Caffeine's grip on your night is governed by one number most people have never computed: the half-life. Drink 200 mg at 3 pm with a 5-hour half-life and at 11 pm — lights out — you still have 100 mg circulating. That is not "a little caffeine in my system"; that is a full espresso shot actively blocking the adenosine receptors your brain uses to build sleep pressure.
The exponential, in plain terms
Level = dose × 2^(−hours since ÷ half-life)
Every half-life halves what remains. The table makes the 3 pm coffee concrete:
| Time | Hours later | mg remaining (t½ = 5 h) |
|---|---|---|
| 3 pm | 0 | 200 |
| 6 pm | 3 | 126 |
| 8 pm | 5 | 100 |
| 11 pm | 8 | 50 |
| 3 am | 12 | 25 |
The lesson hiding in the table: caffeine never really leaves — it dilutes. The question is only at what point the remainder drops below your personal threshold for architectural damage.
Why half-lives range from 2 to 10 hours
- Genetics: CYP1A2 variants make fast vs slow metabolizers; slow metabolizers carry both higher cardiovascular reactivity and stronger sleep disruption.
- Smoking roughly halves the half-life (induced metabolism); pregnancy can triple it — one reason obstetric caffeine limits tighten.
- Oral contraceptives, some antibiotics, liver disease all extend it.
- Tolerance changes the felt effect more than the pharmacology — habitual drinkers feel less buzz but still lose deep sleep.
What caffeine does to a night you "slept through"
Controlled-dosing studies (the Drake et al. work is the classic) found that 400 mg taken six hours before bed reduced total sleep by more than an hour by objective measures — while subjective sleepiness was barely different. Caffeine's signature is quiet: thinner deep sleep, more micro-fragments, a morning that needs more coffee. The loop closes on itself until someone breaks it with arithmetic.
Finding your curfew
- Enter your bedtime and last dose in the curfew calculator; start with a 5-hour half-life.
- Target ≤50 mg at bedtime (≤25 if you are a self-identified sensitive sleeper).
- Run it for a week and watch the second half of the night — that is where the difference shows.
- Adjust the half-life field to your observations, not to the average.
The goal was never zero caffeine — it is caffeine on your schedule instead of in your bloodstream on nobody's. The math takes two minutes and pays out every morning.