Longer than the buzz. Caffeine's half-life is about 4 to 6 hours, so if you drink a coffee at 3pm, roughly half of it is still in your bloodstream through the evening, and a meaningful share can still be there near midnight. That leftover fraction is enough to keep blocking adenosine, the molecule that builds your pressure to sleep, which is how an afternoon cup can quietly thin your deep sleep even when you drop off on time.
How fast you clear it varies more than most people expect. Slow metabolizers — some genetics, older adults, pregnancy, and people on certain medications like some antidepressants or oral contraceptives — can take noticeably longer, while smokers tend to clear it faster. There's no single number that fits everyone, which is exactly why when you take your last cup matters more than counting milligrams.
Rest keeps the rule simple: last cup by 11am, or, if you'd rather time it to your own schedule, at least 8 hours before you plan to sleep.
Full guide: Caffeine Cutoff: When to Stop for Better Sleep.
Trouble sleeping? See how Rest can help »