We most associate yawning with boredom or being sleepy, but new research suggests it can be good for your health - by cooling down your brain.
Scientists at Princeton University found a big yawn can regulate the temperature of the brain and prevent over-heating.
During winter in Tuscon, Arizona, Professor Andrew Gallup and his team asked 80 random pedestrians to look at images of people yawning and then recorded whether they yawned in response.
They then performed the same trial in the summer.
The researchers found that half of the participants yawned in winter while only a quarter yawned in summer.
From this they reasoned that yawning cools the brain, which at first seems counter-intuitive. Surely you would want to cool the brain by yawning more in summer?
But according to the theory, yawning cools the brain via a heat exchange with cooler air drawn in during the process.
This system therefore wouldn't work on a scorching summer's day.
Professor Gallup said: 'Yawning should be counterproductive - and therefore suppressed - in ambient temperatures exceeding body temperature because taking a deep inhalation of air would not promote cooling.
'There should be a 'thermal window' or a relatively narrow range of ambient temperatures in which to expect highest rates of yawning.'
He added that their results were consistent with the theory, even when accounting for humidity, time spent outside and the amount of sleep enjoyed the night before.
'Nearly 40 per cent of participants (both in summer and winter) yawned within the first five minutes outside, but the percentage of summertime yawners quickly dropped to less than 10 per cent afterwards. An inverse effect was observed in the winter,' Professor Gallup said.
'This is the first report to show that yawning frequency varies from season to season.'
It could help explain why humans become confused and disorientated in extreme heat as the brain has limited ways of cooling itself down.
The scientists said that the research adds to our basic physiological knowledge and give a better understanding of conditions such as motor neurone disease or epilepsy, where yawning is a common feature.
The research is published in the journal Frontiers in Evolutionary Neuroscience.