How caffeine clears from your body
Caffeine is eliminated through first-order kinetics: a fixed fraction leaves per unit time, so the amount remaining follows exponential decay. The half-life is the time for half the dose to be cleared. For a healthy adult it averages about 5 hours, though it ranges roughly 3–7 hours.
Because it halves each interval, caffeine lingers longer than people expect: after one half-life half remains, after two a quarter, after three an eighth. Five half-lives (about a day, for most people) is the usual rule for "effectively cleared".
Worked example
You drink a coffee with 95 mg of caffeine at 3 pm. How much is left at 9 pm (6 hours later), assuming a 5-hour half-life?
Caffeine in common drinks
Doses vary widely by brew and serving size; these are typical figures (a useful cross-check is the US FDA's note that a regular 8 oz coffee is about 80–100 mg):
| Drink | Serving | Caffeine (typical) |
|---|---|---|
| Brewed coffee | 8 oz (240 ml) | 80–100 mg |
| Espresso | 1 shot (30 ml) | 60–75 mg |
| Black tea | 8 oz | 40–70 mg |
| Green tea | 8 oz | 25–45 mg |
| Cola | 12 oz (355 ml) | 30–40 mg |
| Energy drink | 8–16 oz | 70–160 mg |
Caffeine, sleep and what changes your half-life
The worked example shows the problem with a late coffee: caffeine still circulating at bedtime delays sleep onset and reduces deep sleep, even if you fall asleep fine. Sleep researchers often suggest stopping caffeine 6–8 hours before bed for this reason.
Your personal half-life isn't fixed — adjust the slider toward what fits you:
- Faster clearance (shorter half-life): regular smoking can roughly halve caffeine's half-life.
- Slower clearance (longer half-life): pregnancy can extend it to many hours, and some medications (certain antibiotics, oral contraceptives) slow caffeine metabolism. Liver function and genetics matter too.