Caffeine clears from your blood with a half life near five hours, so an afternoon coffee is still partly on duty at midnight. Enter the dose and the gap to bedtime to see what remains when your head hits the pillow.
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The five hour clock in your cup
Caffeine blocks adenosine, the chemical pressure that builds through the day and makes you sleepy. Your liver dismantles it at a rate that halves the blood level roughly every five hours in a typical adult. Two hundred milligrams from a generous morning coffee becomes one hundred by mid afternoon, fifty by evening, and a stubborn twenty five near midnight. The blockade weakens as levels fall, but sensitive sleepers feel the residue long after the buzz fades.
Why your half life is not everyone else
The five hour figure is a population average with wide honest scatter. Pregnancy stretches it to ten hours or more. Oral contraceptives slow clearance meaningfully. Smokers clear caffeine nearly twice as fast. Genetics at the CYP1A2 enzyme split people into fast and slow metabolizers, which is why one friend drinks espresso after dinner and sleeps fine while another regrets a two oclock latte at two in the morning. Your own sleep is the only meter that matters.
What residue does to the night
Even caffeine that no longer feels stimulating trims deep slow wave sleep, the stage that does physical restoration. Studies giving caffeine six hours before bed measured an hour of lost sleep that subjects never noticed consciously. The damage shows up in next day sleepiness rather than midnight wakefulness, which makes the connection easy to miss and the habit easy to keep.
Finding your personal cutoff
Work backward from bedtime with this calculator and set a daily caffeine curfew that leaves under about fifty milligrams circulating at lights out. For most people that lands between eight and ten hours before bed. Slow metabolizers and light sleepers do well to push it earlier still, and two weeks of honest trial will tell you more than any average can.