Caffeine
The world's most-used performance aid, with real daily limits and a sleep cost if timed too late.
Not medical advice — pregnant people, those with anxiety or heart rhythm conditions, and anyone sensitive to stimulants should be more conservative than the ranges below.
How It Works
Caffeine blocks adenosine receptors in the brain — adenosine is a compound that builds up over the day and promotes drowsiness, so blocking it produces alertness and reduced perceived effort during exercise. It's also a mild stimulant of the central nervous system more broadly, which is behind its effects on focus, reaction time, and endurance performance.
The Research
Performance: Guest NS, et al. "International Society of Sports Nutrition Position Stand: Caffeine and Exercise Performance," Journal of the International Society of Sports Nutrition, 2021. Concludes caffeine reliably improves exercise performance, most consistently for aerobic endurance, at doses of 3–6mg/kg body mass, typically taken about 60 minutes before exercise; doses beyond roughly 9mg/kg increase side effects without added benefit.
Read the position stand →Tolerance and timing: Drake C, Roehrs T, Shambroom J, Roth T. "Caffeine Effects on Sleep Taken 0, 3, or 6 Hours before Going to Bed," Journal of Clinical Sleep Medicine, 2013. In this trial, 400mg of caffeine taken even 6 hours before bedtime measurably reduced total sleep time by about an hour — notably, participants often didn't subjectively notice the disruption at that distance from bedtime, which is worth knowing if you drink coffee in the afternoon and don't feel like it's affecting your sleep.
Read the study →Tolerance to caffeine's stimulant effects builds with regular use, which is why habitual users often report needing more to feel the same alertness — the performance benefits documented above are generally still present even in habitual users, but the subjective "kick" fades faster than the underlying effect.
Typical Dosing & Safety Limits
Effective performance dose: 3–6mg/kg body mass, per the ISSN position stand above.
Daily upper limit: the European Food Safety Authority's formal safety opinion puts
habitual daily intake up to 400mg as not a safety concern for healthy adults, with single doses up to
200mg also considered safe; pregnant and breastfeeding women are advised to limit intake to 200mg/day,
and adolescents to about 3mg/kg/day. U.S. FDA public guidance is broadly consistent with the 400mg/day
figure for healthy adults.
Read the EFSA scientific opinion on caffeine safety →
Who Should Be Cautious
Per EFSA guidance, pregnant and breastfeeding people should limit intake to 200mg/day. Anyone with anxiety disorders, heart arrhythmias, or high sensitivity to stimulants should stay well under the general guidance above, and anyone with sleep difficulties should be cautious of any caffeine within roughly 6+ hours of bedtime, per the research above.