Why caffeine makes some people buzz - and others barely notice

A significant story is unfolding on the international scene. Why caffeine makes some people buzz - and others barely noticeImage caption, Emily, out with her friends in London, says that coffee and caffeinated energy drinks don't have much effect on body or her sleepByJim ReedHealth reporterPublished33 minutes ago"Coffee just doesn't have much effect on me," says 20-year-old Emily, who is leaving a cafe in central London at 18:00 BST. "I'll normally have two cups a day and then maybe an energy drink too. And then I can drink coke all night and feel completely fine.

"We've all got a friend like Emily. The one who can knock back a double espresso or even an energy drink in the evening and still drift off to sleep an hour or so later. For many people, though, the effects of caffeine can linger for far longer.

The Details

Why then can some people drink coffee before bed and sleep like a baby, while others lie awake staring at the ceiling? What exactly is caffeine? Caffeine is by far the most used psychoactive substance in the world.

More than two billion cups of coffee are drunk each day and that's before we start talking about cola, energy drinks, plus some chocolate, chewing gum and sports supplements, which also contain the chemical. It can make us feel more alert, improve concentration and stave off tiredness. Too much can cause headaches, jitters, a racing heart and restless nights.

Caffeine doesn't give your body extra energy, it helps mask tiredness, experts say. It does this by blocking the action of adenosine, a chemical that builds up in the brain throughout the day and makes us feel sleepy. Why does it affect people differently?

What Experts Say

One key factor is how long caffeine stays in the body. After drinking a cup of coffee, it is absorbed into the bloodstream, where it quickly reaches the brain. All this time it's also being broken down – or metabolised – by an enzyme in the liver and then flushed out in our urine.

That clean-up process can take between six and 20 hours depending on the individual, says Dr Marilyn Cornelis, an associate professor of nutrition at Northwestern University in Chicago. "It's possible to have a first cup of coffee and maybe not feel the effects," she says. "But then two or three hours later, the caffeine is still lingering in the system, and, if you add a second cup, that might be enough to tip the scales and you start feeling a little anxious.

" Figure caption, Why caffeine makes some people buzzHow quickly the liver can processes the caffeine depends on a single gene – known as CYP1A2. Intriguingly it might be influenced, at least to some extent, by your ancestry. A study published this year, external found that Europeans and South Americans tend to break down caffeine more quickly on average.

The development has drawn wide international attention, with diplomatic circles watching closely.