Why caffeine makes some people buzz - and others barely notice

Sleep researchers often recommend avoiding caffeine for at least six hours before bedtime.

Dr Lindsay Browning, a psychologist and sleep expert, says the evidence suggests caffeine might not just stop you dozing off but can affect the quality of sleep as well.

"I'll suggest to people that they might want to avoid caffeine after about 2pm if they are going to bed around 10pm," she says.

"But a blanket hard-and-fast rule isn't helpful, because it doesn't really allow for the intricacies and the individualisation of what's really going on."

The NHS advises pregnant women, external to keep caffeine intake below 200mg a day, about two mugs of filter coffee, as studies have linked it to low birth weight and other complications.

For other adults the key message is moderation, as sensitivity to caffeine varies so much from person to person.

"Caffeine is relatively safe," says Peter Rogers, emeritus professor of nutrition at the University of Bristol.

"If you consume a great deal of it, you just start feeling jittery and unpleasant and you start realising, 'I need to stop this, there's no point taking any more'."

Caffeine overdoses are very rare and severe poisonings almost always involve concentrated caffeine products, where several grams can be swallowed in a matter of minutes.

For coffee lovers the good news is that, if anything, that regular (if expensive) morning habit is linked to better health outcomes.

While researchers cannot prove that coffee is the only reason, decades of studies suggest that people who drink it in moderation tend to have lower risks of several major diseases and may even live slightly longer than non-drinkers.