Caffeine half-life calculator: how much is left at bedtime?
Had an energy drink, coffee or pre-workout? Enter your caffeine amount and the time you drank it to estimate how much might remain when you plan to sleep.
Estimate your caffeine timeline
Estimated caffeine at bedtime
Estimated amount remaining after your drink
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This model estimates caffeine remaining in the body, not whether you'll fall asleep or how alert you'll feel.
How long does 200 mg of caffeine last?
With an assumed five-hour half-life, a 200 mg energy drink has an estimated 100 mg remaining after five hours and 50 mg after ten hours. Other people may clear caffeine faster or slower. This is why an afternoon drink may still matter at bedtime.
The calculator uses the equation: caffeine remaining = dose × 0.5(hours since drink ÷ half-life). It assumes one dose with immediate absorption and first-order elimination. In reality caffeine takes time to absorb and its effects vary by person. It's an educational estimate, not a blood test.
Can an energy drink affect sleep even hours later?
Yes. How much you consume and how close it is to bedtime both matter. In a small randomized trial, a single 400 mg dose disrupted several sleep measurements even when taken up to 12 hours before bed; the same trial did not find significant sleep effects from a 100 mg dose at the tested times. That doesn't mean 100 mg is harmless to everyone's sleep or that 400 mg affects everyone identically.
There is no universal number of milligrams remaining at bedtime that guarantees good sleep. People differ in sensitivity, medications, health conditions and caffeine metabolism. As a practical starting point, many sleep resources recommend avoiding caffeine for roughly eight hours before bed, with longer intervals if you're sensitive.
What about multiple energy drinks or coffee?
This simple calculator models one drink. For more than one, calculate each drink separately using the same planned bedtime and half-life, then add the estimated remaining milligrams. The total is only an approximation. Drink caffeine amounts vary widely, so use labels whenever they're available.
Is 400 mg of caffeine a daily limit?
The FDA cites 400 mg per day as an amount not generally linked to negative effects for most healthy adults, not a guaranteed safe amount for everyone and not a recommended daily target. People who are pregnant, take interacting medications or experience side effects may need much less. Caffeine amounts in energy drinks vary significantly.