Caffeine has a half-life of about 6 hours, so half of what you drink at 3pm is still in you at 9pm. Full clearance takes far longer, roughly 30 hours, which means your morning coffee is not completely gone until the following afternoon.
Both of those facts are true and neither is the number you actually need. The useful figure is how much is left at the moment you try to sleep, and that depends on your whole day, drink by drink.
Contents
- The two questions hiding in one
- How fast it gets in
- How fast it leaves
- The part nobody calculates
- Why the 2pm rule breaks
- Why your number is not the average
- Work out your own figure
The short version
Caffeine is absorbed almost completely within 45 minutes and peaks in the blood inside two hours. It then halves roughly every 6 hours, so a 3pm coffee is at half strength by 9pm and takes about 30 hours to clear properly. Because every drink decays on its own clock, they stack. A small drink taken late can leave more in you at bedtime than a large one taken at breakfast.
The two questions hiding in one
"How long does caffeine stay in your system" gets asked as one question and answered as one number, which is why the answers online disagree so much. There are two separate things being asked.
- How long does it keep affecting me? Hours. The stimulant effect fades as the level falls, and most people stop noticing it long before it is gone.
- How long until it has actually left? Roughly 30 hours, using the standard rule that a drug is effectively cleared after five half-lives. At that point about 3% of the original dose remains.
The gap between those two answers is where the trouble lives. You stop feeling caffeine at a level that is still high enough to change your sleep, which is a point we covered in detail in why your afternoon coffee is keeping you awake.
How fast it gets in
Quickly, and almost entirely. Caffeine is absorbed in the small intestine with 99% taken up within 45 minutes of drinking it, and essentially none of it is lost on first pass through the liver (Institute of Medicine, Pharmacology of Caffeine).
Peak blood concentration lands somewhere between 15 and 120 minutes after you drink it. That spread is mostly down to what else is in your stomach and how fast your own liver enzymes work.
Practically, this is why the 20-minute pre-meeting coffee is badly timed. You are drinking it about an hour before it does its best work.
How fast it leaves
Slowly, and along a curve that flattens as it goes.
Caffeine follows exponential decay. Each half-life removes half of whatever is left, so the amount falls fast at first and then trails. The Institute of Medicine puts the mean elimination half-life in healthy adults at about 5 hours, with a reported range of 1.5 to 9.5 hours.
Our caffeine calculator works on 6 hours. That sits slightly above the average on purpose. A longer half-life produces a higher bedtime estimate, so the tool errs toward telling you there is more left than you might hope.
Here is what 6 hours does to a single 200 mg coffee drunk at 8am.
| Time | Hours elapsed | Caffeine remaining |
|---|---|---|
| 8am | 0 | 200 mg |
| 2pm | 6 | 100 mg |
| 8pm | 12 | 50 mg |
| 2am | 18 | 25 mg |
| 8am next day | 24 | 12.5 mg |
| 2pm next day | 30 | 6.25 mg |
One coffee, and it is still faintly detectable a day and a half later. Nobody feels 6 mg. It is there anyway.
The part nobody calculates
Every table like the one above shows a single drink. Almost nobody has a single drink.
Each one you have starts its own decay curve, and at any moment your actual blood level is the sum of all of them. That sum behaves in a way that catches people out.
Take an ordinary day, with an 11pm bedtime.
| Time | Drink | Dose | Left at 11pm | Share of bedtime total |
|---|---|---|---|---|
| 7:30am | Flat white | 130 mg | 21.7 mg | 19% |
| 10:30am | Americano | 160 mg | 37.8 mg | 33% |
| 2pm | Filter coffee | 95 mg | 33.6 mg | 29% |
| 4:30pm | Mug of tea | 48 mg | 22.7 mg | 20% |
That is 433 mg across the day and about 116 mg still circulating at lights out.
Now look at the first and last rows. The flat white was 130 mg. The tea was 48 mg, less than half as much. The tea leaves more caffeine in you at bedtime.
