Caffeine half-life, drawn
One 200 mg dose — a strong cold brew — plotted hour by hour for three metabolizer types. The dashed lines are bedtime limits. Where a curve crosses a line, that's the cutoff.
| Hours after dose | Fast · T½ 3 h | Average · T½ 5 h | Slow · T½ 8 h |
|---|---|---|---|
| 1 h | 159 mg | 174 mg | 183 mg |
| 2 h | 126 mg | 152 mg | 168 mg |
| 4 h | 79 mg | 115 mg | 141 mg |
| 6 h | 50 mg | 87 mg | 119 mg |
| 8 h | 31 mg | 66 mg | 100 mg |
| 10 h | 20 mg | 50 mg | 84 mg |
| 12 h | 12 mg | 38 mg | 71 mg |
| 16 h | 5 mg | 22 mg | 50 mg |
Questions this chart answers
Why does advice range from 6 to 13 hours?
Because nobody publishing the advice knows your rate or your cup. A 95 mg coffee taken under a 50 mg bedtime limit lasts 2.8 h if you're a fast metabolizer (T½ 3 h) and 7.4 h if you're slow (T½ 8 h); a 200 mg cold brew lasts 6 h fast and 16 h slow. Advice quotes one time because it can't ask you first.
How do I know my half-life?
Self-test: if one morning coffee still fends off sleep at midnight, assume slow. Smoking roughly halves clearance time; estrogen contraceptives nearly double it; third-trimester pregnancy can triple it. The cutoff calculator lets you set these.
What's the safe bedtime residual?
Research that gave 400 mg at 0, 3, and 6 hours before bed found about an hour of lost sleep even in the 6-hour group — roughly 100 mg still circulating. 50 mg is a common standard cutoff; 25 mg if you're sensitive.
Method
Curves: remaining = 200 mg × 0.5(t ÷ T½). Half-life range 1.5–9.5 h in healthy adults, mean ≈ 5 h. Thresholds 25 / 50 / 100 mg correspond to sensitive / standard / tolerant bedtime limits.
Get your own number: Caffeine cutoff calculator