Yawning is an interesting phenomenon. For many people it’s almost impossible to see someone yawning without doing it themselves – in fact I bet you’ll have yawned at least once by the time you reach the end of this article! But why do we yawn and what makes it so contagious?
Nearly 2,500 years ago, the Greek physician Hippocrates suggested that we yawn in order to rid our bodies of what he called “noxious air”. He wrote that ““Like the large quantities of steam that escape from cauldrons when water boils, the accumulated air in the body is violently expelled through the mouth when the body temperature rises“. Many other scientists have come up with other ideas over the years, including the theory that yawning is contagious because it helps individuals in a social group to maintain the same body clock and have the same routine of sleeping and waking.
One of the most popular suggestions has been that we yawn when there isn’t enough oxygen in our blood; the reasoning is that by yawning we take in more air than in a normal breath and boost our oxygen levels. It sounds sensible but experiments have shown that it’s wrong; people don’t yawn any more when they’re in a low-oxygen environment that when in an oxygen-rich one. Another idea has been that we yawn when we’re bored but this has also been shown to be wrong.
So why does it happen? It seems that it may be linked to the temperature of our brains. When we’re tired or sleep deprived the temperature of our brain rises slightly. Yawning increases the heart rate and blood flow, so that warmer blood is carried away from the brain and replaced with cooler air. This cool air is also introduced into the sinus cavities, reducing the temperature even further. Several researchers have looked into the idea and discovered that people who feel the urge to yawn can stop it by holding a cold compress to their forehead, or inhaling cold air. Similarly more people yawned when holding a warm compress to their foreheads.
But why is it contagious? If this theory is correct, when one member of a group has a slightly overheated brain due to tiredness or the environment, it’s a reasonable assumption that the rest of the group probably does too. So when someone yawns it sets off an automatic response in the people around them and increases alertness is the whole group. Even thinking about yawning can set people off – I’ve lost count of how many times I’ve yawned while writing this!