Most animals do it. You’ve been doing it since before you were even born. And it’s quite likely looking at these photos has made you do it now. Yawning. But why do we yawn and why is yawning so contagious? Starting with Hippocrates, many great minds have pondered the mystery of yawning. There have been plenty of theories over the years and recent research has suggested a few new ones.

Is reading this post making you yawn? Yep, yawns are contagious. Images credit: Flickr 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12

Why are you yawning?

Yawning is involuntary and involves opening your mouth wide and drawing in a deep breath, followed by an exhalation. The average yawn lasts for six seconds and also stretches your eardrums. That’s why people advise yawning to pop your ears to reduce the pressure when flying or driving up or down a mountain.

Everyone agrees that yawning happens more often when we are tired. The most common explanation has always been that it is a physiological response to needing more oxygen or to get rid of built-up carbon dioxide. The idea is that yawning helps to give us a big jolt of oxygen when we are breathing shallowly and feeling drowsy as a result.

But the theory doesn’t hold up. Research published in 1987 got people to do a variety of things that manipulated the levels of oxygen and carbon dioxide in their bloodstreams. Participants in the study breathed 100% oxygen or compressed air with higher-than-normal levels of carbon dioxide. In another experiment, people exercised to the point of doubling their breathing rate, which definitely results in us needing more oxygen. But none of these things had any effect on the rate of yawning. Chuck out that theory.

Others have suggested yawning is simply a response to boredom. One experiment showed volunteers either a 30-minute rock video or a 30-minute colour bar test pattern without any audio. Unsurprisingly, people yawned more when watching the extremely boring test pattern.

But there are also plenty of examples of people yawning when they aren’t bored. For example, many Olympic athletes yawn just before competing.

It’s about being cool

In 2007, a new theory came to light. Two psychologists suggested yawning could act as a way to cool the brain. The idea is that when we yawn and stretch our jaws, more blood flows to our skull. Because we inhale at the same time, the blood going to the skull is cooler. In a study of mice, an increase in brain temperature resulted in yawning. And after yawning, the brain temperature of the mice went down again.

How about in humans? When people held a warm pack to their foreheads, they yawned 41% of the time. With a cold pack held to the forehead, yawning dropped to 9%.

The next obvious test was to see if people were more or less likely to yawn when in different temperatures. As predicted, yawning was much more common in summer than in winter.

Why would your brain be hotter than normal? When you’re sleep deprived, exhausted or stressed. Our brains function better when cooler so it makes a lot of sense we have evolved a simple way to keep things cool up there.

Have I made you yawn yet?

Hopefully you are sitting on the train or in an open-plan office right now and can do a little experiment. Let out a big yawn and watch to see how many of the people around you follow suit. Chances are, it will be about half of them.

One man’s yawning makes another yawn. Desiderius Erasmus, 1508

We’ve known for centuries yawning is contagious among humans and research has found yawning is equally contagious in other primates, for example in chimpanzees, and also in baboons.

Given how skilled your pet dog is at reading your communication, you won’t be surprised to hear dogs also ‘catch’ human yawns.

Do you feel what I feel?

Why is yawning contagious? Research in primates has suggested that contagious yawning is a form of empathy, acting to strengthen the bonds within a group. Animals that rely on complex social relationships need to be able to tune into others’ emotions, and many studies have shown links between contagious yawning and empathy. Animals that yawn together, survive together.

A study published last year showed wild wolves also yawn contagiously. The wolf study was interesting because such empathy was thought to be the domain only of humans and other primates. Perhaps empathy evolved earlier in our mammalian history than we previously thought.

One of the lines of evidence for yawning being strongly tied to empathy is the fact you are more likely to ‘catch’ a yawn from a family member or friend than from an acquaintance or stranger.

Research also shows children with autism are much less likely to yawn in response to other peoples’ yawns, perhaps because autistic children miss some of the facial cues that make yawning contagious for the rest of us.

But research in 2014 concluded that yawning has nothing to do with empathy after all and the reason we catch yawns is still a mystery.

All I know is that writing this post has made me yawn a ridiculous number of times and it feels like I need to sleep. Now.

Links and stuff

• The Yawn-O-Meter: How long can you last before yawning?
• Why do we yawn? 2014 video from AsapSCIENCE
• Why is yawning contagious? Video from VSauce

Radio on demand

 This post accompanies a radio segment on Triple R’s Breakfasters program on Wednesday 4 March 2015.