All vertebrates yawn, or indulge in a behaviour that’s at least recognisable as yawn-adjacent. Sociable baboons yawn, but so do semi-solitary orangutans. Parakeets, penguins and crocodiles yawn – and so, probably, did the first ever jawed fish. Until relatively recently, the purpose of yawning wasn’t clear, and it’s still contested by researchers and scientists. But this commonality provides a clue to what it’s really all about – and it’s probably not what you’re expecting.
“When I poll audiences and ask: ‘Why do you think we yawn?’, most people suggest that it has to do with breathing or respiration and might somehow increase oxygen in the blood,” says Andrew Gallup, a professor in behavioural biology at Johns Hopkins University. “And that’s intuitive because most yawns do have this clear respiratory component, this deep inhalation of air. However, what most people don’t realise is that that hypothesis has been explicitly tested and shown 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 both gases in air inhaled by volunteers – and they found that while changes did significantly affect other respiratory processes, they didn’t influence the regularity of yawns. There also doesn’t seem to be any systematically measurable difference in the yawning behaviour of people suffering from illnesses associated with breathing and lung function – which is what you would expect if yawns were respiration-related.
This, more or less, was where Gallup came to the subject. “When I was pursuing my honours thesis, my adviser at the time said, well, why not study yawning, because nobody knows why we do it?” he says. “That was intriguing – we knew it had to serve some underlying physiological function. So I started to examine the motor action pattern it involves – this extended gaping of the jaw that’s accompanied by this deep inhalation of air, followed by a rapid closure of the jaw and a quicker exhalation. And it occurred to me that this likely has important circulatory consequences that are localised to the skull.”
This, in fact, seems to be exactly what’s happening: several reviews of the medical literature suggest that yawning increases arterial blood supply to the cranium, and then venous return (the rate at which blood flows back from the head to the heart).
“We can think of the gaping of the jaw as a localised stretch, similar to stretching muscles in other areas of the body,” says Gallup. “In the same way that stretching helps circulation in those extremities, yawns seem to do the same for the skull.”
From here, Gallup and his fellow researchers started to develop the idea that yawning helps regulate heat in and around the skull. Your brain’s temperature is mainly determined by three variables – the rate of arterial blood travelling to the brain, the temperature of that blood, and metabolic heat production that occurs within the brain, based on neuronal activity – and yawning, theoretically, can alter the first two. When you yawn, you take in a deep breath of air that moves across the moist surfaces of your mouth, tongue and nasal passages, a bit like air flowing across a car radiator – carrying heat away through evaporation and convection.
Studies seem to bear this out: ambient temperature has a pretty predictable effect on yawn frequency, which goes up when it’s just slightly too toasty (when it gets really hot, the air temperature is too high for the radiator effect to work, so other cooling mechanisms such as sweating kick in and yawning settles down again) and down when it’s colder.
This also seems to explain why certain medical conditions are associated with excess yawning: either the conditions themselves, or the drugs used to treat them, cause elevated brain or body temperature. The “neuronal activity” explanation is also borne out by animal studies – the mammals and birds with more neurons in their brains yawn for longer durations, irrespective of the size of their actual brains.
This isn’t to say that other hypotheses have been entirely put to rest. The one best supported by the evidence is the “arousal change” theory: basically, that yawning helps the brain transition between states – from sleeping to wakefulness, boredom to alertness, and so on. “One possibility is that yawning helps the brain switch between using its ‘default mode network’ – the regions associated with daydreaming, recalling memories and self-reflection – and the attentional network responsible for preparing the body for action,” says the historian of medicine Dr Olivier Walusinski, who has authored several papers on the subject. “One proposed mechanism for this would be that it helps with the circulation of cerebrospinal fluid, the liquid that surrounds and cushions your brain and spinal cord.”
It could be the case, in fact, that this function evolved first, with the thermoregulation effect emerging as a useful after-effect: something we’ll have better evidence for as studies are done on a wider array of animals. It could also be that the two explanations are directly related: these state changes probably mean changes in brain activity and temperature, meaning a need for enhanced blood flow and neural cooling. This would explain why you yawn when you’re bored: your brain’s activity level may upshift as it starts to think of ways to move you into a more stimulating situation, and so do its circulatory needs.
Wait a minute, though: what about contagious yawning? We’re all aware of the phenomenon of one person in a room – or even on a TV screen – taking a gulp of air, only for everyone else to do the same. Some researchers have suggested that this sort of infectious behaviour brings groups together, perhaps because it’s a hard-to-fake signal of sleepiness, boredom or vigilance – though it’s unlikely to be yawning’s main purpose, as plenty of solitary animals are regular yawners.
“It could be that contagious yawning doesn’t have a function and is just a byproduct of advanced social cognitive mechanisms within highly social species,” says Gallup.
To put that slightly more simply, lots of animals – including humans – have various ways of improving their empathy, including “mirror neurons”, which fire when an individual performs an action and when they see someone else perform a similar action. It might be, then, that seeing someone else yawn simply makes your mirror neurons kick in, prompting you to yawn yourself. But contagious yawning may also play a role in group coordination through mechanisms related to the arousal change theory: helping every animal in the group switch states from relaxed to active.
One 2021 study that tested this effect in lions found that other behaviour can be contagious among yawners, so if one lying lion yawns and then gets up for a walk, other yawners follow.
Contagious yawning may also promote group vigilance: if one baboon in a troop triggers others to yawn, they may all become more alert. This may also work the other way around – helping to downregulate arousal before sleep.
So, in other words: yawning’s probably good for you, and it’s probably helping your brain function better. Oh, and if you’ve been conspicuously yawning to get a five-year-old to go to sleep, don’t stop – there’s a chance it’s really working.
