Caffeine Half-Life: How Much Is Still in Your System at Bedtime?
October 29, 2026
Caffeine is cleared from the body at a rate proportional to the amount present, so it decays exponentially. The time for the amount to fall by half is the half-life, about 5 hours in healthy adults, although it ranges from about 2 to 10 hours. The amount remaining after t hours is: remaining = initial × 0.5^(t ÷ half-life). A 200 mg dose (about two cups of coffee) at 2 p.m. leaves 200 × 0.5^(8÷5) = 66 mg at 10 p.m. and 50 mg at midnight.
Even a quarter of the original dose can affect sleep in sensitive people. Studies have found that caffeine taken six hours before bed reduced total sleep time by more than an hour.
Skip the math and get your answer instantly:
Open the Water Intake Calculator →Decay Table
The table shows the caffeine remaining from a 200 mg dose at a 5-hour half-life. Plot it and you will see that the amount falls quickly at first and then slowly. After 10 hours, a quarter of the dose remains, and after 15 hours, one eighth. The remaining caffeine from several doses adds up, so the evening level depends on every cup during the day.
A morning pair of cups at 8 a.m. (400 mg total) leaves about 100 mg at 6 p.m. and 50 mg at 11 p.m.
| Hours after dose | Remaining (5 h half-life) | Remaining (3 h half-life) | Remaining (8 h half-life) |
|---|---|---|---|
| 0 | 200 mg | 200 mg | 200 mg |
| 4 | 115 mg | 79 mg | 141 mg |
| 8 | 66 mg | 31 mg | 100 mg |
| 10 | 50 mg | 20 mg | 84 mg |
| 12 | 38 mg | 12 mg | 71 mg |
| 16 | 22 mg | 6 mg | 50 mg |
Why Half-Life Varies
Caffeine is metabolized in the liver by the enzyme CYP1A2, and its activity differs between people because of genes and other factors. Smokers clear caffeine faster (a half-life of about 3 hours); oral contraceptives roughly double the half-life; pregnancy extends it to 9 to 11 hours in the third trimester; liver disease can lengthen it dramatically. Certain medications, such as some antibiotics and antidepressants, slow clearance, and grapefruit juice may have an effect as well. If you are sensitive to caffeine or take medications, allow more time before bed.
Practical Cutoff Times
A common rule is to avoid caffeine for at least 8 hours before bed. For a 10:30 p.m. bedtime, that means a last cup by 2:30 p.m. People with a longer half-life or sleep problems may need a cutoff around noon. To keep the remaining caffeine under about 25 mg at bedtime with a 5-hour half-life, a 200 mg dose should be taken at least 15 hours earlier, in the early morning, while a 100 mg cup could be taken 10 hours before bed. Switching afternoon drinks to decaf (which still contains 2 to 15 mg per cup) or herbal tea helps.
Remember hidden sources: tea, chocolate, cola, pre-workout supplements, and some pain relievers.
Caffeine and Sleep Quality
Even if you fall asleep easily, caffeine in the evening reduces the amount of deep sleep and fragments rest. You may feel you are tolerating it, yet sleep tracking may show shorter sleep. Tolerance develops to the alerting effect but is incomplete for the sleep effect. If you are tired and rely on caffeine to get through the day, review your sleep habits first. When cutting down, taper gradually to avoid withdrawal headaches, which can last several days.
Frequently Asked Questions
What is the half-life of caffeine?
About 5 hours in healthy adults, with a range of roughly 2 to 10 hours.
How much caffeine is left after 10 hours?
About a quarter of the original dose with a 5-hour half-life.
When should I have my last coffee?
At least 8 hours before bed is a common guideline, earlier if you are sensitive.
Does decaf have caffeine?
A small amount, typically 2 to 15 mg per cup.
Why do smokers clear caffeine faster?
Smoking induces the liver enzyme that metabolizes caffeine.