 2024..SourceUsed to support: Not PubMed-indexed: FDA consumer update. Cites 400 mg a day, about two to three 12-ounce cups of coffee, as an amount not generally associated with negative effects for most adults; advises people who are pregnant, trying to become pregnant or breastfeeding to talk to their provider; lists symptoms of too much caffeine; notes that some conditions and medications increase sensitivity; reports that medical experts advise against energy drinks for children and teens; and warns that pure and highly concentrated caffeine products can cause serious harm including death, with toxic effects such as seizures at around 1,200 mg taken rapidly, less than half a teaspoon of pure caffeine.
  12. European Food Safety Authority (EFSA). Caffeine EFSA topic page. 2015;Summary of the EFSA Scientific Opinion on the safety of caffeine, published 27 May 2015, doi: 10.2903/j.efsa.2015.4102; topic page accessed 2026..SourceUsed to support: Not PubMed-indexed (EFSA topic page). States that single doses up to 200 mg (about 3 mg per kg) and intakes up to 400 mg a day (about 5.7 mg per kg) do not raise safety concerns for healthy adults other than pregnant women; that up to 200 mg a day from all sources does not raise safety concerns for the fetus; proposes 3 mg per kg a day for children and adolescents; and notes that single doses of 100 mg may affect sleep in some adults, particularly close to bedtime.
  13. Graham TE, Spriet LL. Metabolic, catecholamine, and exercise performance responses to various doses of caffeine. J Appl Physiol (1985). 1995;78(3):867-74. doi: 10.1152/jappl.1995.78.3.867.PubMedUsed to support: Placebo-controlled trial in 8 well-trained endurance athletes. 3 and 6 mg per kg caffeine taken 1 hour before running at 85% of maximal oxygen uptake each extended time to exhaustion by about 22%, while 9 mg per kg had no significant effect; only the higher doses raised epinephrine.
  14. Guest N, Corey P, Vescovi J, El-Sohemy A. Caffeine, CYP1A2 Genotype, and Endurance Performance in Athletes. Med Sci Sports Exerc. 2018;50(8):1570-1578. doi: 10.1249/MSS.0000000000001596.PubMedUsed to support: Randomized double-blind placebo-controlled trial in 101 competitive male athletes. 4 mg per kg caffeine cut 10 km cycling time by 3% overall; AA carriers of the CYP1A2 variant improved at 2 and 4 mg per kg, AC carriers saw no effect, and CC carriers were 13.7% slower at 4 mg per kg.
  15. Kamimori GH, McLellan TM, Tate CM, Voss DM, Niro P, Lieberman HR. Caffeine improves reaction time, vigilance and logical reasoning during extended periods with restricted opportunities for sleep. Psychopharmacology (Berl). 2015;232(12):2031-42. doi: 10.1007/s00213-014-3834-5.PubMedUsed to support: Randomized placebo-controlled trial in 20 Special Forces soldiers. Four 200 mg doses a night over three nights of sustained wakefulness maintained psychomotor vigilance speed and improved vigilance and logical reasoning; live-fire marksmanship was unchanged.
  16. Childs E, de Wit H. Subjective, behavioral, and physiological effects of acute caffeine in light, nondependent caffeine users. Psychopharmacology (Berl). 2006;185(4):514-23. doi: 10.1007/s00213-006-0341-3.PubMedUsed to support: Randomized double-blind within-subject trial of 0, 50, 150 and 450 mg caffeine in 102 light, nondependent users. Caffeine raised blood pressure, arousal, positive mood and feelings of high, improved vigilance hits and reaction time, and impaired a memory task, showing effects without withdrawal reversal.
  17. Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. J Clin Sleep Med. 2013;9(11):1195-200. doi: 10.5664/jcsm.3170.PubMedUsed to support: Randomized double-blind placebo-controlled crossover trial in 12 healthy sleepers. 400 mg caffeine at bedtime, 3 or 6 hours before each disturbed sleep versus placebo; at 6 hours, monitored sleep fell by more than 1 hour while the diary reduction (41 minutes) was not significant. Funded by a grant from Zeo Inc, maker of the sleep monitor, and one author was listed with Zeo Inc (full text on PMC).