How long does 100 mg of caffeine last?
About 7 and a half hours. 100 mg of caffeine peaks near 91 mg 45 minutes after you take it, halves every 5.7 hours, and falls below the 40 mg sleep threshold about 7 and a half hours later.
| Peak level, 45 minutes in | 91 mg |
| Time to fall below 40 mg | 7 and a half hours |
| Left after 6 hours | 49 mg |
| Left after 12 hours | 24 mg |
| Latest hour for an 11 pm bedtime | 3:16 pm |
| Share of the 400 mg daily limit | 25% |
Hour by hour
| Time after the dose | If taken at 8 am | Still active | For sleep |
|---|---|---|---|
| 1 hour | 9 am | 90 mg | still working |
| 2 hours | 10 am | 80 mg | still working |
| 4 hours | 12 pm | 63 mg | still working |
| 6 hours | 2 pm | 49 mg | still working |
| 8 hours | 4 pm | 39 mg | clear |
| 10 hours | 6 pm | 30 mg | clear |
| 12 hours | 8 pm | 24 mg | clear |
| 16 hours | 12 am | 15 mg | clear |
| 24 hours | 8 am | 6 mg | clear |
What 100 mg looks like
A hundred milligrams is the working unit of caffeine: one mug of filter coffee, a moka pot serving, or a little more than a can of Red Bull. Most guidance is easier to read once you count in these.
It is also the dose at which timing starts to matter. The European Food Safety Authority notes that a single 100 mg dose can lengthen the time it takes to fall asleep in some adults when it is taken close to bedtime. Taken before 3:16 pm, it is out of the way by 11 pm.
One serving of these holds about 100 mg: filter coffee (96 mg), cortado (96 mg), moka pot (100 mg).
| Drink | One serving | To reach 100 mg |
|---|---|---|
| Filter coffee | 96 mg per 8 oz cup | 1 |
| Espresso | 64 mg per 1 oz shot | 1.6 |
| Black tea | 48 mg per 8 oz cup | 2.1 |
| Red Bull | 80 mg per 8.4 oz can | 1.2 |
| Coca-Cola | 32 mg per 11.2 oz can | 3.1 |
How late can you take it?
Count back 7 and a half 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 100 mg |
|---|---|
| 9 pm | 1:16 pm |
| 9:30 pm | 1:46 pm |
| 10 pm | 2:16 pm |
| 10:30 pm | 2:46 pm |
| 11 pm | 3:16 pm |
| 11:30 pm | 3:46 pm |
| midnight | 4:16 pm |
| 1 am | 5:16 pm |
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 | 5 and a half hours | 5:21 pm |
| Average | 5.7 hours | 7 and a half hours | 3:16 pm |
| Slow | 7.9 hours | 10 and a half hours | 12:25 pm |
Common questions
How long does 100 mg of caffeine last?
About 7 and a half hours for an average adult. 100 mg peaks near 91 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 100 mg of caffeine a lot?
No. 100 mg is about one mug of filter coffee and 25% of the 400 mg daily limit for a healthy adult.
How late can I take 100 mg of caffeine and still sleep?
For an 11 pm bedtime the latest is 3:16 pm. Move it by the same amount as your own bedtime. People who clear caffeine slowly need about 10 and a half hours, so they should stop earlier still.
How much of a 100 mg dose is left after 12 hours?
About 24 mg, for an average adult. After 6 hours about 49 mg is still active, and after 24 hours about 6 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.
