Every few years, a person who can’t stop yawning is referred to sleep specialist David Cunnington. One patient, suffering from a neurological condition, counted only minutes between yawns throughout much of each day. “It would just go on all the time,” Cunnington says. They would be “sitting, having a conversation with friends, and it was getting in the way”.

If you stop to consider a yawn, you notice several apparent contradictions. We yawn when we’re tired, but also when we’re waking up. We can yawn when we’re stressed or anxious, but also if we’re bored. And though we might try to stifle a yawn to avoid causing offence in the company of others, the action is really beyond our control. Then there’s the contagion factor: all it takes is being in the company of a yawner and we too find our eyes begin to water and close, mouth widens ... Simply talking or reading (or writing) about yawning can trigger the urge.

Those who study yawning – a field sometimes called chasmology – offer theories of social bonding, alertness and even cooling the brain as the reason for it. They might be getting closer to understanding it, they say. Then again, yawning has been a candidate for “the least understood, common human behaviour”, as Robert Provine, a pioneer researcher of innate curiosities, wrote in the 1980s. This rings true decades later for Cunnington, clinical chair of the Australasian Sleep Association, who notes in his patient: “We tried a whole range of different drugs and really nothing shifted.”

So, what is a yawn? And if you’ve yawned since you started reading this – do scientists know why?

Where does yawning start in the body?

Yawns occur in almost all vertebrates, from reptiles and birds to chimpanzees and lions. The yawn, Provine wrote, is “one of the animal kingdom’s most ancient rites”. As babies grow in the womb, doctors have seen them yawn, putting the action among our earliest hard-wired behaviours with hiccupping and swallowing. “Those mechanisms probably developed in an ancestor of ours, perhaps even an amphibian ancestor, or reptilian ancestor – it goes back a long way,” says University of Nottingham professor of cognitive neuroscience Stephen Jackson.

We usually yawn for six or so seconds up to 10 times a day but what is happening in the brain as we do is not precisely known. Experts say the basic neural circuit that controls yawning appears to be in the brain stem – the area connecting the brain and spinal cord involved in regulating functions such as blood pressure, breathing and body temperature. “Unfortunately, there are a small number of babies born without fully developed brains, but [who] have developed a brain stem,” says Yossi Rathner, an anatomy and physiology researcher at the University of Melbourne. “These babies can demonstrate a full yawning reflex.”

Yawns also appear to recruit other areas of the brain involved in motor skills and the nervous system. Jackson found heightened excitement in the motor cortex has meant people are more likely to “catch” a contagious yawn (more on that later). Meanwhile, researchers in Japan in the ’90s showed that stimulating a rat’s hypothalamus – an area of the brain involved in metabolism, growth and autonomic functions via our nervous systems – resulted in the animal yawning.

Other scientists have found animals injected with dopamine (associated with pleasure), glutamate (involved in memory and mood regulation) and oxytocin (associated with social bonding) all yawn as a result. Cortisol, a hormone involved in stress and fatigue, can also increase yawning. “Quite clearly, there’s a neural basis to yawning,” Jackson says. “It’s linked to a bunch of brain systems and a bunch of neurotransmitters.”

People can yawn chronically as a sign of excessive sleepiness, a side effect of medication or an underlying condition such as pressure build-up in the brain, says Adrian Guggisberg, a neurologist from the University Hospital of Bern, Switzerland.

In one of its most mysterious effects, yawning appears to cause some people paralysed down one side of the body to involuntarily lift their immobilised arm. The oddity – called parakinesia brachialis oscitans – is often an indication of severe paralysis. Guggisberg says the effect seems to show that yawning activates different routes in the body than those used in voluntary movement. “People can’t use these pathways to induce a voluntary activation of the arm.”

When do we yawn most, and do we know why?

