How the calculator works
Caffeine leaves your body gradually. Each half-life, the amount still in your blood drops by half. In healthy adults the half-life is about 5 hours, with a normal range of roughly 2 to 8 hours. So a coffee at 3 p.m. still has a meaningful share of its caffeine working at 10 p.m.
The calculator takes the caffeine in your drink, applies that decay, and finds the latest time you could finish it and still be down to about 25 mg at bedtime. It never suggests a time later than 6 hours before bed.
| Assumption | Value we use | Where it comes from |
|---|---|---|
| Half-life | 5 hours by default (choose 3 to 8) | CDC Yellow Book; EFSA |
| Bedtime target | About 25 mg left | Matches the cutoffs in a 2023 meta-analysis: coffee (107 mg) at least 8.8 hours before bed |
| Minimum gap | 6 hours | In a 2013 sleep-lab study, 400 mg taken 6 hours before bed still cut sleep by about an hour |
| Caffeine per drink | Typical values (coffee 96 mg per 8 oz) | Mayo Clinic caffeine table and USDA FoodData Central; the FDA notes real drinks vary widely, so check the label |
Why the cutoff is earlier than you might expect
The common advice is to stop caffeine 6 hours before bed. That came from the Drake study, which found a large dose still disrupted sleep at 6 hours. A later review of 24 studies estimated that a standard cup of coffee should be finished at least 8.8 hours before bed, and a large pre-workout dose more than 13 hours before. Caffeine can also make sleep lighter without you noticing, so “I sleep fine after an evening coffee” isn’t a reliable test.
Things that change your half-life
- Pregnancy slows caffeine clearance a lot, to as long as 16 hours late in pregnancy.
- Oral contraceptives can double it.
- Smoking speeds it up.
- Liver disease and some medications can slow it considerably. Ask your pharmacist if you’re unsure.
The FDA says that for most adults, 400 mg a day is an amount not generally associated with negative effects. That’s a daily safety ceiling, not a sleep target: timing matters as much as the total.
Want the rest of your evening planned? Use the sleep calculator to pick a bedtime first, then come back here.