Caffeine half-life

Caffeine clears by first-order kinetics: a constant fraction is removed per unit time, not a constant amount. That produces exponential decay, so each half-life halves whatever is left.

remaining = dose × 0.5^(hours / half-life)

Why an afternoon coffee reaches midnight

With a 5-hour half-life, a 95 mg coffee at 3pm still leaves about 24 mg at 11pm and 12 mg at 4am. Controlled studies have found measurable sleep disruption from caffeine taken six hours before bed even when people report no subjective effect — the disruption shows up in sleep architecture rather than in how long it takes to fall asleep.

Tolerance is not clearance

Regular drinkers feel less of a jolt, but the caffeine is still present and still acting on adenosine receptors while you sleep. “Coffee doesn't affect me” usually means the alerting effect has faded, not the pharmacology.

Daily limits

The EFSA and FDA both put 400 mg a day as generally safe for healthy adults, and 200 mg in a single dose. Pregnancy guidance is 200 mg a day total. Roughly four cups of brewed coffee reaches the daily figure.