Your intake
Uses an average half-life of 5 hours. Individual clearance ranges from about 1.5 to 9.5 hours.
Half every five hours
Caffeine has a half-life of roughly five hours in a typical adult, so a 150 mg coffee still leaves about 75 mg after five hours and 37 mg after ten. An afternoon coffee is therefore still measurably present at midnight, whether or not you feel it.
Feeling nothing is not the same as clearing it
Tolerance blunts the sensation long before the caffeine leaves. Sleep studies find that caffeine taken six hours before bed still shortens total sleep and cuts deep sleep, even in people who report no difficulty falling asleep. What you notice and what it does are different questions.
Five hours is an average, not your number
Individual half-lives run from around 1.5 to 9.5 hours. Smoking speeds clearance up; pregnancy and some oral contraceptives slow it dramatically, sometimes doubling it. If caffeine seems to hit you unusually hard or long, the metabolism explanation is a real one.
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Frequently asked questions
Q. What is the 50 mg threshold?
A common rule of thumb for the level below which caffeine is unlikely to disturb sleep much. It is a rough marker rather than a clinical cutoff, and people sensitive to caffeine will notice less than that.
Q. How much caffeine is safe in a day?
Most health authorities put the figure for healthy adults at around 400 mg — roughly two to three brewed coffees. Lower limits apply in pregnancy, generally around 200 mg.
Q. Does decaf have none?
It has a little — commonly a few milligrams per cup rather than none at all. Enough to matter only if you are drinking a lot of it late.