How long does caffeine actually stay in your system?

Longer than most people assume. The half-life averages around five to six hours in healthy adults, meaning an afternoon coffee at 4pm still has roughly a quarter of its caffeine active at midnight. Individual variation is large and genetic — some people clear it in three hours, others take nine, which explains why some can drink coffee at night and others cannot. It does not necessarily prevent you falling asleep, but it measurably reduces deep sleep even when you do. A practical rule is no caffeine within eight hours of bed, and note that chai counts.