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.
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