It turns out we really do yawn more when we are sleepy or bored. In the 1980s and ’90s, Provine asked people to take note of their yawning and found they tended to yawn more during the hour after waking and the hour before bed. In another experiment, he asked subjects to watch a television test card for 30 minutes. They yawned about 70 per cent more than those who watched music videos instead. “However you feel about music videos, you will find them more interesting … than a test pattern,” he wrote.

The troopers displayed “copious yawns” before leaping from aircraft.

He found yawning is also associated with stress. He asked a special forces soldier to observe paratroopers before their first jump, finding the troopers displayed “copious yawns” before leaping from aircraft. The behaviour extended to sportspeople and performers too. Speed-skater Apolo Ohno yawned before a podium finish in a Winter Olympic race in 2010. “It gets the oxygen in and the nerves out,” he said at the time.

So yawning in humans, scientists suspect, is to do with changes in state or mental processing. But what function yawning serves us, or communicates to those around us, isn’t known for sure. Complicating things further, Jackson says, is that it might have a primitive function that’s been lost in time. “It’s hard to explain that in terms of human behaviour now,” he says. “You might just think of spontaneous yawning as this vestigial remains of an earlier brain mechanism.”

One popular belief is that yawning is linked to a need for increased oxygen intake. Provine experimented with gases of different oxygen concentrations. Theoretically, if humans yawned to boost their blood oxygen levels, yawning rates would increase for those who consumed less oxygen. But they didn’t. Then, Provine asked study participants to try differing levels of exercise to produce varied levels of oxygen demand. Still, there was no difference in yawning patterns. “The Provine findings are very significant in falsifying that respiratory hypothesis,” says Andrew Gallup, an evolutionary biologist at the State University of New York Polytechnic Institute.

Instead, Gallup says yawning has a unique physical function: cooling the brain. In warm-blooded organisms, the brain is always undergoing small fluctuations as the body maintains a constant core temperature. When the brain’s temperature rises, Gallup proposes, a yawn is one mechanism that flushes warmer blood away from the skull and introduces a cooler supply. He and colleagues found yawns and stretches were more frequent in rats as their brain temperature rose. In people who held packs at different temperatures to their heads, Gallup found yawns were less likely with cold temperatures, more likely at room temperature or warmer. Brains capable of more neural activity generate more heat, and Gallup also found – from videos he sourced of dozens of animal species – yawns were longer for those whose brains had more neurons.

Guggisberg is sceptical of the brain cooling theory, believing sweating and breathing sufficiently regulate the brain’s temperature. “It’s not sufficiently effective to really ... regulate brain temperature,” he says. “To really show the causality is difficult.” Given some people don’t yawn very often, he also wonders if it has any role. “It’s not that they have a very hot brain.”

Another belief is that yawning, rather than sending us to sleep, could wake us up. Olivier Walusinski, a French family doctor who occasionally sees patients who yawn more than 200 times a day, is among several researchers who believe “yawning stimulates alertness during wakefulness” or prevents us from falling asleep. He proposes yawning causes a flow of cerebrospinal fluid that switches our default-mode network (a pattern of brain activity that occurs when the mind is at rest) to our attentional networks (several brain regions involved in regulating concentration).

However, Guggisberg recorded people’s brain activity before and directly after yawns, confirming they yawned when drowsy but not finding any change in their alertness afterwards. “We could not show that yawning itself keeps us awake.” Still, even if yawning doesn’t make us more alert, it can tell us we’re tired. Says Rathner: “It’s the subconscious telling the conscious [mind] that something has to change.” A yawn might help someone driving become aware they’re losing concentration, for example. “Yawning is alerting me to the fact that I actually have to take notice, either move or fidget to get into a new position,” he says. “But, ultimately, it means I should be thinking about pulling over and having a nap.”

Is yawning contagious?

Chasmologists joke that simply having a conversation about their work can cause other people to yawn. “My reputation as a yawn sleuth has conferred a curious kind of charisma – I’ve become a yawn stimulus,” Provine wrote several years before his death in 2019. Sure, maybe it’s not everyone’s favourite topic of conversation. But he found when people watched filmed repetitions of others yawning, they yawned more than double as often as those who watched a series of smiles. Studies of people reading about yawning found a similar effect.

