What is caffeine half-life?
Half-life is how long your body takes to remove half of a substance. For caffeine, the average adult's half-life is about 5 hours — drink 200 mg at 3 PM, and you still have ~100 mg at 8 PM, ~50 mg at 1 AM. The 5-hour figure varies widely: smokers metabolize caffeine about twice as fast (~3 h), slow metabolizers and oral-contraceptive users ~7 h, and pregnancy slows it to ~11 h.
The 50 mg sleep threshold
Research on sleep and caffeine suggests that even ~50 mg of caffeine in your system near bedtime can reduce sleep quality — it lightens deep sleep even if you fall asleep fine. That's roughly: coffee at 3 PM with a 5-hour half-life → 60 mg left at 11 PM. For an 11 PM bedtime, the last coffee time is usually 2–4 PM for average metabolizers, later if you're fast, earlier if you're slow.
How the calculator works
- Add your drinks with their times (defaults: coffee 8 AM, tea 2 PM).
- Caffeine remaining = dose × 0.5^(hours since drink ÷ half-life), summed across all drinks.
- The result shows total caffeine at bedtime, how much will remain at midnight, and whether you're above the ~50 mg sleep threshold.
- This is an estimate — individual genetics, liver function and other drugs change the numbers. When in doubt, stop caffeine earlier.