Caffeine has a half-life of roughly five to six hours in a healthy adult. That single sentence explains more evening restlessness than any other fact about sleep, and almost nobody is told it.
Half-life means the time it takes for half the dose to clear. So a large coffee at two in the afternoon — call it 200 mg — is not gone by bedtime. Around eight in the evening, roughly 100 mg is still circulating. Around two in the morning, roughly 50 mg. That last figure is about the caffeine in a can of cola, arriving in the middle of the night, uninvited.
Why it matters more than the jitters
Caffeine does not create energy. It blocks a molecule called adenosine, which builds up in the brain across the waking day and produces the growing sense of sleepiness that makes bedtime feel natural. Caffeine sits in the same receptors and stops the message arriving.
The adenosine does not stop accumulating while this happens. It piles up behind the blockade. Which is why the crash, when it comes, arrives all at once — and why a person can be exhausted and wired in the same hour.
The practical version
- Work backwards from the hour you want to be asleep and stop eight to ten hours before it. For an eleven o'clock bedtime, that puts the last cup somewhere around one in the afternoon.
- Half-life varies a great deal between people. Some clear caffeine in four hours, others take eight. Age, certain medications and the contraceptive pill all slow it down.
- Decaf is not caffeine-free. A cup carries somewhere between 2 and 15 mg.
- Tea, dark chocolate and many painkillers count towards the total.
The test worth running: move the cut-off two hours earlier for a week and change nothing else. One variable at a time is the only way to learn anything from your own body.
None of this makes coffee a problem. It makes the timing of coffee a decision — which is a far more useful thing to know.