Why Do We Yawn?


No one knows why we yawn. There are lots of theories, some talk about it signalling tiredness or getting oxygen to the brain, others mention clearing out stale air from the lungs and reducing blood carbon dioxide levels. Most are baloney. But, one thing that is certain, yawns can be infectious. Catch sight of someone yawning, and nine times out of ten, you will yawn yourself within a few seconds. But, that still doesn’t really answer the question, why do we yawn? Is such an infectious yawn a message to others in the group that it’s time for bed? Probably not, otherwise why yawn first thing on getting out of bed? Either way researchers have found that people with autism spectrum disorder don’t tend to succumb to an infectious yawn.

Atsushi Senju of the Centre for Brain and Cognitive Development at Birkbeck, University of London has shown for the first time that children with some degree of autism are not susceptible to contagious yawning. Autism Spectrum Disorder is a developmental disability that severely affects social interaction and communication including empathy. Report published in the August issue of the Royal Society journal Biology Letters.

This would seem an obvious result given that contagious yawning is thought to share similar cognitive and neural mechanisms as empathy.

Senju and colleagues from the University of Tokyo showed videos of people yawning or making mouth movements to 24 children with autism spectrum disorder and to 25 non-ASD children. Both groups of children yawned the same number of times while watching the video of general mouth movements, but the non-ASD children yawned more when watching the video of people yawning.

“This is the first report that a neuropsychological or psychiatric condition can selectively impair contagious yawning, sparing spontaneous yawning,” explains Senju, “Our study confirms the prediction of ’empathy theory’, by demonstrating that individuals with autism, who show atypical developments in empathy, also show selective impairment in contagious yawning.”

None of this answers the question of why do we yawn in the first place? Apparently, yawning becomes contagious at around one to two years of age, although unborn fetuses also yawn (presumably not contagiously though!) and can be triggered in animals by stimulating the hypothalamus in the brain with injected dopamine, excitatory amino acids, nitric oxide, and neuropeptides. None of this really explains why we yawn. The empathy angle perhaps points to an ancient benefit in group behaviour, but what that benefit is, science does not yet know.

For more on simple experiments and the power of yawn, check out the neuroscience for kids page at Washington U.

By the way, did you notice while reading this whether you yawned? Hopefully, it was not merely boredom that did it…

30 thoughts on “Why Do We Yawn?”

  1. Unfortunately Dennis, yawning is much more complicated than that. People yawn after yawning, people yawn even when their lungs are fully inflated, people yawn when they see other people yawn. There are confounding factors in making an attempt at a definitive statement such as yours. You might think you have the answer, but don’t you think others have already suggested and tested your hypothesis many times?

    Indeed, people yawn while exercising even though they may be pounding a treadmill, breathing heavily and get lots of oxygen into fully expanded lungs.

  2. Comment sent by email from Dennis Regan:

    “A Yawn is a Stretch of the muscles around the lungs. The same way that we stretch when we wake up and get out of bed, our lung muscles do the same thing. From the shallow breathing that we do while sleeping or getting tired our lung muscles feel the need to stretch and a yawn is an uncontrollable stretch. Have you ever tried to stop a yawn from happening? It is impossible. The lungs feel the need to stretch and they do it whether we like it or not!

    And there is your answer. Hope you and your readers find it fun and informational.”

  3. @Absta “We” vomit when “we” see someone else vomit? Speak for yourself. Not everyone reflexively vomits at the site or smell of another person’s vomit. Certainly vomiting is not anywhere near as contagious as yawning. But, what’s the “danger” that reflexive yawning is protecting us from, I’d be interested to know…

  4. We yawn when we see someone else yawn just like we vomit when we smell/see someone else’s vomit. It’s most likely a ‘just in case’ response that our body does to protect itself from possible dangers.

  5. Courney – add air pressure changes, oxygen/CO2 levels changing, air movements, temperature change, any other external stimulus that would affect everyone (and other mammals) in a shared environment…

  6. Im trying to figure out how we could explan yawning is contagious [if it is.] But I cant do that if people dont even know why we yawn. My first step would have to be this, and people dont even have a straight answer. All I really have so far is that there are many scientists that have a lot of very different theories. In some which include, tired, drowsiness, or boredem.

  7. hey guys there is a fact that you heart rate rises by 30% so maybe people yawn because their heart isn’t going fast enough for the activities they are about to do although this doesn’t answer the question if your bored but maybe people do it when they wake up for that reason.

