Caffeine half-life in plain English
Short answer: about 5 hours for half, much longer to clear completely. Half-life varies a lot between people, from roughly 2 to 10 hours, depending on genetics, pregnancy, some medications and smoking.
| Time after drinking | Caffeine still active |
|---|---|
| 0 hours | about 95 mg |
| 5 hours | about 48 mg |
| 10 hours | about 24 mg |
| 15 hours | about 12 mg |
That's why a 3 PM coffee can still be working at 11 PM. The calculator starts from your bedtime and works backwards so the leftover amount is small when you lie down.
Common questions
How long does one cup of coffee keep you awake?
The alertness peaks about 30 to 60 minutes after drinking, but about half the caffeine is still in your body 5 hours later, and it can affect deep sleep even when you don't feel wired.
Is it bad to drink coffee 6 hours before bed?
Research has found that caffeine taken 6 hours before bed can still noticeably reduce sleep. Most people do better with an 8-hour gap.