Caffeine half-life: how long does caffeine last?
Caffeine half-life is the time it takes for the amount in your body to fall by half. It takes several half-lives to remove most of a dose, so “the boost has worn off” and “the caffeine is gone” are different statements.
How long does caffeine stay in your system?
There is no exact duration that applies to everyone. Research documents substantial variation in elimination. The National Academies' pharmacology chapter describes the factors behind that variation. Our calculator uses five hours as an illustrative starting assumption that you can change; it is not a measurement of your metabolism.
A worked half-life example
For a hypothetical 200 mg dose and a five-hour half-life, the simplified calculation gives:
| Elapsed time | Estimated amount |
|---|---|
| 0 hours | 200 mg |
| 5 hours | 100 mg |
| 10 hours | 50 mg |
| 15 hours | 25 mg |
| 20 hours | 12.5 mg |
The model approaches zero gradually. A rounded display of 0 mg is not evidence that a biological sample would contain absolutely none.
The formula
remaining = dose × 0.5^(elapsed hours / half-life hours)
For several drinks, calculate each drink's remaining contribution separately and add them. A newer dose has had less time to decay. The half-life and bedtime calculator does this for multiple entries, including drinks taken on a previous day.
What happens when you change half-life?
For 100 mg taken ten hours before bedtime, a five-hour assumption gives 25 mg remaining. A ten-hour assumption gives 50 mg. Changing the assumption changes the estimate without changing the drink. This is why presenting one value as a precise personal prediction would be misleading.
Do not adjust a slider until it “proves” what you suspect about a bad night. A sleep diary alone cannot establish your biological half-life. Our methodology page sets out the calculator's limits, including its simplified treatment of absorption.
Half-life is not a sleep countdown
The same estimated amount does not produce the same effect in everyone. Nor does the calculation account for every reason someone may sleep poorly. Use it to explore dose and timing, then read the sleep evidence and bedtime timing guide.