Caffeine: energy, sleep and safety

Where does caffeine come from?

Coffee and tea naturally contain caffeine. It can also be found in chocolate, energy drinks, pre-workout products, some medicines and supplements. Guarana and other ingredients can add caffeine too, even when a product is labelled “natural.”

Amounts vary by product and serving. Decaffeinated coffee is not necessarily caffeine-free. Check the label and serving size, but do not assume a product contains none just because a number is missing.2

Does caffeine improve exercise performance?

The US National Institutes of Health describes evidence that caffeine can help some endurance and intermittent activities and reduce perceived fatigue. Responses vary and it does not help everyone. More is not automatically better: unwanted effects or disturbed sleep may offset a perceived benefit.

A supplement blend may contain other active ingredients. Evidence about caffeine alone does not establish the benefit or safety of the whole product. Compare creatine and its different evidence.4

Can caffeine affect sleep many hours later?

A 2025 randomised crossover study analysed 23 healthy young men who received a placebo or single amounts of 100 mg or 400 mg of caffeine at different times before bed. These were study amounts, not recommendations. The larger amount affected sleep even when taken 12 hours before bedtime. The smaller amount did not show a statistically significant effect in that group at the times tested.3

This does not establish a safe cutoff for everyone. The study was small and all participants were male; individual sensitivity, usual intake and the amount consumed matter. Persistent sleep problems deserve assessment. See melatonin and sleep timing.

Does coffee cause an irregular heartbeat?

Caffeine can cause palpitations—a noticeable or racing heartbeat—in some people. Different heart rhythms and products need different evidence.

In the 2023 CRAVE trial, 100 adults were randomly instructed to drink coffee or avoid caffeine on different days over two weeks. The main comparison found no statistically significant difference in early beats from the upper heart chambers. A secondary analysis found more early beats from the lower chambers on coffee-assigned days. An extra beat is not the same diagnosis as atrial fibrillation, and this short study cannot establish long-term outcomes.

In the 2026 DECAF trial, 200 current or former coffee drinkers with atrial fibrillation or related flutter after a procedure to restore heart rhythm were randomly assigned caffeinated coffee or avoidance for six months. The rhythm problem was detected again in 47% of the coffee group and 64% of the avoidance group. Participants knew their assignment. The results do not establish coffee as a treatment or automatically apply to other heart conditions, energy drinks or concentrated caffeine. The authors reported public and charitable investigator funding and disclosed several industry relationships.

Discuss recurring palpitations or a known heart condition with a clinician. The NHS advises emergency help for palpitations that do not go away or occur with chest pain, shortness of breath, feeling faint or fainting.678

How much is too much?

The US FDA cites 400 mg per day as an amount not generally associated with negative effects for most adults, while emphasising that people vary substantially. It is not a goal or guarantee of safety. UK NHS guidance advises no more than 200 mg a day during pregnancy, counting drinks, chocolate and supplements. Children, health conditions, medicines and individual sensitivity require particular care. Ask a clinician or pharmacist about your circumstances.

The FDA notes that medical experts advise against energy drinks for children and teenagers because of their caffeine and sugar. Discuss a child’s caffeine intake with a healthcare professional.

Caffeine can pass into breast milk and may make a baby restless or disrupt sleep; sensitivity varies. One NHS breastfeeding page advises no more than 200 mg a day and suggests reducing intake particularly when a baby is under six months old. Advice differs between sources, so discuss your circumstances with a clinician or health visitor.125

What signs and product risks matter?

Too much caffeine can cause a racing heart, palpitations, anxiety, nausea and insomnia. Concentrated powders or liquids can be particularly dangerous: a small measuring error may deliver a very large amount. Seek urgent medical or poison-service advice for suspected excessive exposure or serious symptoms.

Withdrawal can be unpleasant when reducing regular intake; the FDA notes that reducing gradually may help. Read how to read a supplement label and check medicine interactions.2