The short answer
Stop drinking coffee about 8–10 hours before bed for the safest sleep, and treat 6 hours as the bare minimum. A well-known study found that caffeine taken even 6 hours before bedtime significantly reduced total sleep time — often without the drinker noticing. If you go to bed at 11 p.m., that means your last coffee is ideally by 1–3 p.m.
The reason is simple math: caffeine's long half-life means an afternoon cup is still partly active at bedtime.
Why a mid-afternoon cutoff works
Caffeine has a half-life of about 5 hours (see how long caffeine lasts). So a 200 mg afternoon coffee at 3 p.m. still leaves roughly 100 mg in your system at 8 p.m. and ~50 mg at 1 a.m.
Caffeine keeps you awake by blocking adenosine, the molecule that builds up during the day and makes you sleepy. Even a modest amount at bedtime can blunt that signal, reduce deep sleep, and delay how long it takes to fall asleep — even if you *feel* tired enough to drop off.
Find your personal cutoff
Your ideal curfew depends on your bedtime, how much you drink, and how fast you metabolize caffeine. To dial it in:
• Use the math. Enter your dose and bedtime into the caffeine half-life calculator to see how much caffeine will still be active when you turn in.
• Count the real total. A large café coffee can be 200–300 mg, not 95 — check yours with the caffeine intake calculator.
• Adjust for sensitivity. Slow metabolizers, pregnant people, and the caffeine-sensitive should push the cutoff earlier.
If you slip past your cutoff
Had a late coffee? A few things help: switch to decaf or half-caff after your cutoff, keep the evening dose small, and give yourself a longer wind-down. Over days, moving your last cup earlier is one of the highest-impact changes you can make for sleep quality — no other habit is required.