There's a decent chance you'll yawn while reading this. Yawns have a strange power: see one, hear one, or even just think about one, and your own jaw starts to stretch. We do it every day, yet the humble yawn is still one of the body's little mysteries.
Here's the honest answer. Scientists don't fully agree on why we yawn. Scholars have proposed around twenty possible reasons, and one popular idea is that yawning helps cool the brain. Contagious yawning is a bit better understood: it seems to be linked to social connection, and we catch yawns most easily from the people we're closest to.
What Happens in a Yawn
A yawn is a reflex. It starts with a long breath in as the mouth slowly opens wide, peaks with a big muscle stretch, and ends with a quick breath out as everything relaxes. The whole thing usually lasts just a few seconds.
During that stretch, the airway opens up to three or four times its usual size. A tiny muscle in the middle ear tightens too, which is why you sometimes hear a rumbling sound inside your head mid-yawn. Many people stretch their arms, neck, and back at the same time. Yawning and stretching together even has a name: pandiculation.
So Why Do We Do It?
Adults yawn most often right before and after sleep and during dull activities. That's why yawns get linked with tiredness and boredom, though stress and even hunger have been connected to them too.
A long-standing belief is that we yawn to grab more oxygen. But giving people extra oxygen, or reducing carbon dioxide in the air, doesn't make them yawn less, so that explanation has a problem.
Another idea has gained attention: brain cooling. In 2007, researchers suggested yawning may help keep the brain at the right temperature. In experiments, people with cold packs on their foreheads, and people told to breathe only through their noses, yawned less while watching videos of others yawning.
Why Yawns Spread
People have noticed contagious yawning for centuries. In 1508, the scholar Erasmus wrote, “One man's yawning makes another yawn.” A French saying puts it even better: one good yawner makes seven others yawn.
One study from the University of Pisa found that social bonds were the strongest predictor of catching a yawn. People yawned most in response to family, then friends, then acquaintances, and least of all to strangers. The pattern held even when people only heard a yawn rather than saw it. Some researchers see this as a sign that contagious yawning is tied to empathy, though the debate is still open.
There may also be an old survival angle. The psychologist Gordon Gallup has suggested that if everyone in a group yawns after seeing one person yawn, the whole group may become more alert and better at spotting danger.
Not Just a Human Habit
Almost all animals with a backbone yawn, including mammals, birds, reptiles, amphibians, and even fish. Contagious yawning has been seen in chimpanzees, dogs, cats, birds, and reptiles, and it can even jump between species. Pet dogs, for example, have been shown to yawn in response to human yawns.
So if you've yawned a few times while reading this, don't take it personally. It's an ancient reflex, a little social signal, and, if the research holds up, maybe a small sign that you care about the people around you.
