Caffeine half-life, with the actual numbers
The half-life of caffeine in a healthy adult is about 5 to 6 hours. This site and the Cira app use 5.7. Studies put the average near 5 hours, with individual values from about 1.5 to 9.5 hours, and specific circumstances push it further.
Half-life means the time for half of what is present to be eliminated. It is not how long caffeine lasts, which is a question about thresholds rather than fractions.
Try it with your drink
Half-life 5.7 h · peak at 45 min · sleep threshold 40 mg
The formula
Elimination is first order, so a constant fraction leaves per unit time:
remaining = dose × 0.5 ^ (hours ÷ 5.7)
A 96 mg mug of filter coffee therefore goes:
| Time after the cup | Still active | |
|---|---|---|
| 45 minutes (peak) | 88 mg | still working |
| 5.7 hours | 49 mg | still working |
| 11.4 hours | 25 mg | below the sleep threshold |
| 17.1 hours | 12 mg | below the sleep threshold |
| 24.0 hours | 5 mg | below the sleep threshold |
Two refinements make the curve above match what a blood test would show.
Absorption. Caffeine is not all present the moment you swallow it. It climbs for about 45 minutes before peaking, so the true peak is slightly below the nominal dose, since some has already been eliminated while the rest was still being absorbed. The standard description is the Bateman function, a difference of two exponentials, one for absorption and one for elimination. It is what the calculator on this page uses.
The threshold. For sleep the useful figure is not zero. Cira uses 40 mg of active caffeine as the line that is low enough to sleep on. It is a working threshold of the model, not a clinical standard, and with it the model lands close to published cut-off times.
Half-life is a rate, not a duration. After one half-life you still have half a dose in you, and for a 200 mg energy drink that remainder is larger than an entire espresso was.
What changes your half-life
The enzyme CYP1A2 in the liver handles about 95 percent of caffeine metabolism, and its activity varies enormously.
| Who | Half-life |
|---|---|
| Healthy adult, as used here | 5.7 hours |
| Fast metaboliser | about 4 hours |
| Slow metaboliser | 8 to 9 hours |
| Smoker | about 3 hours |
| On oral contraceptives | 10 hours or more |
| Pregnancy, third trimester | up to 15 hours |
| Newborn | over 80 hours |
Three of these are worth dwelling on.
Oral contraceptives roughly double clearance time, and nobody is told this when they start. The same 3 pm coffee that was fine becomes an 11 pm problem.
Smoking cuts the half-life to around 3 hours, which is why smokers often drink far more coffee than non-smokers. Quitting reverses it within a few days, so people give up cigarettes and then lie awake blaming the nicotine withdrawal when the cause is that their normal coffee habit now behaves like a double dose.
Pregnancy slows clearance progressively, reaching 15 hours or more by the third trimester. This is a large part of why the limit in pregnancy drops to 200 mg a day.
Genetics set the baseline. Variants in the CYP1A2 gene split people roughly into fast and slow metabolisers, and it is the main reason two people can drink the same espresso after dinner with completely different nights.
What does not change it
Tolerance. Regular use adapts the brain, not the liver. A daily drinker feels less and clears it at the same speed.
Body weight, mostly. Caffeine distributes through total body water, so a larger person reaches a slightly lower peak concentration from the same dose, but the half-life is close to unchanged. Dose per kilogram matters for children and adolescents, which is why their guideline is written that way.
Age, within adulthood. A healthy 70 year old clears caffeine at close to the rate of a 30 year old. Newborns are the dramatic exception, at over 80 hours, which is why caffeine crosses into breast milk as a real consideration.
Food, water and exercise. Eating slows absorption, so the peak is later and lower, but the total and the elimination rate are the same. Nothing you can drink speeds up clearance, and the "flush it out with water" advice does not work: caffeine is reabsorbed in the kidneys rather than simply filtered out.
Using it
The practical move is to work backwards from bedtime rather than forwards from the cup. Decide when you want to be under 40 mg, then find how many half-lives it takes to get your drink there, and multiply by 5.7. For a 160 mg energy drink that is two half-lives to 40 mg, so about eleven hours.
If you know you are slow, use 8 hours instead of 5.7 and everything moves out by a third.
Common questions
What is the half-life of caffeine?
About 5 to 6 hours in a healthy adult, meaning half of any dose has left your body after that time. Individual values run from roughly 1.5 to 9.5 hours. This site uses 5.7.
How do you calculate caffeine half-life?
Remaining caffeine equals the dose multiplied by 0.5 raised to the power of hours divided by 5.7. A 96 mg coffee leaves about 48 mg after 5.7 hours and 24 mg after 11.4.
What makes caffeine half-life longer?
Oral contraceptives roughly double it, pregnancy extends it to as much as 15 hours, and liver disease lengthens it further. Smoking cuts it nearly in half.