What a five-hour half-life means for an evening
Caffeine is eliminated with a half-life that averages around five hours in healthy adults, meaning roughly half of a dose remains in the bloodstream after each five-hour interval. A 200 mg dose taken at noon would, on this average, leave about 100 mg by 5 pm, 50 mg by 10 pm, and a residue still present near midnight. This slow tail is why afternoon coffee can disturb sleep even when a person does not feel wired.
The half-life is an average, not a fixed number; it can roughly double in pregnancy or with certain medications and shorten in heavy smokers.
Reading the decay curve for your own timing
Because elimination is exponential rather than abrupt, the practical takeaway is that timing matters as much as total amount, and individual clearance varies widely with genetics, liver enzymes, and other drugs. People sensitive to caffeine often benefit from an earlier daily cutoff.
This is general educational information, not medical advice. Those who are pregnant, on medication that affects liver enzymes, or managing a heart condition should consult a qualified professional about caffeine.