This post is also available in Dutch.
Odds are, you’ve yawned a couple of times today; everyone yawns every day. But why do we do it? Some researchers think we yawn to cool our brains. By yawning, warm blood would flow away from the skull and would be replaced by fresher, cooler blood. A fascinating idea, but how do I find out if it’s true?
First, there are a couple of remarkable findings that seem to fit this idea. People who suffer from illnesses where they struggle with keeping up their temperature yawn abnormally often. And the warmer people are, the sleepier they become, and the more they yawn. It could be that you’re not yawning because you’re sleepy but because your brain is warm. Such correlations show that temperature and yawning are related to each other, but we cannot use them to conclude that we yawn in order to cool our brains. For example, in the case of illness, it might be that a brain area is affected which has multiple functions: yawning and regulating temperature.
Do you yawn more if your brain is warmer?
A better method is to show that people yawn less if their brains are cooler. And indeed: if people press an ice pack of 4 degrees Celsius against their forehead, they yawn less than if the ice pack is room temperature. If the ice pack is pressed against their carotid arteries (that bring blood to your head), people yawn less than if the ice pack is room temperature. We don’t know for sure that the brain’s temperature actually changes by pressing an ice pack against your forehead or neck, but it does show that temperature plays a role in this process.
Does your brain cool down if you yawn?
Imagine that you yawn when it’s warm, but your brain doesn’t cool down. Then it’s clear that something else must be going on. It’s therefore important to know if your brain actually cools down if you yawn. The temperature of the brains of rats can be measured by implanting a miniature thermometer in their brains. As it turns out, in the run-up to a yawn, the brain’s temperature rises, and after the yawn, it decreases again. Even this evidence does not yet fully convince me: a relation in time does not imply causality! We don’t know what else happened in that time, so we can’t say that the brain cooled down because of the yawn. Ideally, we researchers would decide the exact moment someone yawns and then directly measure the effects.
There are quite a few pieces of evidence for the idea that we yawn to cool our brains, but it’s not yet completely conclusive. Anyway, that probably would not be the only function of yawning. We know for example that yawning also has a social function: because it’s so contagious, yawning could cause the behaviour of a group to synchronise. At least to me, it’s a calming thought that I might not have yawned so much during lockdown because I was bored but because I needed to keep a cool head in such trying times 😉