Caffeine Cutoff
Caffeine has a ~5-hour half-life. If you go to bed at 11pm and have an espresso at 5pm, ~50% is still in your system at lights-out. Plan backward from when you want to sleep.
Latest caffeine time
Cutoff
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Half-lives to reach target
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Hours before bed
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Why caffeine wrecks sleep even when you can "fall asleep fine"
Caffeine blocks adenosine receptors. Adenosine is what builds up through the day and tells your brain it's time to sleep. Even if you don't feel wired, residual caffeine reduces deep sleep and REM. Studies show 200mg (≈ one big coffee) 6 hours before bed cuts total sleep by ~40 minutes and reduces sleep quality measurably.
The half-life math
Caffeine clears the body roughly first-order: half goes away every ~5 hours in a typical adult. So 200mg at 1pm leaves:
- 1pm → 200 mg
- 6pm (5h later) → 100 mg
- 11pm (10h, ≈ bedtime) → 50 mg
- 4am → 25 mg
The "acceptable residual" setting on this calculator lets you pick how much you want left at bedtime. 25% is a sensible default; 12.5% (3 half-lives) is stricter and pushes the cutoff much earlier.
Your half-life varies
- Genetics — CYP1A2 variants make some people "fast metabolizers" (clear in 3-4h) and others "slow" (8-10h). Slow metabolizers feel jittery from a single cup.
- Pregnancy — half-life roughly doubles in the third trimester (~10h).
- Smoking — induces CYP1A2, so smokers metabolize caffeine faster (~3h).
- Hormonal birth control — slows it down (~7-8h).
- Liver function — anything that hits the liver (alcohol, certain meds) slows caffeine clearance.
A useful rule of thumb
"Don't drink coffee after lunch" maps to roughly the 25% / 5h-half-life calculation for an 11pm bedtime. If you're a slow metabolizer or value deep sleep, push the cutoff to mid-morning.