How long does 400 mg of caffeine last?
About 19 hours. 400 mg of caffeine peaks near 365 mg 45 minutes after you take it, halves every 5.7 hours, and falls below the 40 mg sleep threshold about 19 hours later.
| Peak level, 45 minutes in | 365 mg |
| Time to fall below 40 mg | 19 hours |
| Left after 6 hours | 198 mg |
| Left after 12 hours | 95 mg |
| Latest hour for an 11 pm bedtime | 3:52 am the night before |
| Share of the 400 mg daily limit | 100% |
Hour by hour
| Time after the dose | If taken at 8 am | Still active | For sleep |
|---|---|---|---|
| 1 hour | 9 am | 360 mg | still working |
| 2 hours | 10 am | 321 mg | still working |
| 4 hours | 12 pm | 252 mg | still working |
| 6 hours | 2 pm | 198 mg | still working |
| 8 hours | 4 pm | 155 mg | still working |
| 10 hours | 6 pm | 121 mg | still working |
| 12 hours | 8 pm | 95 mg | still working |
| 16 hours | 12 am | 59 mg | still working |
| 24 hours | 8 am | 22 mg | clear |
What 400 mg looks like
400 mg is the daily limit for a healthy adult, the figure used by both the European Food Safety Authority and the US Food and Drug Administration. It is about four mugs of filter coffee, two cans of Celsius, or two and a half cans of Monster.
The limit is meant as a day's total, not a single dose. Taken at once, 400 mg peaks near 365 mg and takes about 19 hours to clear: you would have to take it before 3:52 am the night before to be under 40 mg by 11 pm. Spread from 7 am to noon, the same 400 mg behaves far better.
| Drink | One serving | To reach 400 mg |
|---|---|---|
| Filter coffee | 96 mg per 8 oz cup | 4.2 |
| Espresso | 64 mg per 1 oz shot | 6.2 |
| Black tea | 48 mg per 8 oz cup | 8.3 |
| Red Bull | 80 mg per 8.4 oz can | 5 |
| Coca-Cola | 32 mg per 11.2 oz can | 12.5 |
How late can you take it?
Count back 19 hours from the time you go to bed. Each bedtime below has its own page with the cutoff for every drink.
| Bedtime | Latest hour for 400 mg |
|---|---|
| 9 pm | 1:52 am the night before |
| 9:30 pm | 2:22 am the night before |
| 10 pm | 2:52 am the night before |
| 10:30 pm | 3:22 am the night before |
| 11 pm | 3:52 am the night before |
| 11:30 pm | 4:22 am the night before |
| midnight | 4:52 am the night before |
| 1 am | 5:52 am |
If you clear caffeine faster or slower
Half-life runs from about four hours in fast metabolisers to eight or more in slow ones, and the difference compounds over a day.
| Metabolism | Half-life | Clears in | Latest for 11 pm |
|---|---|---|---|
| Fast | 4.1 hours | 14 hours | 9:09 am |
| Average | 5.7 hours | 19 hours | 3:52 am the night before |
| Slow | 7.9 hours | 26 and a half hours | the day before |
Common questions
How long does 400 mg of caffeine last?
About 19 hours for an average adult. 400 mg peaks near 365 mg active 45 minutes after you take it, then halves every 5.7 hours until it falls below 40 mg, the line Cira uses as low enough to sleep on.
Is 400 mg of caffeine a lot?
As a single dose, yes. 400 mg is above the 200 mg that the European Food Safety Authority considers safe to take at once. As a day's total it is exactly the 400 mg limit for a healthy adult.
How late can I take 400 mg of caffeine and still sleep?
For an 11 pm bedtime the latest is 3:52 am the night before. Move it by the same amount as your own bedtime. People who clear caffeine slowly need about 26 and a half hours, so they should stop earlier still.
How much of a 400 mg dose is left after 12 hours?
About 95 mg, for an average adult. After 6 hours about 198 mg is still active, and after 24 hours about 22 mg.
Other doses
Look it up another way
Where these numbers come from
Every figure is calculated with the model the Cira app uses: caffeine peaks 45 minutes after you take it and halves every 5.7 hours, a typical value for healthy adults. The 40 mg sleep line is a working threshold of the model, and the 400 mg daily limit is the figure used by the European Food Safety Authority and the US FDA. These are averages. Smoking, pregnancy, the contraceptive pill and some medicines change how fast you clear caffeine. How the numbers are worked out.
