Caffeine is the most widely used stimulant in the world, and for good reason: it works. It sharpens attention, lifts mood and gets plenty of night-shift workers through 4 a.m. The catch is that it keeps working long after the cup is empty, and most of us underestimate how long.
How caffeine keeps you awake
Throughout the day your brain builds up a chemical called adenosine. The more of it there is, the sleepier you feel. Caffeine fits into the same receptors as adenosine and blocks them, so the "you're tired" signal doesn't get through. The adenosine is still there, which is why some people crash when the caffeine wears off.
Caffeine reaches its highest level in your blood roughly 30–60 minutes after you drink it. Then your liver slowly clears it.
The half-life problem
In healthy adults, it takes on average around five hours for the body to get rid of half the caffeine you've had. That's an average across a very wide range: for some people it's two or three hours, for others closer to ten. It tends to be longer during pregnancy and for people taking certain medicines, including hormonal contraceptives, and shorter in smokers.
Put simply: if you have a strong coffee at 4 p.m., a good part of it may still be active at 9 p.m., and some of it at midnight.
A sleep-lab study published in 2013 gave people a large dose of caffeine (400 mg, roughly four cups of filter coffee) at bedtime, three hours before, or six hours before. Even taken six hours before bed, it cut their total sleep by more than an hour. Many of them didn't notice. They felt they'd slept normally.
How much is in your drink?
Amounts vary a lot by brand and preparation, but the European Food Safety Authority (EFSA) uses these typical figures:
- Filter coffee, 200 ml cup: about 90 mg
- Espresso, 60 ml: about 80 mg
- Energy drink, 250 ml can: about 80 mg
- Black tea, 220 ml cup: about 50 mg
- Cola, 355 ml can: about 40 mg
- Dark chocolate, 50 g: about 25 mg
Big coffee-shop drinks can contain two or three times the amount of a home-made cup.
How much is too much?
EFSA's assessment is that, for healthy adults, single doses of up to 200 mg and up to 400 mg a day don't raise safety concerns. For pregnant and breastfeeding women the daily figure is 200 mg. Children and teenagers should have much less. Common signs of having had too much include jitteriness, a racing heart, anxiety, an upset stomach and, of course, poor sleep.
A cut-off time that works for night owls
The usual advice is "no coffee after lunch", but that assumes you go to bed at 11. A better rule is to count back from your bedtime:
- Stop around 6–8 hours before you plan to sleep. If you go to bed at 2 a.m., an early-evening coffee is probably fine. If you sleep at 10 p.m., make 2 p.m. your last one.
- Night-shift workers: use caffeine early in the shift, and stop well before the drive home and your day sleep.
- If you sleep badly, try moving your cut-off two hours earlier for a week and see if it changes anything. It's a free experiment.
The "coffee nap"
Because caffeine takes 20–30 minutes to kick in, some people drink a coffee and immediately take a short nap of 15–20 minutes. They wake up just as the caffeine arrives. It's a handy trick before a long drive or the second half of a night shift. It's not one for the evening before you want to sleep.
Cutting down without the headache
Regular caffeine users who stop suddenly often get headaches, tiredness and low mood for a few days. If you want to cut down, reduce gradually, for example by half a cup every few days, or switch some cups to decaf or half-caf.
This article is general information, not medical advice. If you're pregnant, have a heart condition or take medication, ask a doctor or pharmacist about caffeine.
