In brief. Caffeine temporarily blocks adenosine, a substance that builds sleepiness throughout the day. It is absorbed quickly and has a half-life of around three to five hours in adults, although this varies considerably from person to person. EFSA considers an intake of up to around 400 mg a day from all sources and around 200 mg in a single dose to raise no safety concerns for healthy adults; during pregnancy, the reference level is around 200 mg a day. These are reference levels for safe intake, not recommendations to consume that amount. This is information, not medical advice.
How does caffeine work in the brain?
Throughout the day, a substance called adenosine builds up in the brain. As adenosine binds to its receptors, you gradually feel sleepier. Caffeine closely resembles adenosine in shape, so it can temporarily fit into the same receptors without activating them. In this way, caffeine temporarily prevents adenosine from binding to its receptors. Caffeine does not create new energy; it mainly postpones the feeling of fatigue. Once the caffeine has been broken down, the accumulated adenosine can still do its job.
Under European rules, certain descriptive claims about caffeine and alertness or concentration may be used only under strict conditions, including a minimum amount of 75 mg per serving and specific label wording. That is why we mainly explain how caffeine works here, instead of turning it into a promise. Caffeine is not a remedy for fatigue as a medical condition or for any disease.
How quickly is caffeine absorbed and how long does it remain active?
After consumption, caffeine is absorbed quickly and generally reaches its highest concentration in the blood within around thirty to sixty minutes. Its half-life, the time the body takes to break down about half of it, is often around three to five hours in healthy adults. This varies considerably from person to person and depends on factors such as genetics, liver function, smoking, certain medicines and pregnancy. As a result, the same cup of coffee can keep one person awake for hours while appearing to have hardly any effect on someone else.
What are tolerance and habituation?
With regular use, the body can adapt, a phenomenon known as tolerance. The alert feeling you noticed with your first cup of coffee may become less pronounced over time, partly because the body adjusts the number of adenosine receptors. People who consume caffeine every day and suddenly stop may experience temporary withdrawal symptoms, such as headaches, irritability and fatigue. These symptoms are usually mild and generally disappear within a few days. Gradually reducing your intake often helps limit them.
How much caffeine is too much?
The European Food Safety Authority (EFSA) has assessed the safety of caffeine and published reference levels. For healthy adults, EFSA considers an intake of up to around 400 mg a day from all sources combined to raise no safety concerns. A single dose of up to around 200 mg is likewise considered safe for healthy adults. It is important to understand that these are reference levels for safe intake, not targets.
| Situation | EFSA reference level for safe intake |
|---|---|
| Healthy adult, per day (all sources) | Up to around 400 mg raises no safety concerns |
| Healthy adult, single dose | Up to around 200 mg raises no safety concerns |
| During pregnancy, per day | Up to around 200 mg as a reference level |
Caffeine is found not only in coffee, but also in tea, cocoa, cola, energy drinks and some supplements. Add all these sources together when estimating your total intake. Higher amounts can cause symptoms such as restlessness, palpitations, nervousness and disturbed sleep.
Caffeine, timing and sleep
Because caffeine can remain active for hours, it may disturb sleep when consumed late in the day. Many people find that caffeine in the afternoon or evening makes it harder to fall asleep or results in less deep sleep, even if they do not feel distinctly wide awake. A common practical approach is to plan caffeine earlier in the day and limit it later on. Sensitivity varies from person to person and partly depends on how quickly your body breaks caffeine down.
Responsible use and who should limit caffeine
Many healthy adults tolerate moderate amounts of caffeine well, but not everyone responds in the same way. Some people are naturally more sensitive and notice restlessness or palpitations even with small amounts. People who are pregnant or breastfeeding are advised to limit their intake; EFSA gives around 200 mg a day as the reference level during pregnancy. People with cardiovascular complaints, high blood pressure, an anxiety disorder, sleep problems or certain stomach complaints may also benefit from caution. Caffeine can also interact with some medicines. Do not exceed the recommended dose of caffeine-containing supplements, and take caffeine from other sources on the same day into account. If in doubt, or if you have a medical condition or use medication, speak to a doctor or pharmacist first. Do not give caffeine-containing products to children without professional advice. Supplements are no substitute for adequate sleep, a varied diet or medical care.
Caffeine and L-theanine: what do we know?
Caffeine is sometimes combined with L-theanine, an amino acid naturally found in tea. This combination is popular and is being studied in preliminary research, but there is no authorised health claim for L-theanine in the European Union. We therefore describe the combination solely in factual terms and make no promises about it. Our own formulation with caffeine and L-theanine is described alongside our FocusFuel with caffeine, L-theanine, B6 and B1. Whether a combination is suitable for you depends on your sensitivity, your total caffeine intake and your personal preference.
Frequently asked questions
How much caffeine per day is safe?
For healthy adults, EFSA considers an intake of up to around 400 mg a day from all sources combined to raise no safety concerns, along with a single dose of up to around 200 mg. These are reference levels, not targets. During pregnancy, around 200 mg a day is the reference level. Add all sources together and take your own sensitivity into account.
How long does caffeine stay in your body?
The half-life is often around three to five hours in healthy adults, but this varies considerably from person to person. Factors such as genetics, smoking, certain medicines and pregnancy can speed up or slow down its breakdown. Caffeine may therefore remain active much longer in one person than in another.
What is caffeine?
Caffeine does not create new energy. Adenosine is a signalling molecule involved in sleep regulation, and caffeine acts as an antagonist at adenosine receptors. Caffeine is no substitute for adequate sleep.
Why does caffeine sometimes disturb sleep?
Because caffeine can remain active for hours, it may disturb sleep when consumed late in the day. Many people then find it harder to fall asleep or sleep less deeply. Planning caffeine earlier in the day and limiting it later is common.
Who should limit caffeine?
People who are pregnant or breastfeeding, people who are naturally sensitive, and people with certain conditions such as cardiovascular complaints, an anxiety disorder or sleep problems may benefit from caution. Caffeine can also interact with some medicines. If in doubt, speak to a doctor or pharmacist.
Sources
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on the safety of caffeine. EFSA Journal. 2015;13(5):4102.
- EU Register of nutrition and health claims made on foods (claims concerning caffeine and alertness or concentration, subject to conditions). Regulation (EU) No 432/2012. ec.europa.eu
- Regulation (EC) No 1924/2006 on nutrition and health claims made on foods.
- EFSA. Scientific Opinions on the substantiation of health claims related to caffeine (alertness and attention). EFSA Journal.
Written by the NOTFORTOMORROW editorial team. How we research: we base factual statements on EFSA's scientific advice and the official EU Register of health claims, cite our sources and date our reviews. This article provides information, not medical advice; consult a qualified professional about your circumstances. Last reviewed: 2026-06-06.
