Why do we yawn, and why is it contagious? (You will probably yawn reading this)

Try this: while you read this article, count how many times you yawn. Chances are it will be at least once. Yawning is one of the most common and, at the same time, most puzzling behaviors of the human body. We do it before we are even born, we share it with many animals, and it has an almost magical property: it spreads just by seeing it, hearing it or even thinking about it.

A very old gesture

A typical yawn lasts about six seconds. We open our mouth wide, take a deep breath in, stretch the muscles of the jaw and face, and finish with a shorter breath out. Sometimes it comes with watery eyes or a full-body stretch.

Ultrasound scans show that human fetuses already yawn in the womb during the first trimester of pregnancy. And we are not alone: dogs, cats, monkeys, lions, many birds and even some fish and reptiles yawn too. A behavior this widespread among vertebrates suggests it serves some important purpose, although scientists are still debating what that purpose is.

The oxygen myth

The most common explanation is that we yawn because we lack oxygen or have too much carbon dioxide in our blood. However, this idea was tested in the 1980s by the psychologist Robert Provine. In his experiments, volunteers breathed air with more oxygen or more carbon dioxide than normal, and the number of yawns did not change. Exercise, which greatly increases the body’s need for oxygen, does not make us yawn more either. So although the theory is popular, the evidence does not support it.

A fan for the brain?

One of the most studied hypotheses today suggests that yawning helps regulate brain temperature. Opening the mouth wide and stretching the jaw increases blood flow in the head, and the incoming air may exchange heat. Several studies by the researcher Andrew Gallup and his colleagues found that people yawn more when the outside temperature is moderate, and less when it is very hot (the air would not cool anything) or very cold (cooling would not be needed).

Another study, published in 2016, compared yawn duration across many mammal species and found that animals with larger brains and more neurons tend to yawn for longer. That fits the idea that yawning has something to do with brain activity, although on its own it does not prove what yawning is for.

Yawning is also linked to changes in state: we yawn when we wake up, before going to sleep, when we are bored, or at moments of transition, such as before an exam or a competition. Some researchers believe it may help «reboot» alertness.

Why it is contagious

Contagious yawning is very real. In laboratory experiments, a large share of people yawn while watching videos of other people yawning. Even hearing a yawn or reading about one can be enough, as is probably happening to you right now.

Interestingly, babies do not catch yawns. Contagious yawning usually appears around the age of four or five, when children become better at understanding what others are feeling. That is why many scientists have linked it to empathy and social bonds. Some studies found we catch yawns more easily from family and friends than from strangers, although other researchers argue the empathy link is less clear and that attention and age also play a role.

Chimpanzees and bonobos catch yawns from each other too. In dogs, some studies suggest they can catch their owners’ yawns, although the results are not entirely consistent.

When to worry

Yawning a lot usually just means we are sleepy, slept badly or are bored. Even so, doctors recommend getting checked if you suddenly start yawning excessively for no obvious reason, especially if it comes with other symptoms such as dizziness, extreme tiredness or neurological changes. Some medicines, including certain antidepressants, can also increase yawning as a side effect.

How to stop a contagious yawn

If you need to avoid yawning in a meeting, a few tricks have been explored in studies: breathing through your nose instead of your mouth, or cooling your forehead, appeared to reduce contagious yawning in experiments, which fits the temperature hypothesis. And if you have already yawned a couple of times by the end of this text, do not worry: your brain is simply doing something humans have always done.

Wheeler Cowperthwaite / Wikimedia Commons (CC BY 2.0) · source