Caffeine occupies the adenosine receptor without firing it; sleepiness is only postponed.
Full mechanism, in text
How caffeine blocks sleepiness
1 · Every waking hour, adenosine presses the brake
Adenosine binds receptors on neurons, damps activity, and makes you progressively drowsy.That is one way the body tracks time to sleep.
2 · Caffeine takes the seat but does not push
Caffeine can sit in the same receptor without activating it, blocking real adenosine.The sleepiness brake is occupied, and you feel alert. Fatigue is not erased, only postponed.
3 · Adenosine keeps accumulating
The key is that adenosine keeps accumulating: once caffeine is metabolized, the suppressed drowsiness returns together.
4 · Half-life sets the afternoon cutoff
Caffeine usually has a 4 to 5 hour half-life, so about half of a noon cup still works by early evening, and the cutoff is counted back from bedtime.In healthy adults the elimination half-life of caffeine is usually 4 to 5 hours, longer in slow metabolizers. A noon cup still has about half its caffeine working by early evening. Set the cutoff by bedtime: in Drake 2013, a small trial in 12 people, 400 mg of caffeine given 0, 3 or 6 hours before bed disrupted sleep at all three times, so the authors advise no sizeable caffeine within at least 6 hours of bed.