Caffeine Timing & Half-Life Calculator
See how much caffeine is still circulating at bedtime, at your half-life and at a slow one, and how long a dose takes to fall below a threshold you choose.
At bedtime
To fall below your threshold by bedtime at the half-life you entered, have this drink at least 4.7 hours before you sleep, or 9.3 hours if your half-life is 10 hours. Being under the threshold does not guarantee sleep: caffeine left in your system can still lighten it, and sensitivity differs from person to person.
Daily totals: the FDA cites 400 mg a day as an amount not generally associated with negative effects for healthy adults. In pregnancy, ACOG advises under 200 mg a day. Children, and anyone with a heart condition or anxiety, or taking medication, should ask a doctor about caffeine.
How the estimate is built
Caffeine follows exponential decay. The amount remaining after a given time is the dose multiplied by one-half raised to the power of the elapsed hours divided by your half-life. To find how long to wait, the calculator runs that backwards: it solves for the number of half-lives it takes to fall from the dose to your threshold, using a base-two logarithm, and multiplies by your half-life. It repeats both at a slow, 10-hour half-life, and it rounds amounts and hours up, never down.
The presets are rough averages — a real espresso or energy drink can vary widely by brand and size — so treat the milligram figures as a starting point and adjust to the actual product.
Frequently Asked Questions
How does caffeine leave the body?
By first-order decay, which means a fixed fraction clears in each equal stretch of time. After one half-life, half the dose remains; after two, a quarter; after three, an eighth. The amount left after t hours is the dose times one-half raised to the power of t divided by your half-life. That is the exact formula this calculator uses.
What is a caffeine half-life?
It is the time your body takes to clear half of a caffeine dose. The average for a healthy adult is about five hours, but that is an average: it ranges from roughly 1.5 to 9.5 hours between people, is shorter in regular smokers, and is longer in pregnancy, with certain medications (including oral contraceptives), and with reduced liver function. Because nobody knows their own figure from a calculator, every result here also shows a slow, 10-hour half-life.
Why does an afternoon coffee affect sleep?
Because a five-hour half-life is long. A 200 mg cold brew at 3pm still leaves about 100 mg in your system at 8pm and 50 mg at 1am. Even when you fall asleep, lingering caffeine can reduce deep sleep and make the night less restorative. The calculator shows exactly how much is left at your bedtime so you can judge the trade-off.
What is the bedtime threshold?
It is a level you choose to be under by the time you go to bed, set to 50 mg by default, roughly half a cup of coffee. It is a rule of thumb, not a guarantee: falling below it does not promise good sleep, and sensitivity differs, so a cautious sleeper might set it lower. The tool works out how many hours your dose takes to decay to that level, at your half-life and at a slow one.
How much caffeine a day is too much?
The FDA cites 400 mg a day, about two to three 12-ounce cups of coffee, as an amount not generally associated with negative effects for healthy adults. In pregnancy, ACOG advises keeping caffeine under 200 mg a day. Children, and anyone with a heart condition or anxiety, or taking medication, should ask a doctor about caffeine rather than rely on a calculator.
Educational estimate only — not medical or dietary advice. Caffeine metabolism varies widely, and caffeine affects health conditions, pregnancy, and medications differently. Follow product labels and consult a doctor or pharmacist about your own intake.