The half-life, simply explained
Caffeine is broken down by your liver, and the standard way to describe that is its half-life: the time it takes for half of it to be removed. For a healthy adult, that is usually 5–6 hours.
That works out roughly like this for a 200 mg cup:
- 5–6 hours later: about 100 mg left
- 10–12 hours later: about 50 mg left
- 15–18 hours later: about 25 mg left
The practical point: the caffeine from an afternoon coffee is still in your system when you go to bed, even if you no longer feel "buzzed".
Why it can still keep you up
Caffeine blocks the sleep-promoting chemical adenosine. Even a small amount of leftover caffeine can delay sleep onset, shorten deep sleep, and make you wake more tired. Some people are much more sensitive than others.
What changes the half-life
- Genetics — some people clear caffeine fast, others slowly.
- Regular use — tolerance can change how it affects you.
- Body weight and size.
- Some medications and pregnancy slow caffeine clearance.
Two common mistakes
The first mistake is assuming that because you do not feel a buzz, the caffeine is gone. Caffeine works below the level you notice, especially after regular use builds a tolerance. You can be calm and alert-looking while still carrying enough caffeine to lighten your sleep.
The second mistake is comparing yourself to a friend. Two people of the same size can clear caffeine at very different speeds because of genetics. Some people feel a late coffee as a restless night; others sleep fine. Your own pattern is the only one that matters.
How to find your own half-life
You do not need a lab test. A simple home experiment gives you a useful answer: pick a few evenings and stop caffeine progressively earlier — noon, then 10 am, then none — and keep everything else the same. Note how quickly you fall asleep and how rested you feel. Most people find their cutoff within a week of testing.
If you want a faster answer, the half-life calculator uses the standard 5-6 hour range, which fits most adults closely enough for planning.
Use the calculator to see when your caffeine wears off, then plan the latest time you can drink without hurting your sleep.