Caffeine is in coffee, tea, chocolate, energy drinks and most pre-workout products, and most adults take it daily without thinking of it as a supplement. In sport it is one of the few substances with enough trials to support an umbrella review: a review of the reviews.

The performance evidence is about as settled as evidence in sports nutrition gets. The interesting part of this guide is the other side of the ledger. The same trial methods that measure the benefit have measured what caffeine does to the following night's sleep, and the answer is larger and lasts longer than most people assume.

Caffeine blocks adenosine receptors in the brain. Adenosine accumulates during waking and signals tiredness; blocking it reduces perceived effort and fatigue, which is the main route to better performance. Smaller contributions come from effects on muscle calcium handling and on the release of adrenaline.

The sleep effect is the same mechanism running at the wrong time. Caffeine's half-life in adults is around five hours and varies several-fold between people depending on liver enzyme genetics, so a dose taken in the afternoon is still partly active at bedtime, where it blocks the very signal that sleep pressure depends on.

Performance: established across the board. A 2020 umbrella review in the British Journal of Sports Medicine assessed 21 published meta-analyses and found caffeine ergogenic for aerobic endurance, muscle strength, muscle endurance, power, jumping and exercise speed, with moderate-quality evidence for endurance, strength, muscle endurance and anaerobic power. Effects were generally larger for aerobic than anaerobic tasks, and most underlying trials were in young men.

The size of the effect is modest and consistent. A 2018 meta-analysis of 46 endurance studies found moderate doses improved mean power output by about 3% and time-trial completion by about 2%, with a minority of studies finding slower performance in some people. A 2018 meta-analysis of strength and power found small but significant gains in maximal strength and vertical jump, with the strength benefit in the upper body rather than the lower.

Sleep: a measurable cost. A 2023 meta-analysis of 24 studies found caffeine reduced total sleep time by 45 minutes and sleep efficiency by 7%, lengthened the time to fall asleep and the time awake during the night, and cut deep sleep by about 11 minutes. To avoid a reduction in total sleep time, the authors estimated a cup of coffee needed to be taken at least 8.8 hours before bed, and a standard pre-workout dose at least 13 hours before.

Caffeine's common effects are the familiar ones: jitteriness, raised heart rate, stomach upset, anxiety in susceptible people, and withdrawal headache when a regular intake stops. Trials of supplement doses in healthy adults report these and little else. The sleep effect above is the one most people underestimate and the one with the clearest trial data.

The real danger is concentrated caffeine. Pure powdered caffeine and highly concentrated liquids have caused deaths from doses that look like a teaspoon, which is why the US FDA has acted against their sale to consumers. A pre-workout product is a measured dose; a bag of powder is not. Caffeine in pregnancy, with certain heart conditions, and alongside other stimulants is a conversation for a clinician.

Caffeine is a food constituent and a food additive, and is regulated as such in every major market. It is not a medicine at the doses in drinks and supplements, though it is used as one in hospitals for premature infants. Energy drinks and supplements carry caffeine content on the label in most jurisdictions, with warnings required in some.

In sport, caffeine was removed from the World Anti-Doping Agency's prohibited list in 2004 and remains on its monitoring program. The US FDA has taken enforcement action against bulk pure caffeine products sold directly to consumers.