Caffeine does not “wear off in an hour”. It leaves your blood slowly, and an evening cup works all night — even if you fell asleep just fine.

Half-life: six hours

Six hours after a cup, half the caffeine is still in you. Six hours after that, a quarter. Coffee at four in the afternoon is about a quarter of a cup at midnight and an eighth towards morning. Not much? Enough to change the structure of your night.

What exactly it breaks

Falling asleep with caffeine usually works, especially when you are tired. What suffers is not falling asleep but depth: there is less deep sleep across the night, and deep sleep is what handles physical repair. Hence the familiar “I slept eight hours and I am still wrecked”. In the morning it shows up in your recovery and resting heart rate.

Where your time comes from

We count eight hours back from your bedtime today — the one the “When to turn in” widget works out. So the deadline is not fixed: on a day when you came up short on sleep and go to bed earlier, your last coffee moves earlier too.

Everyone metabolises it differently

Caffeine is broken down by the liver, and the speed is largely genetic: some people clear it twice as fast as average, some twice as slow. Eight hours is a cautious margin that suits most people. If you fall asleep confidently after an evening espresso, do not look at falling asleep — look at the share of deep sleep on nights like that. It answers more honestly.

What counts as coffee

Not only coffee. There is caffeine in black and green tea (two to three times less per cup), in energy drinks, cola, dark chocolate and in pre-workout — where there is often more of it than in a double espresso.

What to do after the deadline

Water, herbal tea, decaf. If it is the ritual you want, keep the cup but drop the caffeine: the habit works either way.