How does caffeine work?

How does caffeine work? The short answer

Caffeine doesn't give your body energy. It works by blocking a sleepy signal in the brain. While you are awake, a chemical called adenosine slowly builds up in the brain, and the more there is, the sleepier you feel.³⁴ Caffeine has almost the same shape as adenosine, so it sits on adenosine's landing spots, called receptors, without switching them on. The tired message is blocked, and you feel more alert.¹⁴

What is adenosine, and why does it make you sleepy?

Your brain cells use energy all day. As they work, a substance called adenosine is left over.⁴ Scientists have measured that adenosine rises in parts of the brain the longer an animal stays awake and falls again during sleep.³ This growing need for sleep is called sleep pressure. In the video we draw it as a jar that fills during the day and empties at night. That's a picture, not a real jar: adenosine isn't stored in one place.

How does caffeine block adenosine? (the lock and key)

1. Receptors are the locks. Brain cells have adenosine receptors on their surface. There are four kinds, and caffeine blocks all of them.¹
2. Adenosine is the key. When adenosine binds, it calms down cells that help keep you awake.³
3. Caffeine is a look-alike key. Caffeine is very close in structure to adenosine, so it attaches to the same receptors but doesn't trigger the sleepy signal.⁴
4. The result. With caffeine in the way, adenosine can't land, so you feel less tired. Scientists call a blocker like this an antagonist.¹²

Caffeine dissolves in both water and fat, which helps it pass easily from the blood into the brain.¹

How long does caffeine last?

Caffeine is taken up quickly: nearly all of it is absorbed within about 45 minutes, and blood levels peak somewhere between 15 minutes and 2 hours after a drink.² The liver then breaks it down and the kidneys clear it out.²

In an average adult, about half the caffeine is gone after roughly 5 hours. That time, the half-life, varies a lot, from about 1.5 to 9.5 hours, depending on things like smoking, pregnancy and other factors.¹² So a cup at 8 a.m. is about half gone by 1 p.m., and about a quarter is still there at 6 p.m.

Why does coffee make me sleepy?

Coffee is meant to wake people up, so feeling sleepy afterwards is a puzzle. There are a few likely reasons:⁷

• The crash. Caffeine blocks adenosine, but it doesn't remove it. Adenosine keeps building up in the background, so when the caffeine wears off, the tiredness can arrive all at once.⁷
• Not enough sleep. Caffeine can hide tiredness for a while, but it doesn't replace sleep. If you're short on sleep, the sleepiness comes back.⁷
• Caffeine late in the day. In a review of 24 studies, caffeine cut total sleep by about 45 minutes and reduced deep sleep.⁸ Even a large dose 6 hours before bed disturbed sleep.⁹ A worse night means a sleepier next day.
• Tolerance. Regular, heavy caffeine use can make the brain add more adenosine receptors, so the same amount works less well, and people feel tired or get headaches without it.²

Where does caffeine come from?

Caffeine is made by plants. It's found in coffee, tea, guarana, maté, kola nuts and cacao.⁴ In total, more than 60 kinds of plants make it, in their leaves, seeds or fruit.⁶ Coffee beans are the seeds of the coffee plant's red fruit, often called coffee cherries.⁴

Why would a plant make caffeine? Caffeine tastes bitter,⁴ and in 1984 a Harvard scientist reported in the journal Science that caffeine can act as a natural pesticide: it harms insects that feed on the plant.¹¹

Is caffeine bad for kids?

Kids' bodies are smaller, so the same amount of caffeine has a bigger effect. Children's doctors advise against energy drinks for children and teens because of their sugar and caffeine,⁵ and they suggest skipping coffee too: caffeine raises heart rate and blood pressure in teens.¹⁰ The U.S. Dietary Guidelines say water and plain milk should be the main drinks for children and teens.⁵ Kids' health experts say kids under 12 should probably skip caffeine altogether.¹² For most healthy adults, the U.S. Food and Drug Administration cites 400 mg a day (about two to three 12-ounce cups of coffee) as an amount not generally linked to harmful effects, but people's sensitivity varies a lot.⁵ If you have questions about caffeine and your family, ask your doctor.

How much caffeine is in drinks and food?

Misconceptions

Keep exploring

Curious what happens while you sleep? See how dreams work. And where does chocolate come from? How chocolate is made. ← More explainers

FAQ