Why caffeine makes some people buzz - and others barely notice

Image caption, Emily, out with her friends in London, says that coffee and caffeinated energy drinks don't have much effect on body or her sleep

"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.

Why then can some people drink coffee before bed and sleep like a baby, while others lie awake staring at the ceiling?