Your caffeine cutoff.
Caffeine leaves the body slowly - about half is gone every five hours. Enter your bedtime and last drink, and we estimate the latest you should stop so most of it has cleared before you sleep.
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Method & sources
Caffeine clears by first-order decay: the amount left is dose × ½^(hours ÷ half-life). Using an average ~5-hour half-life (adjustable for smoking, oral contraceptives, or pregnancy, which are known to change it), we find the latest time for your last dose so that under ~50 mg remains by bedtime - a level low enough to be rest-friendly for most people. Controlled studies show even moderate caffeine taken 6 hours before bed can measurably disrupt sleep.
- Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. J Clin Sleep Med. 2013;9(11):1195–1200.
- Gardiner C, et al. The effect of caffeine on subsequent sleep: a systematic review and meta-analysis. Sleep Med Rev. 2023;69:101764.
- Institute of Medicine. Pharmacology of Caffeine (half-life ~5 h, range ~1.5–9.5 h; metabolism factors).
- FDA. Spilling the Beans: How Much Caffeine is Too Much? (~400 mg/day for healthy adults).
- ACOG. Moderate caffeine consumption during pregnancy (≤200 mg/day).
How caffeine actually works.
Caffeine has a "half-life" of roughly five hours in most adults - the time it takes for half of a dose to clear. So a 200 mg coffee at 3 pm leaves about 100 mg in your system at 8 pm, 50 mg at 1 am, and 25 mg at 6 am. The cutoff time here is simply when the amount left by bedtime should be low enough not to get in the way of sleep.
Why the five hours is only an average
Real half-life ranges from roughly 1.5 to 9.5 hours depending on the person. A few things move it a lot:
- Genetics - the CYP1A2 liver enzyme does most of the work, and there are common fast and slow versions.
- Smoking - speeds clearance, roughly halving the half-life.
- Oral contraceptives / estrogen - can roughly double it.
- Pregnancy - slows it dramatically, up to ~15 hours in late pregnancy.
- Age and liver health - clearance slows gradually with both.
Why afternoon caffeine hurts sleep
Even when you fall asleep on time, leftover caffeine can reduce deep (slow-wave) sleep - you may log the same hours but wake less rested. In a controlled study, 400 mg taken six hours before bed still measurably worsened objective sleep. As a rough rule, the bigger the dose, the earlier the cutoff.
What this tool isn't
The cutoff is a reasonable estimate, not a prescription, and it can't measure your actual caffeine levels. If you're sensitive, you may need to stop earlier than the number suggests. Persistent sleep problems are worth discussing with a doctor - no calculator replaces a sleep specialist.