Caffeine Half-Life: How Long Caffeine Stays in Your System

By Jack Wallner. Updated October 12, 2026.

What a half-life means for your coffee

A half-life is the time it takes your body to clear half of a dose. Caffeine clearance follows that pattern, so the amount left drops by half every half-life: 200 mg of caffeine becomes 100 mg after one half-life, 50 mg after two, and 25 mg after three. A review of caffeine safety (Temple and colleagues, Frontiers in Psychiatry, 2017) reports caffeine concentrations peaking in blood about 2 hours after ingestion and a half-life of 3 to 7 hours in adults. Five hours is the usual middle figure for planning.

This table applies the formula remaining = dose × 0.5^(hours ÷ half-life) to a single 200 mg dose, roughly one 12-ounce cup of coffee going by the FDA's figure that 400 mg is two to three such cups, at three half-life values:

Model arithmetic, not a measurement. Real absorption takes time, so the first hour or two does not follow the curve.

A 2 pm coffee with a 10 pm bedtime is 8 hours of clearance. On a 5-hour half-life, about a third of it (66 mg of 200) is still around at bedtime, and the plausible range on this table runs from 50 to 79 mg.

What changes your half-life

The 3 to 7 hour range is wide because several things move it. The same review and a pooled analysis of caffeine pharmacokinetic studies (Frontiers in Pharmacology, 2021) report these directions:

• Smoking speeds clearance. The 2017 review says it roughly doubles the rate.
• Oral contraceptives lengthen the half-life, which the review says tend to double it.
• Pregnancy lengthens it, on average by 8.3 hours in the review and by as much as 16 hours in some cases.
• Liver disease and heavy alcohol intake slow clearance.

The FDA makes the same point in plainer terms: there is wide variation in how sensitive people are to caffeine and in how fast they eliminate it. If you are pregnant, take medication or have a medical condition, ask your clinician how much caffeine suits you.

Why bedtime matters

In a placebo-controlled study of 12 healthy sleepers (Drake and colleagues, Journal of Clinical Sleep Medicine, 2013), 400 mg of caffeine taken 0, 3 or 6 hours before bedtime each disrupted sleep. Even at 6 hours, caffeine cut sleep by more than an hour. The authors concluded that avoiding substantial caffeine for at least 6 hours before bed is supported by the data. The study was small, so treat it as a clear signal at a high dose rather than a rule for every cup.

For daily totals, the FDA cites 400 milligrams a day, about two to three 12-ounce cups of coffee, as an amount not generally associated with negative effects for most adults.

How to use a half-life estimate honestly

1. Treat the output as an estimate. It assumes one fixed half-life, and your real one may differ by hours.
2. Count every source: coffee, tea, soda, energy drinks, and some bars, gum, supplements and over-the-counter medications. The FDA notes that caffeine from natural and added sources is handled the same way by the body.
3. Compare against your own sleep. A model cannot know how sensitive you are, but your sleep record can show whether late-day caffeine lines up with shorter nights.

How Caffeine Tracker: Bedtime helps

Caffeine Tracker: Bedtime applies the formula above to every drink you log, with a default 5-hour half-life you can adjust and a 4 to 6 hour reference range shown in the app. It adds the doses into a current estimate and a bedtime estimate, and a preview lets you see what a drink at a given time would do to your bedtime estimate before you save it. With the optional Body Insights setting on, it compares your recorded sleep on higher- and lower-caffeine nights from Apple Health and, after 21 nights, may show a personal cutoff or say there is no clear difference. Those are observations from your own records. They do not show that caffeine caused a change, and they are not a safe limit or medical advice.

Frequently asked questions

What is the half-life of caffeine?

In healthy adults, caffeine has a half-life of about 3 to 7 hours, and 5 hours is the usual planning figure. After one half-life, half of the dose is left; after two, a quarter.

How long does caffeine stay in your system?

On a 5-hour half-life model, a 200 mg dose falls to 100 mg after 5 hours, 50 mg after 10 hours and 25 mg after 15 hours. Small amounts remain longer, and your own half-life may be shorter or longer.

Does caffeine at 2 pm affect sleep at 10 pm?

It can. A model with a 5-hour half-life leaves about a third of a 2 pm dose at 10 pm. In a small study, 400 mg of caffeine taken 6 hours before bed reduced sleep by more than an hour, but responses vary by person and dose.

What makes caffeine last longer?

Oral contraceptives, pregnancy, liver disease and heavy alcohol intake are reported to lengthen the half-life, while smoking shortens it. Genetics also play a role.

How accurate is a caffeine half-life calculator?

It is an estimate. A calculator uses one fixed half-life for everyone, while real half-lives vary by hours between people and across conditions. It is best used to compare options, not to measure caffeine in your body.

Sources

1. Temple et al. The Safety of Ingested Caffeine: A Review. Frontiers in Psychiatry, 2017 (PMC)
2. Drake et al. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine, 2013 (PMC)
3. Grzegorzewski et al. Pharmacokinetics of Caffeine: A Systematic Analysis of Reported Data. Frontiers in Pharmacology, 2021
4. U.S. Food and Drug Administration: Spilling the Beans, How Much Caffeine Is Too Much?

This page is general information, not medical advice. Caffeine Tracker: Bedtime provides estimates, not blood measurements or safe limits, and does not diagnose, treat or prevent any condition. Talk to a clinician about caffeine if you are pregnant, take medication or have a medical condition.