Revealed, why coffee keeps us awake

Research shows that drinking a strong coffee in the evening makes the body think it is around an hour further west than it actually is.

Some 200mg of caffeine – the amount found in some High Street coffees – slows down the body clock to such an extent that the brain is a whole time zone behind.

This, not surprisingly, means it takes longer to get to sleep. It also makes it harder to get up in the morning.

The British and US research doesn't just reinforce the advice to avoid caffeine in the evening. It also suggests that the stimulant could be used to treat jet lag.

To pin down the effect of caffeine on the body, a group of men and women lived in a windowless, clockless laboratory for seven weeks.

Some nights they were given a caffeine pill before going to bed, other nights they took a dummy drug. 

The amount of light they were exposed to at bedtime was also altered as the weeks went on.

The pills contained 200mg of caffeine. A mug of instant coffee contains around 100mg and a cup of tea has 50mg.

The average single espresso, the base of many High Street coffees, contains 80mg of caffeine. 

However, previous research has shown that many drinks in British cafes are stronger than this, with some espressos packing more than 200mg in a single shot.

The volunteers had their saliva tested regularly for levels of melatonin, a hormone is made as night draws in, making us feel sleepy.

The combination of a caffeine pill and a dimly-lit bedroom delayed the melatonin rise by 40 minutes on average – roughly the equivalent of crossing one time zone when flying west.

But in some people, the caffeine hit turned the body clock back two hours.

The overall effect was the same as being exposed to bright light for three hours when trying to get to sleep.

Researcher John O'Neill, of the Medical Research Council in Cambridge, said: 'It is perhaps not surprising that caffeine has an effect on the biological clock but that is not something that has ever been tested.

'Our findings provide a more complete explanation for why it's harder for some people to sleep if they've had a coffee in the evening – because their internal clockwork thinks they are an hour further west.'

Writing in the journal Science Translational Medicine, the researchers said that caffeine's ability to turn back the body clock means it could be used to help people flying west avoid jet lag.

However, the scientists' main aim is not help longhaul travellers sleep better but to understand more about the workings of the body clock.

A chaotic body clock raises the risk of a host of ills, from cancer and heart disease to diabetes and Alzheimer's disease. 

(dailymail.co.uk)