Caffeine

Summary

Caffeine raises alertness for several hours, most clearly when you are short on sleep. In mostly simulated shift-work trials in young adults, it reduced errors and improved attention and memory; trial quality was low. Meta-analyses show small gains in endurance, strength and power. It shortens and lightens sleep if taken late, and higher intake is linked to more anxiety, most clearly above 400 mg/day.

Mechanism

Caffeine blocks adenosine receptors. Adenosine builds up while you are awake and drives sleepiness, so blocking it increases alertness. Half-life is about 4–6 hours in healthy adults, longer in pregnancy, liver disease or with some medicines.

Typical studied dose

Exercise studies typically used 3–6 mg per kg body weight before exercise. A cup of brewed coffee holds roughly 80–100 mg, but content varies widely. In a sleep meta-analysis, a 107 mg coffee needed to be taken at least 8.8 hours before bed to avoid shortening sleep.

Upper limit

EFSA (2015): habitual intake up to 400 mg/day and single doses up to 200 mg raise no safety concerns for healthy adults; up to 200 mg/day in pregnancy and breastfeeding. The FDA cites about 400 mg/day for healthy adults. Pure caffeine powder is risky: a teaspoon can contain several grams.