Some people can finish an evening coffee, a cola and even an energy drink and still be asleep within the hour. Others feel the effects for hours, lying awake and staring at the ceiling.

Caffeine is the most widely used psychoactive substance in the world, consumed in more than two billion cups of coffee every day. It also appears in tea, cola, energy drinks, chocolate and some medications.

It works by blocking adenosine, a substance that builds up in the brain and signals tiredness. Effects typically begin within 30 minutes and can last five hours or longer.

How the Body Breaks It Down

After a cup of coffee is drunk, caffeine reaches the bloodstream and brain quickly. The liver then metabolises it, a process that can take between six and 20 hours, according to Dr Marilyn Cornelis of Northwestern University.

Caffeine's half-life, the time needed to clear half a dose, ranges from two to 12 hours. That range helps explain why one person's cup wears off fast while another's lingers into the evening.

Dr Cornelis said a first cup may not seem to have an effect, but caffeine can still be lingering hours later. A second cup at that point may be enough to tip the balance into jitters or anxiety.

How quickly caffeine clears also depends on a gene known as CYP1A2. Some research suggests ancestry may play a role in how quickly people break caffeine down, though findings in this area are still developing.

Genetics and Lifestyle Factors

Smoking appears to speed up how quickly the liver processes caffeine, with some studies putting the increase at around 50 per cent, though vaping does not seem to have the same effect. This may explain why smokers drink more coffee, and why quitting can make a cup feel far stronger.

Certain antibiotics and contraceptive pills have the opposite effect, keeping caffeine in the body for longer. Pregnancy slows clearance further still, substantially extending caffeine's half-life as pregnancy hormones suppress the liver enzyme involved.

Dr Sergi Ferre, a brain scientist at the US National Institutes of Health, said adenosine builds back up once caffeine is removed. This is why stopping suddenly can bring on withdrawal headaches and extra sleepiness.

Falling Asleep Is Not Enough

Falling asleep quickly does not mean caffeine had no effect on sleep. It can still reduce deep, slow-wave sleep, and survey data from Sleep Foundation suggests a large majority of people who use caffeine in the afternoon go on to report a sleep problem.

What Experts Recommend

Advice on timing varies. Psychologist and sleep expert Dr Lindsay Browning suggested cutting off caffeine around six hours before bed, while other guidance recommends a minimum of eight hours for sensitive sleepers.

Dr Browning said caffeine can affect not just how quickly someone falls asleep, but the quality of that sleep too. She cautioned against a strict rule, since individual responses vary so widely.

The NHS advises pregnant women to keep caffeine intake below 200mg a day, roughly two mugs of filter coffee, due to links with low birth weight. For other adults, moderation remains the key message.

The US Food and Drug Administration considers up to 400mg a day generally safe for most adults, around four cups of coffee. Individual tolerance still varies widely, so this is not a fixed target for everyone.

Peter Rogers, emeritus professor of biological psychology at the University of Bristol, said caffeine is relatively safe overall. He noted that excessive intake tends to cause jitteriness and discomfort that prompt people to stop on their own.

Frequently Asked Questions

  • What is caffeine?
    Caffeine is a psychoactive substance found in coffee, tea, cola, energy drinks, chocolate, and some medications.
  • How does caffeine affect sleep?
    Caffeine can reduce deep, slow-wave sleep and may cause sleep problems if consumed in the afternoon.
  • How long does caffeine stay in the body?
    Caffeine's half-life ranges from two to 12 hours, depending on individual metabolism and genetic factors.
  • What factors influence caffeine metabolism?
    Genetic factors, smoking, certain medications, and pregnancy can all influence how quickly caffeine is metabolised.
  • What is the recommended caffeine intake?
    The NHS advises pregnant women to keep caffeine intake below 200mg a day, while the FDA considers up to 400mg a day generally safe for most adults.
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