Half-life is why the afternoon coffee matters
A five-hour half-life means that five hours after a 200 mg coffee you still have 100 mg on board — a full cup's worth. After ten hours, 50 mg. After fifteen, 25 mg. Caffeine does not switch off; it fades.
This is why a 3pm coffee is a sleep problem even for people who insist it is not. At 11pm you still have around a quarter of it circulating, and research consistently shows caffeine reduces slow-wave sleep even when it does not stop you falling asleep. You sleep, but worse — and you do not notice, which is precisely the trap.
Your half-life is not everybody's
Five hours is an average that hides enormous variation. Caffeine is metabolised mainly by the liver enzyme CYP1A2, and both genetics and circumstance change how fast it works:
- Smoking roughly halves the half-life — which is why people often feel over-caffeinated after quitting.
- Pregnancy can extend it to 15 hours or more in the third trimester.
- Oral contraceptives can roughly double it.
- Genetics. "Fast metabolisers" clear caffeine in 2–3 hours; slow metabolisers can take 8–10.
- Liver conditions and some medications, including certain antibiotics, slow clearance substantially.
If coffee at noon keeps you awake, you are probably a slow metaboliser. Set the half-life field to 8 and the picture will make a lot more sense.
How much is too much
The FDA and EFSA both put the general adult limit at around 400 mg a day — about four filter coffees. Pregnancy guidance is 200 mg. Adolescents are usually advised to stay under 100 mg.
| Drink | Serving | Caffeine |
|---|---|---|
| Filter coffee | 240 ml | 95 mg |
| Espresso | single shot | 63 mg |
| Instant coffee | 240 ml | 62 mg |
| Black tea | 240 ml | 47 mg |
| Green tea | 240 ml | 28 mg |
| Energy drink | 250 ml | 80 mg |
| Cola | 355 ml | 34 mg |
| Decaf coffee | 240 ml | 2–5 mg |
Tolerance to the alerting effect builds within days, but tolerance to the sleep disruption is much weaker — so regular drinkers often feel no buzz while still sleeping badly. A practical rule: nothing caffeinated within eight hours of bed.