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Caffeine feels like it wears off in a few hours — but its half-life is about five, meaning that 3pm coffee is still half a coffee in your system at 8pm and a quarter of it at 1am. Even that 'leftover' measurably lightens deep sleep. This calculator works out precisely how much of your dose remains at bedtime and gives you the cutoff time that genuinely protects your night.
Caffeine's half-life is about 5 hours — this calculator shows how much is still in your system at bedtime and the cutoff time that actually protects your sleep.
Caffeine's half-life is roughly 5 hours, so half is gone after 5h, three-quarters after 10h — but sensitivity varies 2-5x between people.
For most people a 3pm coffee still leaves meaningful caffeine at midnight. If you sleep lightly or wake often, treat noon as your cutoff.
Under 400mg a day is the common guideline (about 4 small coffees), but timing matters as much as the total for sleep.
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