Caffeine

What the evidence says

Caffeine blocks adenosine receptors and can acutely increase alertness, attention, and reaction speed. A 2025 meta-analysis of randomized trials found small-to-moderate improvements in several attention domains, not a general increase in intelligence or durable learning capacity.^[1]

The effect is noticeable at ordinary beverage doses: a randomized trial found that 60 mg improved sustained attention and alertness. During sleep loss, caffeine also improves multiple performance measures, although it cannot replace the restorative functions of sleep.^[4][3]

Timing is the limiting factor. A meta-analysis estimated that caffeine reduces total sleep time and sleep efficiency and increases sleep-onset latency; its models suggested that even coffee consumed many hours before bedtime can matter. Individual metabolism, dose, pregnancy, medications, and habitual use shift that window.^[2]

Regular use produces tolerance and withdrawal, including headache, fatigue, and reduced concentration. The FDA says up to 400 mg per day is not generally associated with negative effects for most healthy adults, but that is a population guide rather than a target and does not apply to everyone.^[5][1]