Caffeine half-life
How Long Does Caffeine Stay in Your Body?
Most people think caffeine wears off after they stop feeling alert. It doesn't. This guide covers how caffeine half-life works, what affects it, why an afternoon coffee can still impact sleep, and how long different drinks stay active.
Alertness fades before caffeine does
Feeling normal again isn't the same as caffeine being gone. The jittery, wired feeling peaks fast and drops well before the caffeine itself clears your system. Half of it can still be active hours after you feel done with it.
What half-life actually means
Caffeine's half-life is roughly 5 hours - the time it takes your body to clear half of what you drank. A 200 mg coffee at 2 PM leaves about 100 mg active at 7 PM, and about 50 mg still active at midnight. It doesn't drop to zero. It halves, over and over, until what's left is negligible.
What changes your half-life
5 hours is an average, not a fixed number. Your own clearance can run a lot faster or slower.
- Genetics: some people metabolize caffeine twice as fast as others - same cup, very different clearance.
- Pregnancy: half-life roughly doubles, sometimes more.
- Smoking: speeds clearance. Smokers often process caffeine faster than non-smokers.
- Certain medications: some birth control pills and antibiotics slow clearance significantly.
- Liver function: a slower liver means slower clearance, full stop.
Why an afternoon coffee still hits your sleep
A coffee at 4 PM feels fine by 9 PM. It isn't gone. At a 5-hour half-life, roughly half of it - call it 45 mg from a 90 mg cup - is still active at bedtime. That's enough to delay how fast you fall asleep and cut into deep sleep, even if you don't notice yourself lying awake. OnBoard tracks this against your actual bedtime, not a generic cutoff, so it warns you before the drink, not after a bad night's sleep.
How much caffeine is left, hour by hour
| Drink | At 0h | At 5h | At 10h |
|---|---|---|---|
| Brewed coffee | ~95 mg | ~47 mg | ~24 mg |
| Energy drink | ~80 mg | ~40 mg | ~20 mg |
| Espresso | ~63 mg | ~32 mg | ~16 mg |
| Black tea | ~47 mg | ~24 mg | ~12 mg |
| Cola | ~34 mg | ~17 mg | ~8 mg |
| Green tea | ~28 mg | ~14 mg | ~7 mg |
Frequently asked questions
Does tolerance change caffeine's half-life?
No. Tolerance changes how strong it feels, not how fast your body clears it.
Why does caffeine feel gone way before it actually is?
The alertness effect fades faster than the caffeine itself clears. Your body adapts to the stimulation before the substance leaves your system.
Does drinking water speed up caffeine clearance?
No. Caffeine clears through liver metabolism on its own timeline, not through hydration.
Is 5 hours the same for everyone?
No. Genetics, pregnancy, smoking, and certain medications can cut it in half or double it.
How long until caffeine is completely gone?
Never fully to zero on a clean line, but after about 4 to 5 half-lives - 20 to 25 hours - what's left is negligible.