Caffeine Is a Longer Story Than You Think

The most common thing people say about caffeine is that it doesn't affect their sleep, because they fall asleep fine after an evening coffee.

That's probably accurate. It's also not the mechanism.

What caffeine is actually doing

While you're awake, a molecule called adenosine accumulates in the brain. The more of it there is, the sleepier you feel. That build-up is what people mean by "sleep pressure" — the sense, by ten at night, of being genuinely ready.

Caffeine doesn't remove adenosine. It sits in the receptors so the adenosine can't be read. The pressure is still building; you just can't feel it.

Which is why caffeine works so well and why the crash is so specific: when it clears, all the accumulated adenosine becomes readable at once.

The half-life is the part people miss

Caffeine has a half-life of roughly five to six hours in most adults, and it varies substantially between people for genetic reasons.

A coffee at four in the afternoon means a meaningful proportion is still circulating at ten. Not enough to keep you awake — enough to reduce the depth of the sleep you do get.

It rarely stops you falling asleep. It quietly lowers the quality of the night, so you wake feeling like you slept badly without any obvious reason why.

Then the loop

Slightly worse sleep produces a slightly more tired morning, which produces a slightly larger coffee, which produces slightly worse sleep. It's an extremely stable arrangement and it can run for years without anyone noticing a problem.

The tell is simple: if you genuinely need caffeine to function in the morning rather than just enjoying it, you are probably inside that loop.

What we do

A cut-off at two in the afternoon. Not a rule about quantity — a rule about timing, which is easier to keep and does more.

Expect a rough four or five days. Then it stops being noticeable, and the first week of sleeping without it is usually enough of an argument.

The things people forget contain it

Green tea, most black tea, dark chocolate, some painkillers, anything marketed for focus. Decaf isn't zero either — it's low, not none.

None of these is a problem on its own. All of them are worth knowing about at seven in the evening.