Caffeine Calculator
Caffeine takes hours to clear your system. Check how much of your afternoon coffee will still be active at bedtime.
Still in your system at bedtime
34 mg
Moderate. May lighten sleep for caffeine-sensitive people.
Uses an average caffeine half-life of 5 hours. Genetics, pregnancy, some medications, and smoking can make your actual half-life much shorter or longer.
How caffeine clearance works
Caffeine has an average half-life of about 5 hours: every 5 hours, the amount in your body drops by half. A 95 mg coffee at 3 p.m. leaves roughly 33 mg circulating at 10:30 p.m., enough to lighten sleep in sensitive people. Caffeine blocks adenosine, the brain's sleepiness signal, so leftover caffeine at bedtime delays sleep onset and reduces deep sleep even when you do fall asleep.
Practical rules of thumb
Keeping your last caffeinated drink at least 8 hours before bed protects most people's sleep, and the FDA considers up to 400 mg per day safe for most healthy adults. If you sleep poorly despite an early cutoff, you may be a slow metabolizer; try moving the cutoff earlier or switching the afternoon cup to decaf, which has only 2 to 5 mg.
Half-life varies widely with genetics, pregnancy, smoking, and some medications, so your actual clearance may differ. This is general information, not medical advice.
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