Is it really possible to catch a yawn? Why do we do it anyway? Weirdly, for such a common behaviour there is still much that science doesn’t know about yawns.
But let’s explore what science does know…
What exactly is a yawn?
A yawn is an involuntary breathing movement. It involves a jaw gaping wide and a deep, long breath. Then your muscles contract briefly, and then you exhale and shut your mouth.
This movement is not unique to people. Animals of all shapes and sizes yawn – not just mammals, but birds, reptiles and fish have all been observed yawning – or at least exhibiting “yawn-like behaviour”. The Siamese fighting fish, for instance, opens its mouth in little bursts that some researchers say are yawns!
In humans, a yawn lasts about 5 or 6 seconds – in other animals it can be shorter or longer. According to a 2021 study in Communications Biology, the length of the yawn is linked to brain size. The researchers analysed 1291 yawns from video footage of 101 different mammal and bird species, finding the mammals had an average yawn length of 3.40 seconds, while birds, with a tiny brain, lasted 1.46 seconds on average.
Because it’s so widespread, there’s probably a good biological reason for yawning. Right?
Why do we yawn?
In fact, we don’t yet have a completely solid answer. This is partly because it’s kind of hard to test yawns in a clinical setting. They’re involuntary, but we all know it’s possible to deliberately go through the motions of a yawn and feel similar – or stifle a yawn you feel coming on. And if you know you’re supposed to be yawning for science, is that going to affect the way you yawn?
That said, there are still a few hundred studies on yawning. That’s not enough to give us bulletproof facts, but it’s more than enough to give us some good theories.
One idea is yawning helps to cool the brain. Ambient air is usually cooler than your body, so pulling in a lot of it in a yawn might help to cool you down. The yawn also triggers blood flowing to the brain, which cools the brain further.
Read more: The reason we sigh
Another part of the puzzle is the time of day we yawn most – just before or after sleep. Yawning might play a role in the change from sleepy to alert, or in switching between other mental states.
Another theory suggests that ear pressure could have a role to play – and, of course, seeing someone else yawn might be a trigger.
One popular theory is that yawning increases our blood oxygen. This was actually debunked in a 1987 study, and since then no-one’s been able to find much evidence to support the theory.
So whatever yawning is for, it’s probably not about getting more oxygen.
Is it actually contagious?
Given “why we yawn” is a tricky question, you can bet that contagious yawning is even harder to untangle.
But in fact, the contagious yawn effect is so well-established it’s often used to induce yawning in yawning studies. So you’re not imagining things: people definitely yawn more when they’ve seen someone else do it.
We’re not the only social animal to “catch” a yawn. Playing videos of yawns to apes like chimpanzees, baboons and bonobos makes them yawn more. There’s some evidence that dogs yawn in response to humans. Interestingly, a 2009 study in Animal Cognition found that dogs don’t catch yawns from other dogs.
Even reading or thinking about yawning is likely to induce it: you’ve likely been yawning a little more while reading this article.
So why is yawning contagious?
There are a few theories for this as well, and nothing completely certain.
A 2010 review in Neuroscience & Biobehavioral Reviews lists a few different theories. One element is social: we often use yawning to indicate boredom or tiredness. If you notice someone else in a meeting seems bored, you might start to become disengaged yourself – and not feel as determined to suppress your own yawns.
But we know it goes deeper than that. MRI scans have shown that watching someone else yawn activates regions in the brain linked to imitation, empathy, and social behaviour. But none of this research is conclusive.
Another theory suggests there’s an evolutionary advantage to yawning. If one person yawns because they’re ready to go to sleep, that indicates to the rest of the group that it’s safe to sleep. Alternatively, rousing yawns can enhance a group’s vigilance or coordinate behaviour.
We’re getting into very speculative territory here. There are likely some social factors to yawning, some biological, and some neurological ones – and some that are a combination.
Ellen Phiddian is a science journalist at Cosmos. She has a BSc (Honours) in chemistry and science communication, and an MSc in science communication, both from the Australian National University.
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