That 2 p.m. coffee isn't gone by dinner. Caffeine leaves your body on a predictable schedule set by its half-life — the most useful thing you can know about your daily habit.
What “half-life” means
Half-life is the time your body needs to clear half of a dose. In a healthy adult it averages about 5–6 hours, but ranges from roughly 1.5 to 9.5 hours depending on genes, age, liver and medications.
Drink 200 mg (a large coffee) at 9 a.m. and you still have about 100 mg circulating by mid-afternoon. It takes roughly 5–6 half-lives — about a full day for caffeine to be effectively gone.
Why your number is personal
- Pregnancy & oral contraceptives slow it down — in late pregnancy the half-life can reach 15 hours.
- Smoking speeds it up, roughly halving the half-life.
- Genetics (CYP1A2) make some people fast metabolizers and others slow.
This is what Caffy models. Instead of a rule of thumb, it uses your half-life to show how much caffeine is active in your body right now.