Black tea before bed: 47 mg of caffeine and how long it lasts

Black tea delivers about 47 mg of caffeine per 8 fl oz (237 ml) — a light dose next to the 400 mg a day the FDA calls generally safe for healthy adults. Published values run from 25 to 48 mg depending on leaf and steeping time.

How long black tea stays in your system

With a 5-hour half-life, 24 mg is still in your system five hours after black tea, 12 mg after ten. It takes about 5 hours to fall under 25 mg — the illustrative threshold used on this site, not a medical one.

The half-life is an average. Between people it runs from about 1.5 to 9.5 hours: smokers clear caffeine faster; pregnancy, oral contraceptives and some medicines slow it. The curve above is a model of a typical adult, not a measurement of you.

Last black tea for common bedtimes

6-hour line from Drake et al. 2013 (400 mg at 6 h before bed cut sleep by >1 h) · 5-hour half-life · as of October 3, 2026

What the 6-hour rule is based on

One controlled trial (Drake and colleagues, 2013) gave 400 mg of caffeine — about four cups of brewed coffee — at bedtime, three hours before and six hours before, and measured sleep in a lab. Even the six-hour dose cut total sleep by more than an hour, and participants did not report feeling the difference. A single black tea is a fraction of that dose, so the effect is likely smaller, but the direction is the same.