How late is too late for caffeine?

It's 4pm and the afternoon has gone flat. The inbox has stopped making sense, so you make a coffee, a proper one, and get back to it.

That night you're asleep by 11.15 with no trouble at all. Then it's 2.40 and you're awake, then 3.50, and from half four you're drifting rather than sleeping. By the alarm you've had something like five hours, in pieces.

You don't connect the two. Why would you? The coffee was eleven hours before the worst of it, and you fell asleep fine.

This isn't carelessness. In the trial that tested caffeine six hours before bed, the people taking it barely reported worse sleep; the monitor on their foreheads said otherwise.

Afternoon caffeine delays you dropping off by minutes. It takes far more off the second half of the night, and you don't notice it going.

Then the day pays for it, and by four o'clock the afternoon has gone flat again. So you make a coffee.

How caffeine works, and why it lingers

While you're awake, a chemical called adenosine builds up in the brain, and it's part of what makes you sleepy by evening. Caffeine's main target in the brain is the adenosine receptor, a 2018 review in Pharmacological Reviews explains. It sits in the lock, so the sleepiness signal can't get through.

The signal hasn't gone. It's queued behind the caffeine, and it waits as long as the caffeine does.

The Royal College of Psychiatrists says caffeine takes 20 to 30 minutes to start working, and hangs around in your body long after. Half-life is the measure that matters. It's the time your body takes to clear half of what you drank.

How long does caffeine last?

In healthy adults the half-life is two and a half to five hours, according to that 2018 review. Take a mug of instant coffee at 2pm and a four-hour half-life. By 6pm half the caffeine is still there, by 10pm a quarter, and at 2am an eighth is still in you.

That's for an average half-life. Half-lives vary a lot. The same review says 54 per cent of people carry a gene variant that clears caffeine more slowly, so for about half of us the sums above are optimistic.

Some things slow it a lot more. The contraceptive pill almost doubles the half-life, from about six hours to nearly eleven in the study the review cites. In late pregnancy it stretches to 11.5 to 18 hours, and the NHS caps caffeine at 200mg a day in pregnancy.

The antidepressant fluvoxamine takes it from five hours to 31. The review also flags a marked interaction with quinolone antibiotics, so if you've started a new medicine, read the leaflet.

Smoking speeds it up, which is the one argument for smoking that nobody should use.

What the trials found about timing

The study everyone quotes is small. In 2013, researchers in Detroit gave 12 healthy sleepers 400mg of caffeine or a placebo. They took it at bedtime, three hours before bed or six hours before, on different nights, and slept at home wearing a headband monitor.

Every timing cut sleep by 1.1 to 1.2 hours on the monitor. On the six-hour nights their own diaries recorded 41 minutes less, a difference too small and uneven to count as significant. They lost an hour and reported not much.

400mg is a lot: roughly four mugs of instant coffee by NHS figures. So the trial describes a heavy afternoon rather than a single cup. It's also 12 people, which is why the next study matters.

In 2023 a review in Sleep Medicine Reviews pooled 24 studies. Across them, caffeine cut total sleep by 45 minutes and took 11 minutes off deep sleep. It added nine minutes to falling asleep and 12 minutes awake in the night.

To avoid losing sleep, their model put a standard 107mg coffee at least 8.8 hours before bed. A 217mg dose needed 13.2 hours.

So the NHS figure of six hours is a floor, not a target. The Royal College of Psychiatrists simply says stop by 2pm, and for a late bedtime that's about the same thing.

How much is in a cup?

The NHS publishes these figures, and they're worth knowing because "a coffee" covers a lot of ground. A cup of instant coffee has about 100mg. Filter coffee and shop-bought coffee have more, and the large sizes have more again.

A cup of tea has about 75mg, so three cups is more than two instant coffees. A can of cola has about 40mg, and a 250ml energy drink about 80mg. The Royal College adds that some fizzy drinks and even mints carry a lot, so the label matters.

What to do

1. Work back from your bedtime. Count eight or nine hours back from lights out and make that your last coffee, with six hours as the absolute floor. For an 11pm bedtime that's 2pm to 3pm, which is why the Royal College's 2pm rule is simpler than the maths.
2. Know your dose. A takeaway cup is far bigger than the NHS's cup, and the NHS says shop-bought coffee is stronger per cup too. If the afternoon coffee is a large one from a shop, it's close to the trial dose. The trial dose cost an hour.
3. Count the tea. Three mugs through the afternoon is more caffeine than the coffee you gave up. Tea feels gentler because each cup is smaller. The afternoon isn't.
4. Move the cut-off earlier if you're on the pill, pregnant or on certain medicines. With the half-life doubled, a 2pm coffee behaves like a 6pm one. The NHS limit in pregnancy is 200mg a day, which is two cups of instant coffee in total.
5. Keep the morning cup. None of this is a reason to quit. The Royal College doesn't ask you to give up the morning cup; it just says stop by 2pm. A coffee at 8am has had 15 hours to clear by an 11pm bedtime.
6. Replace the 4pm coffee with the thing it was standing in for. Usually that's a slump. Bright light, fresh air and physical activity are the Royal College's own list for staying awake. A walk round the block gets you all three.
7. Test it for two weeks, on paper. Move the cut-off, keep a sleep diary, and look at the second half of the night. If nothing changes, caffeine wasn't your problem, and that's worth knowing too.

Where the evidence runs out

The timing evidence is thinner than the confidence around it.

The trial that tested six hours had 12 people and one large dose. The 8.8-hour figure is a model fitted across 24 mostly laboratory studies. And your own half-life could be at either end of a two-fold range.

And no trial shows that cutting caffeine on its own fixes a long-running sleep problem. It removes one cause.

If your nights were broken before the afternoon coffee started, we've gone through the other causes in why you wake at 3am. And there's how to get back to sleep for when it happens.

When to see a GP

The NHS says to see a GP if changing your habits hasn't helped, you've had trouble sleeping for months, or it's affecting your daily life. Two weeks with the caffeine moved is a fair test of a habit. If the nights don't change, take the diary and the question to a GP rather than cutting the morning coffee too.

Caffeine may be the cause of your nights or one of several; the diary will tell you which. The 2pm cut-off, though, you can start tomorrow without anyone's help.