Why Do We Yawn?

Yawning is an involuntary action that causes us to open our mouths wide and take a deep breath. We know it's involuntary because we do it even before we're born: According to Robert Provine, a developmental neuroscientist at the University of Maryland, Baltimore County, research has shown that 11-week-old fetuses yawn.

And while we typically associate yawning with relaxation and drowsiness, your heart rate can rise as much as 30 percent during a yawn. Yawning is also a sign of arousal, including sexual arousal [sources: Alexander, The Stress of Life].

Many parts of the body are in action when you yawn. First, your mouth opens, and your jaw drops, allowing as much air as possible to be taken in. When you inhale, the air taken in is filling your lungs. Your abdominal muscles flex, and your diaphragm is pushed down. The air you breathe in expands the lungs to capacity and then some of the air is blown back out.

Animals Yawn, Too

Humans are not the only ones who yawn. Almost all vertebrates yawn, and some might even start yawning after they see a person yawn.

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.

Interestingly, while all vertebrates (including fish) yawn, only humans, chimps and possibly dogs find yawns contagious. And people don't find them contagious until they're about 4 years old. Recent studies show yawn contagion might tie to one's capacity for empathy [source: Sohn].

In one study, researchers showed autistic and allistic children videos of people yawning and people simply moving their mouths. Both groups of kids yawned the same amount when viewing the video of people moving their mouths. But the allistic kids yawned much more frequently than those with autism spectrum disorder when watching people really yawning.

Since autism is a disorder that can affect a person's social interaction skills, including the ability to empathize with others, the autistic kids' lack of yawning when watching others do so could indicate they're less empathetic. The study also found the more severe a child's autism, the less likely they were to yawn.

On a positive note, someday doctors may be able to diagnose cognitive disabilities in young children more easily by seeing whether or not they can catch a yawn from others [sources: Kahn, Sohn].

So even though we still don't know for sure why we yawn, we do know lots of interesting things about yawning: You start yawning in utero, you yawn when you're aroused, more than half of you will yawn if you see someone else yawn and reading about yawning makes you yawn [source: Sohn].

So, how many times did you yawn while reading this article? We hope not too many.