One of a yawn’s most important jobs appears to be to cool off the brain, which makes a lot of heat. We do yawn more when we are sleepy and brain temperature is increasing.
For many years, scientists thought that, when we were tired or bored, we probably were breathing slowly and not very deeply, so yawning was an automatic response that helped the body take in a big breath of oxygen and then exhale an overload of the waste gas, carbon dioxide. That seemed reasonable. After all, normal breathing takes in oxygen and gets rid of carbon dioxide, and a yawn is just a big breath.
But yawning doesn’t decrease when we breathe extra oxygen or increase when we breathe extra carbon dioxide. More experiments showed that the brain is warmer before a yawn and cooler afterward.
But is brain cooling the complete answer to why we yawn? It doesn't explain how yawning can be contagious: the sight of one person yawning causes others to yawn. Also, people sometimes yawn when they are feeling stress, as in the moments just before a big athletic event. So scientists have more research to do before we can fully explain yawning.
When you feel like yawning, try this experiment: cool your brain by breathing through your nose or holding something cool to your forehead. Does that reduce the urge?
Photo by Deyan Georgiev/Alamy