Caffeine Half-Life Calculator

Caffeine has a biological half-life of about 5 hours in healthy adults — the time it takes for the body to clear half the dose. After 5 hours, half remains; after 10, a quarter; after 20, around 6%. Smokers metabolize faster (3 hours); pregnancy and oral contraceptives slow it (8 to 15 hours).

Given dose, consumption time, and (optionally) a personal half-life, the calculator projects circulating caffeine across the day and flags how much will still be in your system at bedtime — a common cause of poor sleep that drinkers don't connect to afternoon coffee.

The pharmacokinetics follow a simple exponential decay model: amount(t) = dose × 0.5^(t/half_life). Caffeine is metabolized primarily in the liver by the CYP1A2 enzyme. Genetic variation in CYP1A2 produces a wide range of personal half-lives — fast metabolizers clear in 2 to 4 hours, slow metabolizers in 8 to 10. Smoking induces CYP1A2 expression, roughly doubling the rate. Estrogen and pregnancy slow it. Some medications (fluvoxamine, ciprofloxacin) inhibit CYP1A2 substantially. The model assumes single-compartment kinetics, which is close enough to reality at typical doses; at very high doses, absorption rate becomes nonlinear and the model overstates peaks.

A worked example. Two cups of coffee (200 mg total) at 9 AM. With a 5-hour half-life: at 9 AM, 200 mg circulating; at 2 PM, 100 mg; at 7 PM, 50 mg; at midnight, 25 mg. With a 7-hour half-life (slow metabolizer): at 2 PM, 137 mg; at 7 PM, 99 mg; at midnight, 71 mg — still nearly half the original dose at bedtime. Add an afternoon coffee at 2 PM (another 100 mg) and at midnight a slow metabolizer is sitting on 121 mg, more than the morning dose. Sleep researchers consistently find that caffeine within 6 hours of bedtime measurably reduces total sleep time and slow-wave sleep, even when the drinker doesn't notice.

Limitations and edge cases worth noting. The model assumes all caffeine is absorbed at t=0, which exaggerates the early curve — actual absorption peaks 30 to 60 minutes after consumption. Cold brew has 1.5 to 2 times the caffeine of drip per equivalent volume. Tea caffeine effects feel different because L-theanine modulates the rise, not because the caffeine clears faster. Decaf is not zero — typical 8 oz cups contain 2 to 15 mg, negligible for most people but noticeable for slow metabolizers consuming many cups. The model treats every dose as additive, which is correct mechanically but ignores tolerance: regular drinkers metabolize more caffeine without subjective effect, but the chemical curve is the same. The actual sleep-disruption effect is more about peak circulating amount near bedtime than total daily intake.