Caffeine, Sleep Debt and Your Afternoon Coffee

The U.S. FDA cites 400 milligrams a day — about two to three 12-ounce cups of brewed coffee — as an amount not generally associated with negative effects in healthy adults, while noting that sensitivity varies. Per 12-ounce serving, it lists roughly 113–247 mg for brewed coffee, 71 mg for black tea, 37 mg for green tea, 23–83 mg for caffeinated soft drinks and 41–246 mg for energy drinks. Anyone pregnant, trying to become pregnant or breastfeeding should ask their health care provider about limits.

Caffeine’s half-life is commonly cited as 3–7 hours, with about 5 as typical. After one half-life half of it remains, after two a quarter. For one coffee of about 180 mg, finished at 8 a.m.:

By 10 p.m. (14 hours later) a typical 5-hour half-life leaves about 26 mg; a slow metaboliser with a 7-hour half-life still has about 45 mg. A coffee at 4 p.m. leaves far more at bedtime: about 78 mg for the typical case.

Say you need 8 hours and slept 6, 6, 7, 8, 5, 9 and 6 over a week — a debt of 9 hours. Short nights make you sleepier the next day, so you drink more caffeine, which can delay sleep onset and reduce deep sleep, creating the next short night.

The sleep debt calculator tallies this and the caffeine calculator shows how much is left at your bedtime.

Your own need within the range is what counts: if you wake unaided, alert and stay that way, you are probably close.

1. Set a caffeine cut-off 8–10 hours before bed; light sleepers may need more.
2. When tired in the afternoon, try a walk, daylight, water or a short nap (under 30 minutes) before more coffee.
3. Add 30–60 minutes of sleep per night rather than relying on weekend lie-ins.
4. Check labels on energy drinks, pre-workout powders and some medicines.
5. If you are always exhausted despite adequate sleep, talk to a doctor.