Why do we yawn? It’s almost certainly not for the reason you think | Human biology

AAll vertebrates yawn or engage in behavior at least recognizable as adjacent to yawning. Sociable baboons yawn, as do semi-solitary orangutans. Parakeets, penguins and crocodiles yawn – as does probably the very first jawed fish. Until relatively recently, the purpose of yawning was unclear, and it is still disputed by researchers and scientists. But this commonality gives some idea of ​​what it’s really about – and it’s probably not what you’d expect.

“When I poll the public and ask, ‘Why You do you think we’re yawning?’, most people suggest that it has to do with breathing and might somehow increase oxygen in the blood,” says Andrew Gallup, a professor of behavioral biology at Johns Hopkins University. “And that’s intuitive because most yawns have this clear respiratory component, this deep inhalation of air. However, what most people don’t realize is that this hypothesis has been explicitly tested and found to be false.

To test the idea that we yawn to bring in more oxygen or expel excess carbon dioxide, studies published in the 1980s manipulated the levels of these two gases in the air inhaled by volunteers – and found that although the changes did significantly affect other respiratory processes, they do not influence the regularity of yawning. There also appears to be no consistently measurable difference in the yawning behavior of people with diseases associated with breathing and lung function – which is what one would expect if yawns were related to breathing.

That’s more or less where Gallup took up the subject. “When I was pursuing my honors thesis, my supervisor at the time said to me: why not study yawning, because no one knows why we do it?” he said. “It was intriguing – we knew it must serve an underlying physiological function. So I started looking at the pattern of motor action that this involves – this prolonged jaw spreading that is accompanied by a deep inhalation of air, followed by a rapid jaw closing and a more rapid exhalation. And it occurred to me that this probably had important circulatory consequences localized to the skull. “

In fact, that seems to be exactly what’s happening: Several reviews of the medical literature suggest that yawning increases arterial blood supply to the skull and then venous return (the rate at which blood flows back from the head to the heart).

“We can think of opening the jaw as a localized stretch, similar to stretching muscles in other parts of the body,” says Gallup. “In the same way that stretching promotes circulation to these extremities, yawning appears to have the same effect on the skull.”

From there, Gallup and his fellow researchers began to develop the idea that yawning helps regulate heat in and around the skull. The temperature of your brain is primarily determined by three variables – the speed of arterial blood moving to the brain, the temperature of that blood, and the metabolic heat production that occurs in the brain, based on neuronal activity – and yawning, in theory, can change the first two. When you yawn, you inhale a deep breath that travels over the moist surfaces of your mouth, tongue, and nasal passages, much like air flowing through a car radiator – carrying away heat through evaporation and convection.

Studies seem to confirm it: room temperature has a fairly predictable effect on the frequency of yawning, which increases when it’s just slightly too toasty (when it’s very hot, the air temperature is too high for the radiator effect to work, so other cooling mechanisms such as sweating come into play and yawning sets in again) and drops when it gets colder.

This also seems to explain why certain medical conditions are associated with excessive yawning: either the conditions themselves or the medications used to treat them cause a rise in brain or body temperature. The “neuronal activity” explanation is also supported by animal studies: mammals and birds with more neurons in their brains yawn for longer durations, regardless of the actual size of their brains.

This does not mean that other hypotheses have been entirely ruled out. The theory best supported by the evidence is the “arousal shift” theory: basically, yawning helps the brain transition from one state to another – from sleep to wakefulness, from boredom to alertness, etc. “One possibility is that yawning helps the brain switch between using its ‘default mode network’ – the regions associated with daydreaming, memory recall and self-reflection – and the attentional network responsible for preparing the body for action,” explains medical historian Dr Olivier Walusinski, author of several articles on the subject. “A proposed mechanism for this would be that it helps with the circulation of cerebrospinal fluid, the fluid that surrounds and cushions your brain and spinal cord.”

It may in fact be that this function evolved first, with the thermoregulatory effect emerging as a useful side effect: something we will have better evidence for as studies are carried out in a wider range of animals. It could also be that the two explanations are directly related: These state changes likely mean changes in brain activity and temperature, meaning a need for improved blood flow. And neuronal cooling. This would explain why you yawn when you’re bored: your brain’s activity level may increase as it begins to think of ways to put you in a more stimulating situation, just like its circulatory needs.

But wait a minute: what about contagious yawn? We are all aware of the phenomenon where one person in a room – or even on a television screen – takes a gulp of air, only for everyone else to do the same. Some researchers have suggested that this type of infectious behavior brings groups together, perhaps because it is a difficult-to-fake signal of sleepiness, boredom, or alertness – although this is unlikely to be the primary purpose of yawning, as many solitary animals yawn regularly.

“It could be that contagious yawning has no function and is simply a byproduct of advanced social cognitive mechanisms in highly social species,” says Gallup.

To put it more simply, many animals – including humans – have a variety of ways to enhance their empathy, including “mirror neurons,” which fire when an individual performs an action and when they see someone else performing a similar action. So it could be that seeing someone else yawn simply triggers your mirror neurons, prompting you to yawn yourself. But contagious yawns can also play a role in group coordination through mechanisms linked to the theory of arousal change: helping each animal in the group move from a relaxed state to an active state.

A 2021 study that tested this effect in lions found that other behaviors can be contagious in yawners, so if a lying lion yawns then gets up to wander around, other yawners follow.

Contagious yawning can also promote group alertness: if one baboon in a troop tricks the others into yawning, they may all become more alert. It can also work the other way, helping to downregulate wakefulness before sleep.

In other words: yawning is probably good for you and probably helps your brain work better. Oh, and if you’re yawning pointedly to put a five-year-old to sleep, don’t stop — there’s a chance it might actually work.