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Caffeine Half-Life Tracker
Active caffeine level in the body from coffee/tea timing.
How it works
Caffeine does not vanish the moment a coffee wears off; it declines gradually, halving roughly every five to six hours in most adults. This tracker applies that exponential decay to your dose: after one half-life about half remains, after two about a quarter, and so on. The half-life used here, 5.7 hours, is a commonly cited adult average. It keeps the maths transparent, and it explains why an afternoon coffee can still be active at bedtime even when you no longer feel alert.
How to read the result
A 200 milligram coffee, roughly a large brewed coffee, leaves about 97 milligrams active after six hours and about 50 milligrams after eleven to twelve hours. That residual amount is enough to fragment sleep for sensitive people, even if they fall asleep easily. If sleep is a problem, work backwards from bedtime: a dose taken eight hours before bed still leaves roughly a third of its caffeine in your system.
One practical tip
Set a personal caffeine cut-off, such as no caffeine after midday, and test it for two weeks. Sleep quality is the outcome to watch, not just how quickly you fall asleep, since caffeine can lighten sleep long after its stimulating feeling has faded.
Caffeine also hides in tea, chocolate, energy drinks and some painkillers, so add those sources to your total dose before judging how much remains, or the estimate will understate the real amount in your system.
For a rough sleep check, add up every caffeinated drink in a day, not just coffee, and run the largest late dose through this decay; if more than about fifty milligrams remains at your usual bedtime, an earlier cut-off is worth trying.