How late can you drink coffee?

Caffeine is metabolised by the CYP1A2 enzyme in the liver. Genetic variation in CYP1A2 splits the population into roughly three bands: fast metabolisers (low sensitivity, ~3-hour half-life), normal metabolisers (~5 hours), and slow metabolisers (high sensitivity, ~7 hours). Tolerance also shifts the curve — regular drinkers metabolise faster than abstainers.

The cutoff comes from the sleep research rather than a formula. In a controlled trial, a large dose (400mg) taken six hours before bed still cut sleep by more than an hour, so we stop earlier than six hours. The calculator stops caffeine eight hours before bed for normal sensitivity, ten for high, and six for low. The result page shows three reference times — morning coffee, post-lunch latte, late afternoon pick-me-up — so you can see how each one decays through the day.

Caveats. One, this is the average; your actual half-life could be 3 to 9 hours. Two, half-life slows in pregnancy, on oral contraceptives, and with age — and speeds up if you smoke. Three, caffeine sources matter: 200mg in a slow drip coffee absorbs differently than 200mg in an energy drink. The calculator treats all sources as equivalent for simplicity.

The cutoff is the latest clock time to have your last caffeine. It comes from the sleep research, not a decay formula: a large dose six hours before bed still cut sleep in a controlled trial, so the normal band stops eight hours out. High-sensitivity users get an earlier cutoff because their liver clears caffeine slowly, leaving more of it circulating at lights-out.

Two things quietly break the number: hidden sources and decaf. Tea, dark chocolate, pre-workout powders, some sodas, and a few pain relievers all carry caffeine, so count them toward the same daily total. Decaf is not caffeine-free either, at roughly 2 to 5 mg a cup, which is negligible for most people but not for the very sensitive.

In a controlled trial, Drake and colleagues gave people 400 mg of caffeine at 0, 3, or even 6 hours before bed. Caffeine taken a full 6 hours before bedtime still cut total sleep by more than an hour, and crucially, subjects often underestimated the disruption. Falling asleep is not the same as sleeping well.

The mechanism is adenosine. Adenosine builds up in the brain across the day and creates sleep pressure; caffeine blocks its receptors, masking that pressure. It also specifically suppresses slow-wave (deep) sleep, so even when you drop off on schedule the night runs lighter and less restorative than it feels.

Taper rather than quitting cold. Move your cutoff earlier by about an hour a week, and swap the afternoon cup for decaf or a herbal tea so the ritual survives while the dose drops. Abrupt withdrawal from a heavy habit brings 24 to 48 hours of headache and fog that most people blame on the wrong thing.

The real lever is total daily load plus timing, not a single rule. If you drink caffeine all day, an earlier cutoff helps but a lower overall amount helps more. Aim to keep the back half of your day light, and let this calculator set the hard stop.