While fatigue, sleep deprivation or boredom easily bring on yawns, scientists are discovering there's more to yawning than most people think. Some research even suggests that brain and body temperature can prompt you to yawn.
We don't know much about why we yawn or if it serves any useful function, and there is very little basic medical research on the subject. It is also not taught in medical school because of a lack of physiological significance. However, there are several theories about why we yawn. Here are the four most common.
The Physiological Theory
Yawning helps to draw in more oxygen or remove a buildup of carbon dioxide. This theory helps explain why we yawn in groups. Larger groups produce more carbon dioxide, which means our bodies would act to draw in more oxygen and get rid of the excess CO2.
However, if our bodies make us yawn to draw in needed oxygen, wouldn't we yawn during exercise? Robert Provine, a developmental neuroscientist at the University of Maryland, Baltimore County, and a leading expert on yawning, has tested this theory: Giving people additional oxygen didn't decrease yawning, and decreasing the amount of carbon dioxide in a subject's environment also didn't prevent yawning [source: University of Washington].
The Evolution Theory
Some think that yawning began with our ancestors, who used yawning to show their teeth and intimidate others. An offshoot of this theory is the idea that yawning developed from early man as a signal for us to change activities [source: University of Washington].
The Boredom Theory
Although we do tend to yawn when bored or tired, this theory doesn't explain why Olympic athletes yawn right before they compete in their event or why dogs tend to yawn just before they attack. It's doubtful either is bored [source: Patterson].
The Brain-cooling Theory
More recently, researchers proposed another theory: Since people yawn more in situations where their brains are likely to be warmer — tested by having some subjects breathe through their noses or press hot or cold packs to their foreheads — it's a brain cooling mechanism.
Why does brain temperature even matter? Cool brains can think more clearly; hence, yawning cools our brains and might have developed to keep us alert [source: Nagourney].
So now that we have an idea of what causes yawning, let's look at why seeing someone yawn might make you yawn, too.