It's no secret that a coffee before bedtime can keep you awake. But now scientists know why.
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.
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Health