Cola before bed: 33 mg of caffeine and how long it lasts

A 12 fl oz (355 ml) can serving of cola has roughly 33 mg of caffeine (reported range 30–40 mg). That makes it a light source: 8% of the FDA's 400 mg daily guideline in one serving.

How long cola stays in your system

Caffeine leaves the body by halves: five hours after a serving about 17 mg remains, after ten about 8 mg. Crossing below 25 mg takes roughly 2 hours at the average rate.

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 cola 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 cola is a fraction of that dose, so the effect is likely smaller, but the direction is the same.