Caffeine Is Borrowing Against Sleep, Not Replacing It

A six-hour half-life means an afternoon coffee is still active at bedtime. The measurable cost shows up in deep sleep.

Caffeine blocks adenosine receptors, which suppresses the perception of sleep pressure without removing the pressure itself. The half-life in most adults is around five to six hours, meaning a 2 p.m. dose leaves a meaningful fraction circulating at 10 p.m.

Controlled trials that dosed caffeine three and six hours before bed found reduced total sleep time and measurably less slow-wave sleep — even in participants who reported no subjective difficulty falling asleep. Self-report is the wrong instrument here.

Genetics account for a good deal of variation. Slow metabolisers of the CYP1A2 variety clear caffeine over considerably longer windows, which is why blanket cutoff advice fits some people badly.

A workable default: keep intake under about 400 mg a day, stop at least eight hours before your intended sleep time, and delay the first cup an hour after waking if you are prone to an early-afternoon crash.

Editorial note: this article summarises published research for general information. It is not medical advice. Discuss any intervention with a qualified clinician.