By GWYNETH REES

Last updated at 00:32 04 July 2007

It can catch you embarrassingly unawares - as Cherie Blair has discovered.

But rather than being a precursor to sleep, scientists say the yawn is actually designed to keep you awake.

A study has found that when you yawn, the inhaled air reduces the temperature of vessels in the nasal cavity, allowing cooled blood to be sent to the brain.

This chills the brain, making it more alert and able to perform better.

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Those who breathe in through the nose instead of the mouth are much less likely to yawn, as the vessels in the nasal cavity are already cooled.

To avoid embarrassment and quell a fit of the yawns, you can adopt

this method or apply something cold to the forehead, the researchers said - advice that might have been welcomed by the former prime minister's wife.

Mrs Blair failed to suppress a very public yawn during the closing ceremony of last year's Commonwealth Games in Melbourne.

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The scientists believe that 'contagious' yawning - where one immediately mimics another's yawn - is an evolved protective mechanism to make a group more alert.

Their team, from New York State University, spent months watching 44 students, examining why they yawned and what happened when they did.

They found that the physical mechanism of yawning cooled the brain - making it operate more efficiently. Leading researcher Dr Gordon Gallup said: 'According to our hypothesis, rather than promoting sleep, yawning should antagonise sleep.

'We think contagious yawning is triggered by empathic mechanisms which function to maintain group vigilance.'

For the study, volunteers were shown hours of film clips of men and women yawning. The number of 'contagious' yawns they made were recorded.

While watching the films, volunteers were told to breathe in and out in one of four ways - strictly orally, strictly nasally, orally while wearing a nose plug, or just normally.

Fifty per cent of those breathing normally or through their mouths yawned while watching the films. But none of those breathing through their nose was affected.

Those who held a cold pack to their forehead - again cooling the vessels - also avoided yawning, but those who held a warm pack to their forehead were not so lucky.

Researcher Robert Provine, from the University of Maryland in Baltimore, said yawning could mark the body's readiness to become alert. 'Paratroopers report yawning before they jump.

' Yawning signals a transition between the behavioural states of wakefulness and sleepiness, and boredom to alertness.'

Meanwhile, a team at Kyoto University in Japan has discovered that chimpanzees also suffer from contagious yawning.

The scientists believe their findings provide further evidence that apes may possess an advanced level of self-awareness, similar to that found in humans.