How Caffeine Works, and How Much Is Too Much
Coffee, tea, cola and energy drinks all wake you up the same way: by blocking a chemical that makes you feel sleepy. Here’s how caffeine works in the body, how long it lasts, and what health authorities say is a safe amount.
Daily Life Buzz Editorial 5 min readExplainer
General information, not medical advice. If you have a health condition, ask your doctor what is right for you.
In this article6 sections
For many of us, the day doesn’t really start until the first cup of coffee or tea. Within half an hour we feel more alert, more focused and more cheerful. Caffeine is the most widely used mind-altering substance in the world, and most of us take it every day without thinking twice.
So what exactly does it do, and when does a useful habit become too much?
How caffeine wakes you up
From the moment you wake up, your brain produces a chemical called adenosine. As it builds up through the day, it attaches to receptors on brain cells and slows them down, which is one of the reasons you feel increasingly sleepy by evening. Sleep clears it away again.
Caffeine has a shape similar to adenosine. It slots into the same receptors and blocks them, without slowing the cells down. With adenosine blocked, you feel more awake and alert, and other brain chemicals that help with attention and mood can work more freely.
This is important to understand: caffeine doesn’t give you energy, and it doesn’t remove tiredness. It hides the signal. When the caffeine wears off, the adenosine is still there, which is why some people feel a “crash”.
Caffeine doesn’t remove tiredness. It hides the signal.
How long it lasts
Caffeine is absorbed quickly. Its effects usually start within 15 to 45 minutes and peak around an hour after you drink it.
Your liver then breaks it down. For most healthy adults, caffeine’s half-life, the time it takes for half of it to leave your body, is about five hours. It varies a lot between people, from about two hours to more than ten. It is longer during pregnancy, and in people taking some medicines, including the contraceptive pill, and shorter in smokers.
That means a coffee at 4 p.m. can leave about half its caffeine in your body at 9 p.m. In a 2013 study, researchers found that 400 mg of caffeine taken even six hours before bedtime cut people’s sleep by more than an hour. Many people don’t notice the effect, but it shows up in how deeply and how long they sleep.
How much caffeine is in your drink
Amounts vary a lot depending on the bean or leaf, the strength and the size of the cup, so these are rough guides:
| Drink | Typical caffeine |
|---|---|
| Brewed coffee, 1 mug (about 250 ml) | 80–150 mg |
| Espresso, 1 shot | 60–80 mg |
| Instant coffee, 1 mug | 60–90 mg |
| Black tea, 1 mug | 40–60 mg |
| Green tea, 1 mug | 25–40 mg |
| Cola, 1 can (330 ml) | 30–40 mg |
| Energy drink, 250 ml can | About 80 mg |
| Dark chocolate, 50 g bar | About 25–40 mg |
Large coffee-shop drinks can contain much more than a home-made cup, sometimes over 300 mg. Decaffeinated coffee isn’t completely caffeine-free, but usually contains only a few milligrams.
How much is too much?
In 2015, the European Food Safety Authority (EFSA) reviewed the evidence and concluded that, for healthy adults:
- Up to 400 mg a day doesn’t raise safety concerns. The US Food and Drug Administration gives the same figure.
- Single doses up to 200 mg are safe, even shortly before exercise.
- During pregnancy, up to 200 mg a day doesn’t raise safety concerns for the unborn child. The same applies while breastfeeding.
- For children and teenagers, about 3 mg per kilogram of body weight per day is the level EFSA considers safe. For a 40 kg child, that’s about 120 mg.
Too much caffeine can cause jitteriness, a racing heart, anxiety, an upset stomach, headaches and poor sleep. Very large amounts, usually from caffeine tablets or powders rather than drinks, can be dangerous.
Tolerance and withdrawal
If you drink caffeine every day, your brain adapts by making more adenosine receptors. You need more caffeine for the same effect, and without it you can feel worse than before. This is tolerance.
If you stop suddenly, withdrawal symptoms such as headaches, tiredness, low mood and difficulty concentrating usually start within 12 to 24 hours, peak after a day or two, and can last up to about nine days. Cutting down gradually, over a week or two, usually avoids the worst of it.
Using caffeine well
- Set a cut-off time, such as early afternoon, if you have trouble sleeping.
- Count all your sources, including tea, cola, energy drinks, chocolate and some painkillers.
- Don’t use caffeine to replace sleep. It can hide tiredness, but it can’t fix the effects of too little sleep. See how much sleep you really need.
- Ask your doctor if you are pregnant, have a heart condition, high blood pressure or anxiety, or take regular medicines.
Frequently asked questions
Does coffee dehydrate you?
Not in the amounts most people drink. Caffeine has a mild effect on urine, but the water in coffee and tea more than makes up for it. Regular coffee and tea count towards your daily fluids.
Is tea better than coffee?
A mug of tea usually contains less caffeine than a mug of coffee, so it can be a gentler option. Both can be part of a healthy diet.
Why does caffeine affect me more than my friends?
People break caffeine down at different speeds, partly because of their genes, and some are more sensitive to its effects. Your usual intake also matters, because of tolerance.
Sources
- European Food Safety Authority (2015): Scientific opinion on the safety of caffeine
- US Food and Drug Administration. Spilling the beans: how much caffeine is too much? Consumer update.
- Drake, C., Roehrs, T., Shambroom, J. and Roth, T. (2013). Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine, 9(11), 1195–1200.
- Juliano, L. M. and Griffiths, R. R. (2004). A critical review of caffeine withdrawal: empirical validation of symptoms and signs, incidence, severity, and associated features. Psychopharmacology, 176(1), 1–29.
- NHS: Should I limit caffeine during pregnancy?
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