  8. I never said there was something special about human, as opposed to yawning in other animals. If anything it seems to be an innate trait of lots of mammals, although I’m not sure whether I’ve ever seen a pigmy shrew yawn…maybe they do. It could be that yawning is simply a stretch, no more, no less, and perhaps it’s contagious because it acts as a reminder that we should all stretch. But, while dog owners may observe mutual yawn infectiousness, I certainly don’t feel the urge to stretch my back when my dog does her down-face dog posture, and that’s probably just as useful a stretch as a yawn.

  9. While you’re right there’s no way to know if Mech is right, you seem to be presuming here that yawning has a special reason in humans. To me as I suspect Mech, this doesn’t make much sense. We don’t know for sure why animals (includign humans) yawn. The teeth theory is one of them and there are a bunch of others. But since other animals do yawn, including chimps http://www.newscientist.com/article.ns?id=dn6182 there’s a good chance we yawn for the same reason. Presuming our ancerstors’ yawned it’s unlikely we evolved out of that behaviour in such a small space of time only to ‘bring it back’ for another reason. As for the dog things, actually human yawning is contagious to dogs http://news.bbc.co.uk/2/hi/science/nature/7541633.stm

  10. There was news just recently that answered the question “Why do we yawn?” with the simple answer: “to stay awake”. It’s a possible. But it still doesn’t explain why yawning is infectious.


  11. animals yawn because they are tired and humans yawn to stretch their face!!!!!!! Thanx but this website did not help me

  12. thanx for this information it hasn’t helped me as i am doing a project for a science fair. and animals yawn because they are tired so… thats my theory!

  13. Thanks for posting your theory Mech. I’m not sure why you take such an aggressive tone, or maybe I’m just reading too much between the lines, but ending your comment with the phrase “Understood? Good” smacks of fascetiousness does it not?

    As to your actual theory, there is no way that it could be proven that a yawn is as contrived a social dominance function as you suggest. It may well be, but we will never know for certain because we are not the same creatures evolutionarily speaking as we were at the time before written language, clay houses, and civilisation. Your assertion that it is an expression of “I’m tired but I will still fight” does not necessarily follow, for instance. After all, yawning is not and probably never was associated uniquely with tiredness.

    It’s funny, when I see my dog yawn, it makes me react by yawning, but I’ve never observed the opposite effect, although maybe I was too busy stretching to notice.


  14. When yawning we show our teeth. Yawning has evolved just as speech and communication, dress, technology, hunting, socializing, and everything else. When we yawn we show our teeth. Our mouths open and our teeth show (Majority of humans). This is just the evolved version of yawning though, back before civilization, clay houses, kings and queens, and written language yawning was something different. It was an expression of strength and aggression. “I’m tired, but I will still attack and fight”

    It is the same with smiling. Smiling is an evolved social skill. Before it evolved so many times, smiling was the same as growling or showing your teeth. Back before we could actually express joy and happiness in the way that we do today.

    When I yawn, someone else yawns. This is triggered by the deeply rooted mechanism. You yawn in response to growl back. When I yawn in a class room, someone else yawns (most of the time) – not everyone in the class, but at least 1-4 people will yawn back in response. Usually the strongest, or usually the weakest. The weak yawn in response in defense. The strong yawn in response out of dominance.

    And that is why we yawn.

    It has evolved, that is all. We yawn when are tired.

    Back then, if we were tired, a yawn was a “I’m tired, but I will fight” signal.

    If you are tired or exhausted you yawn. You show your teeth. You instinctively show that you are tired but will still fight, defend, claim, and aggress.

    Understand? Good.

  15. News just in of the dangers of yawning too widely Huge yawn locks jaw, chokes man

    Meanwhile, is a pet’s yawn contagious, has anyone noticed? Do you yawn when your pet yawns or vice versa? Couldn’t the answer to why do we yawn be nothing more complicated than the fact that it’s a good way to stretch and stretching is an almost essential activity for all mammals?

  16. Never mind why we yawn, Andrea Anderson reveals a secret way to check out who’s watching you in a crowded room. The theory goes that if you yawn, anyone who is surreptitiously watching you won’t be able to resist yawning too, because it’s contagious, don’t you know?. This gives you an idea of who’s been watching and if you’re on the pull, according to Andrea, marks them out as a susceptible target for wooing. Of course, if it’s a boring lecture you may just find that everyone else is yawning and what if someone is playing this mind game on you and you yawn at just the wrong time you could end up being “wooed” unwittingly yourself.

  17. Before anyone asks, yes, that is my dog, I’ll record her yawning and add it as an mp3 extra to the post if someone requests it…the site of her yawn not only beggars the question why do we yawn, but why do other animals yawn?

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