Caffeine Half-Life Calculator
That 4 PM espresso is half-gone by 10 PM — and half is plenty to fragment deep sleep. See your caffeine decay curve hour by hour.
How to use this calculator
- Enter your dose — tap a preset drink or type the milligrams.
- Say how long ago you drank it, and pick the half-life closest to you.
- Read what is left now, and the timeline until it drops below sleep-relevant levels.
How caffeine decays
Caffeine is eliminated by first-order kinetics — a constant fraction per unit time, not a fixed amount:
remaining = dose × 0.5 ^ (hours ÷ half-life)In healthy adults the half-life averages about 5–6 hours, with a wide 1.5–9.5 h range set mostly by the CYP1A2 liver gene. Smoking roughly halves it; oral contraceptives roughly double it; pregnancy stretches it to 15+ hours by the third trimester.
Why the afternoon dose matters
Sleep research (e.g. Drake et al., 2013) found caffeine taken 6 hours before bed still cost about an hour of objective sleep — even when people could not subjectively feel it. A practical rule: the number at your bedtime matters more than the number at your desk. As a rough marker, effects on sleep become small below about 50 mg remaining.
| Common sources | Typical caffeine |
|---|---|
| Brewed coffee (8 oz / 240 ml) | ~95 mg |
| Espresso (single shot) | ~65 mg |
| Energy drink (250 ml can) | ~80 mg |
| Black tea (8 oz) | ~47 mg |
| Cola (330 ml) | ~34 mg |
Frequently asked questions
How much caffeine is safe per day?
The US FDA and EFSA put healthy-adult guidance at 400 mg/day, and 200 mg per single dose. Pregnant people are advised to stay under 200 mg/day total.
Why does coffee affect me more than my friend?
CYP1A2 genetics create fast and slow metabolisers, and that difference is real: slow metabolisers feel effects longer and may get more sleep disruption from the same cup.
Does caffeine affect sleep even if I fall asleep fine?
Yes — studies show reduced deep-sleep time and more night wakings from afternoon caffeine even in people who report sleeping normally. Falling asleep is not the same as sleeping deep.
Does tolerance change the half-life?
Tolerance blunts the subjective jolt but elimination speed stays roughly the same. Your sleep can still be taxed by a dose you no longer feel.