How long does caffeine last? An hour-by-hour estimate

Key takeaways

• Feeling caffeine and having caffeine in your body are different things: MedlinePlus puts the felt effect at about four to six hours.
• The European Food Safety Authority (EFSA) gives caffeine an average half-life of about 4 hours in healthy adults, with a range of 2 to 8 hours.
• At a 4-hour half-life, 100 mg falls to about 50 mg after 4 hours, 25 mg after 8 and 13 mg after 12, by our arithmetic.
• In a small study, 200 mg taken around 7 a.m. was still measurable at 11 p.m. and still reduced sleep that night.
• Smoking speeds up clearance, while pregnancy, some medicines and liver disease can slow it a great deal.

How long caffeine lasts: feeling it versus having it in your body

Caffeine works fast. According to EFSA, you may start to notice it within 15 to 30 minutes and the lift can last for hours, and MedlinePlus says the level in your blood peaks within about an hour. If you want the detail on the start of that curve, see how long caffeine takes to kick in.

MedlinePlus adds that you may keep feeling the effects for four to six hours. That is the answer most people mean when they ask how long caffeine lasts, but it is not the same as how long caffeine stays in your body. The level falls gradually, and the fall is described by its half-life: the time it takes your body to clear half of what is there.

Not noticing caffeine any more does not mean it has stopped acting. In a 2025 sleep trial, what the sleep recordings showed and how people rated their own sleep did not match, and the researchers concluded that people may not notice how much caffeine is affecting their sleep.

Caffeine left in your body hour by hour: a 100 mg estimate

The table below is an estimate, not a measurement. It takes one 100 mg dose, assumes it is all absorbed at once, and halves it every half-life. The three columns use EFSA's figures for healthy adults: a 2-hour half-life (fast end), 4 hours (EFSA's average) and 8 hours (slow end). Hours are counted from when the caffeine peaks, roughly within an hour of drinking.

For other doses, scale the numbers: double them for 200 mg, halve them for 50 mg. For a sense of size, the NHS estimates about 100 mg in a cup of instant coffee, while the FDA lists 113 to 247 mg for a 12-ounce brewed coffee. Our brewed coffee page shows why cups vary so much.

How long caffeine keeps you awake: what the sleep studies measured

The clearest real-world picture of how long caffeine lingers comes from a small 1995 lab study in 9 healthy men. They took 200 mg at about 7 a.m. Their saliva caffeine fell from a peak of 17 micromoles per litre an hour after the dose to 3 just before bed at 11 p.m., roughly a sixth of the peak. Compared with placebo, their total sleep time and sleep efficiency that night were still significantly lower.

Bigger doses closer to bed do more. In a 2013 trial of 12 healthy sleepers, 400 mg taken at bedtime, 3 hours before or 6 hours before each disturbed sleep compared with placebo. Even at 6 hours, a sleep monitor measured more than an hour less sleep, although the drop in their own sleep diaries was smaller and not clearly significant.

Smaller doses look gentler. In a 2025 randomized trial of 23 healthy young men, 100 mg taken 12, 8 or 4 hours before bed had no significant effect on measured or self-rated sleep, while 400 mg within 12 hours of bed delayed falling asleep and changed sleep structure. Times closer than 4 hours were not tested. To turn this into a time for your last drink, use the caffeine cutoff calculator.

Why caffeine wears off faster for some people than for others

The FDA points out that people differ a lot, both in how strongly caffeine affects them and in how quickly they get rid of it. EFSA lists age, body weight, pregnancy, medicines and liver health among the things that change how long caffeine stays in the body. Some of the measured effects are large:

• Smoking: a review states that cigarette smoking doubles the rate of caffeine clearance.
• Estrogen birth-control pills: in one small study, half-life averaged about 7.9 hours in users versus 5.4 hours in non-users.
• Pregnancy: in a 1981 study, half-life averaged 8.3 hours (range 3 to 16) in pregnant women versus 3.4 hours in non-pregnant adults.
• Some medicines: in small studies, the antibiotic ciprofloxacin raised half-life from 4.5 to 9.7 hours, and the antidepressant fluvoxamine from about 5 to 31 hours.
• Liver disease: two patients with alcohol-related liver disease had half-lives of 60 and 168 hours.

Why a second coffee makes caffeine last longer into the evening

Caffeine from one cup is still in your body when you drink the next, so doses stack. Here is the arithmetic for two 100 mg coffees, one at 8 a.m. and one at noon, with a 4-hour half-life: about 50 mg is left from the first when you drink the second, so you start the afternoon at about 150 mg. By 4 p.m. that is about 75 mg, and by 10 p.m. about 27 mg. The noon cup alone would leave about 18 mg at 10 p.m.

With an 8-hour half-life the same two cups leave about 72 mg at 10 p.m. A single 100 mg coffee at 4 p.m. leaves about 35 mg at 10 p.m. on the average curve and about 59 mg on the slow one. The calculator above runs these numbers for your own drink and bedtime.

If you are working out whether you are on the fast or slow side, our guide to being a fast or slow caffeine metabolizer goes through the factors one by one, and the caffeine half-life explainer shows how the halving works.

When lingering caffeine symptoms need medical help

Once caffeine is in your body, waiting is the main thing that brings the level down; the studies we rely on do not show a way to make your liver clear it faster. If you had too much and cannot sleep, our guide on how to sleep after caffeine covers what you can still do tonight.

The FDA lists a faster heart rate, palpitations, high blood pressure, sleep problems, anxiety, jitters, upset stomach, nausea and headache among the signs of too much caffeine, and advises talking with a health professional if they occur. MedlinePlus says that signs of a caffeine overdose, such as breathing trouble, seizures, or an irregular or rapid heartbeat, need medical help right away.

The NHS advises calling emergency services if heart palpitations come with chest pain, shortness of breath, or feeling faint or fainting. Outside the UK, use your local emergency number. If you take regular medicines, are pregnant or have liver disease, ask your doctor or pharmacist how caffeine fits in, because these can make it last much longer.

Common questions