A mug of tea at half past four outweighs a flat white at half past seven. Timing beats dose, and it is not close.
This is the thing a generic half-life calculator cannot tell you, because it only handles one drink at a time. It is also why people who cut out their morning coffee and keep the afternoon tea are often disappointed with the result.
Why the 2pm rule breaks
The standard advice is to stop drinking caffeine by 2pm. It is a reasonable rule and it has one hidden assumption: that the 2pm drink is the only thing in you.
Work the single-drink case. To have less than 50 mg left at 11pm from one 130 mg coffee, that coffee needs to land by about a quarter to three. So far so good, and the 2pm rule holds with room to spare.
Now run it against the day in the table above. By 2pm that person is already carrying the residue of two earlier coffees. The 2pm filter coffee is not arriving into an empty system, it is arriving on top of about 90 mg that is already there. The rule that worked in isolation does not survive contact with a real day.
A cutoff time is only meaningful once you know what is already in you. That is the calculation, and it is tedious to do by hand, which is why we built a tool that does it.
Why your number is not the average
Everything above uses a 6-hour half-life. Yours may be nothing like that.
Roughly 95% of caffeine clearance runs through a single liver enzyme, CYP1A2, and how active it is varies enormously between people. A systematic analysis pooling 141 published studies covering 4,714 individuals found consistent, decades-old evidence for the main modifiers (Frontiers in Pharmacology, 2021).
What lengthens your half-life:
- Oral contraceptives. Can double it. A 6-hour clearance becomes 12, which moves your cutoff time by six hours.
- Pregnancy. Clearance slows markedly, most of all in later pregnancy, when the half-life can reach about 15 hours (see our post on caffeine in pregnancy). UK guidance is 200 mg a day during pregnancy.
- Liver disease. Cirrhosis produces the largest reduction in clearance of any factor studied.
- Age. Metabolism becomes less efficient over time, so a habit that never troubled you at 25 starts costing sleep in your forties.
What shortens it: smoking, which increases clearance substantially. Worth knowing that clearance returns to normal after quitting, so people who stop smoking often find their usual coffee suddenly feels like too much.
The genetics alone are enough to explain why two people can drink the same flat white and one is asleep by eleven while the other is awake at two.
Work out your own figure
Reading about half-lives will not tell you your number. The arithmetic is simple per drink and annoying across a day, so the caffeine calculator does it. It also helps to know how much caffeine is in your cup, because the same drink can be 89 mg at one chain and 325 mg at another.
You log what you drank and roughly when, from a list of 220 drinks, then set your bedtime. It gives you four things:
- Your total across the day
- Your peak level, and when it hit
- How much is still circulating at bedtime
- The time your next drink would need to land by to stay under the line
It also names the single drink doing the most damage to your bedtime figure, which is rarely the one people expect. The 50 mg line the tool uses is a practical threshold for reading the chart, not a medical limit.
Most people are wrong about their daily total, and wrong in the same direction. The bedtime number is usually the one that changes behaviour.
Then there is the total
Moving your last drink earlier changes tonight. It is the fastest win available and it costs nothing.
Bringing the daily total down is the slower job, and it is the one that changes how much caffeine you need in the first place. Doing that abruptly produces a headache that peaks between 20 and 51 hours in, which we mapped in the caffeine withdrawal timeline. Stepping down avoids most of it.
The Uncaffed 30-day kit runs that step-down by capsule count so it does not rely on you estimating a brew. Reset opens at 160 mg a day and drops 40 mg every five days until there is no added caffeine left. Flow runs alongside at a constant dose for the full 30 days with no added caffeine, carrying magnesium, which contributes to the reduction of tiredness and fatigue, and pantothenic acid, which contributes to normal mental performance.
Preorders are open. Kits ship by 30 October. The reasoning behind the step sizes is on the science page.
Start with tonight though. Run your day through the calculator and find out what you are actually taking to bed. For the morning side, see when to have your first coffee.
