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Caffeine has a half-life of about 5 hours, meaning roughly half of what you drank is still circulating 5 hours later and about a quarter remains after 10. Drink an 8oz cup of brewed coffee (95mg, USDA) at 3pm and something like 45-50mg is still working on you at 8pm, with a residue hanging on near midnight. You feel the peak within the first hour or so, but the tail is long, and the tail is what ruins sleep.
The 5-hour half-life, in practice
| Time after drinking | Roughly remaining | From a 95mg cup |
|---|---|---|
| 0 hours | all of it | 95mg |
| ~5 hours | about half | ~48mg |
| ~10 hours | about a quarter | ~24mg |
| ~15 hours | about an eighth | ~12mg |
Half-life varies from person to person; ~5 hours is a typical figure. The FDA considers up to 400mg per day generally safe for healthy adults.
Why the afternoon coffee hits harder than it should
Doses stack. A 2pm cold brew does not start from zero; it lands on top of whatever is left from the morning. If you had a Starbucks grande Pike Place at 8am (310mg, published menu value), around half of it is still aboard at 1pm, so a modest afternoon top-up puts your circulating level back near where the morning started. People blame the small afternoon cup for their 11pm wired feeling when the real culprit is the stack. Run your own schedule through the caffeine curfew calculator to see when your last cup should land.
What changes the speed
The ~5 hour figure is an average, not a law. Genetics move it a lot: fast metabolizers clear caffeine noticeably quicker, slow metabolizers can feel a lunch coffee at bedtime. Pregnancy and some medications slow clearance substantially, while smoking speeds it up. Sensitivity is a separate dial from speed, which is why one person sleeps fine after espresso with dessert and another cannot touch caffeine past 10am. You cannot change your half-life; you can only change your dose and your timing.
Working with the tail, not against it
The practical move is a personal caffeine curfew: count back from bedtime and put the last full-strength cup 8-10 hours before it, then switch to decaf, which carries only 2-15mg per 8oz. Keeping decent decaf beans around makes the curfew painless because the ritual survives even when the dose stops. How much you drink in total matters too; that budget question is covered in how much caffeine per day is safe.
Related reading
FAQ
How long does caffeine last in your system? Its half-life is about 5 hours: half of the dose remains after 5 hours, a quarter after 10. A morning cup leaves traces well into the evening.
How long before bed should I stop drinking caffeine? A common rule is 8-10 hours before bedtime for the last full-strength cup, since about a quarter of a dose can still be circulating 10 hours later.
Why does caffeine affect some people longer? Half-life varies with genetics, pregnancy, smoking, and some medications. Slow metabolizers can still feel a lunchtime coffee at bedtime.
Sources: USDA FoodData Central (brewed coffee, decaf); Starbucks published menu caffeine values; FDA guidance on caffeine. This page is information, not medical advice.
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