Caffeine

/ˈkæf.iːn/ · KAF-een noun

A naturally occurring stimulant found in coffee beans, tea leaves, cocoa and some other plants. It works mainly by blocking the action of adenosine, a substance that accumulates in the brain during waking hours and promotes sleepiness.

What it is and how it works

From the moment you wake up, a substance called adenosine slowly builds up in the brain. The more there is, the sleepier you feel. This is part of the sleep pressure described under sleep hygiene. Caffeine has a similar shape to adenosine and sits in the same receptors without activating them, a bit like a key that fits a lock but does not turn it. Adenosine cannot do its job, so the feeling of sleepiness is muted.

This is why many people notice they feel more awake after coffee. It is also why caffeine does not remove tiredness: the adenosine is still there, and when the caffeine wears off, the sleepiness returns.

Cup of black coffee on a wooden desk next to a closed laptop and a notebook — A cup of black coffee on a desk. Most adults get their caffeine from coffee or tea.

How long it lasts

Caffeine is absorbed within about 45 minutes. Its half-life, the time it takes the body to clear half of it, is typically around five hours in healthy adults, but varies from about two to ten hours. Genetics, pregnancy, smoking, liver function and some medications all affect it. A coffee at 4 pm can therefore still be partly active at bedtime, which is why caffeine is often mentioned alongside the circadian rhythm.

Where it is found

Amounts vary widely with the product and how it is made. As rough orders of magnitude:

a cup of filter coffee: around 90 mg,
a single espresso: around 60 to 80 mg,
a cup of black tea: around 40 to 50 mg,
a cup of green tea: around 30 mg,
a can of energy drink (250 ml): around 80 mg,
a can of cola (330 ml): around 30 to 40 mg,
dark chocolate: a small amount per square.

How much is generally considered safe

The European Food Safety Authority has concluded that, for healthy adults, intakes of up to 400 mg a day spread over the day, and single doses of up to 200 mg, do not raise safety concerns. For pregnant and breastfeeding women, the reference is up to 200 mg a day. Children and teenagers are more sensitive, and energy drinks are not recommended for them.

People with heart rhythm problems, anxiety, high blood pressure, or who take certain medicines may be advised to limit caffeine by their doctor.

In everyday life

Mind the clock: many people sleep better with no caffeine from early afternoon onwards.
Watch hidden sources: energy drinks, some soft drinks, pre-workout products and some painkillers contain caffeine.
Tea and coffee still hydrate: in moderate amounts they count towards daily fluids; see hydration.
Separate from iron-rich meals if iron is a concern, because tea and coffee reduce non-haem iron absorption; see iron.
Cutting back? Reduce gradually to avoid headaches and irritability.

Common confusions

"Coffee dehydrates you." At usual intakes, the fluid in coffee more than makes up for its mild effect on urine output.

"Caffeine replaces sleep." It masks sleepiness for a while but does not provide what sleep provides.

"Decaf has no caffeine." It has much less, typically a few milligrams per cup, but not none.

"Caffeine affects everyone the same way." Sensitivity varies a lot. Jitters, a racing pulse or a higher resting heart rate are signs to cut down.

Added Oct 14, 2026 · Filed under Rest & recovery