The reason behind why we yawn has been a long-standing biological mystery. The popular thinking as to why we yawn is that we’re tired, and our brains need the extra oxygen that is drawn into our system during the yawn in order to stay awake. But now medical science has found that’s not exactly how it works.
Yawning does impact the brain, but it’s not the rush of oxygen that matters. When you yawn, you’re inhaling more than oxygen, you’re inhaling air that’s cooler than your body temperature. That air gets funneled into your sinuses, which then introduces cooler air to the brain. Yawning is essentially a cooling system for your head.
There have been a number of different theories on why we yawn, but this is the only one that stands up to scientific scrutiny. It also answers the question of the point of our sinuses, besides giving us headaches.
It turns out that the air intake and the physical act of yawning all work together. It isn’t just enough to get the rush of air to lower the temperature of our brains. Opening your mouth to yawn makes the walls of the sinus flex, and they act as bellows to push the air through the cavities in your head.
The theory has been backed up in medical research conducted on rats. Researchers have monitored their brain temperature in conjunction with their yawning, and have confirmed that whenever there is a jump in temperature, the rats will yawn to help lower it. People who have the ability to predict their yawns have also participated in studies that confirmed the hypothesis works in humans, too.
That also raises the question of why we have the overwhelming urge to yawn when we see someone else do it. After all, brain temperature can’t be contagious, can it?
The answer seems to be a vague “Sort of.”
Our brains function most efficiently when they’re at a yawn-cooled temperature. It’s why we might yawn when we first get up in the morning, to help speed the waking process. But it’s also why we might be hard-wired to see yawning as a signal that there’s some reason you need to be awake and functioning at your best. When you see someone yawn, that’s a sign that there’s danger, that something’s approaching, or there’s some other reason that you need to be at the top of your game.
And since yawning is silent, for all but the most dramatic among us, that also works with the theory that yawning is a type of in-the-face-of-danger communication.
Interestingly, chimpanzees are the only other animal that has demonstrated contagious yawning. And while they’re frequently afflicted with it, the idea that humans are all subject to yawning when we see someone else do it is also a myth. In fact, only about half of the human population can’t resist joining in when they see a yawn.