Why do we yawn? It’s not because we’re tired – it helps keep our brains cool

 
Sarah Spickernell
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Even world leaders cannot escape the occasional public yawn (Source: Getty)

Work has wound down, and we’re all feeling a bit sleepy post-Christmas. It’s at times like these you might expect yawning levels to rise through the roof.

It’s a glorious feeling, which perhaps explains why seeing someone else yawn makes you want to yawn, and why you probably want to yawn just by reading this (and if you didn’t, I bet you do now).

But what is it, exactly, that makes us open our mouths as if we are about to roar, only to gasp in a little bit of air? Most people think it happens when they are bored or tired, but the evidence suggests quite the contrary – that yawning happens when we are concentrating hard or need to be particularly vigilant.

It all comes down to thermoregulation – for our brains to work quickly, they need to be at their optimum temperature. If a brain becomes too hot, yawning in some cold air helps to cool down all the blood in that area.

The theory was first put to the test by researchers from Vienna earlier this year. They studied pedestrians’ yawning patterns in Austria and the US and observed a “thermal window” for maximum yawning. When it was cold enough outside for the yawned air to cool the body down, but not so cold (or so hot) that the air would have harmful consequences, people tended to yawn.

Another experiment by researchers at the University of Albany gave similar results, and that contagious yawning could be directly influenced by the presence of hot and cold packs. When subjects of the study were shown a video of people yawning, there was a 41 per cent chance that they would yawn in reaction when holding warm packs to their heads, compared to a nine per cent chance when they held cold packs to their heads.

NOT BECAUSE OF TIREDNESS

Where did the idea that yawning happens when you’re tired come from? It is widely believed that it helps deliver a large dose of oxygen to the head, and that this wakes a person up.

There is a simple explanation for this – sleep deprivation increases the brain’s temperature, and so naturally we end up yawning more when we are tired than at other times – so there is an indirect link.

Yet this cannot be the case, since if it was really about respiration and increasing oxygen intake, we would yawn more during vigorous exercise than at any other time – something that definitely does not happen.

The brain-cooling theory also helps explain our desire to yawn when we see others yawn. When an animal yawns it is because there is some external factor increasing its need for alertness – this makes the other animals in the pack or herd become conscious of their need to also be alert, and so they react by yawning.

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