Jackson, who researches Tourette syndrome, wanted to know if yawning could share anything in common with tics where people can mimic others’ actions or sounds. His research relies on a concept of a mirror neuron where the same neurons that respond when we carry out an action also respond when we see someone else doing that action. These neurons, Jackson says, act as an internalisation of the actions required to copy the same move. “It’s suggested that these mechanisms form part of a system for understanding other people’s behaviour.”

“The more you are friends, the higher the level of yawn contagion you show.”

In people watching videos of others yawning, Jackson found the more excited the motor cortex was, the more likely they were to also yawn, suggesting the same neurons might be playing a role. “You don’t choose to yawn, but when you’re watching someone else yawn, the motor mechanisms involved in producing yawns are activated partially,” he says.

But why? Yawning catching on in humans has puzzled us for millennia. Aristotle or his followers wondered in an ancient text: “Why do men generally themselves yawn when they see others yawn?”

Today, this question remains unresolved. One theory suggests yawning has a social function. Dogs have long been thought to yawn contagiously, even in response to people. Elisabetta Palagi, an ethology professor at the University of Pisa, has found animals catching each other’s yawns extending to chimpanzees, bonobos, gelada monkeys and lions. What’s unusual is this doesn’t seem to happen for animals she’s studied who are less social, such as gorillas in captivity. “So, the more you are friends, the higher the level of yawn contagion you show,” she says.

Puzzling Palagi was what contagious yawning does in groups. Her team followed 19 lions in South Africa for four months, filming every yawn they could and watching how it caught on. They might have found the answer in what kept happening after the first lion yawned: that lion would get up, another would yawn and do the same. Other lions in the pride that didn’t yawn wouldn’t follow, suggesting it was a signal. “Responding to others’ yawns increases the probability [they] do something together; to move in a synchronous way, and to improve group cohesion,” Palagi says.

Palagi, who has also researched yawning in humans, proposes yawns are contagious because they are a sign to others. “I think that yawning is a behaviour that can indicate a shift in the emotional and physiological state of people,” she says. “When you start yawning, you are trying to keep aroused. It is a clear sign to others that you are tired and that you want to do something different.”

Palagi is among researchers who link contagious yawning with empathy. “If we are synchronised and if we yawn a lot together, some empathic relationships can be established,” she says. “What we found is that friends, or kin, infect each other much more compared to strangers or simply acquaintances.”

Other research has found people who score highly on psychopathic traits are less likely to yawn contagiously. This was dramatised in the BBC series Luther, when a detective played by Idris Elba deliberately yawns while interrogating a murder suspect. When she fails to follow suit, he concludes she lacks empathy and, by implication, that she might be some kind of psychopath. (Luther’s boss, yawning as he presents this theory, isn’t so sure though: “So her affect’s off, it could be medication.” ) Gallup, who worked on the study into psychopathy, is among researchers who believe empathy is not a cause of contagious yawning but could be correlated with it. “It’s difficult to disentangle,” he says.

Seeing we can’t control it, how do we deal with yawning?

Yawn at the end of a dinner with friends and it might be a sign of a fun evening out too late. Yawn minutes into a colleague’s presentation and you might come across as rude or dismissive. The upper classes of Victorian-era Britain saw yawning as one of several signs of being too at ease, up there with taking the most comfortable seat in a room, placing feet on a chair or standing with your back to a fire, according to Routledge’s Manual of Etiquette from 1860.

Liz Wyse, an etiquette adviser at Debrett’s in London, believes yawning implies unconcealed boredom – rude in most circumstances today. “Of course, this is not necessarily the case and sometimes overwhelming fatigue can cause a bout of yawning,” she says. In any case, given a yawn can’t be stifled, covering the mouth, apologising, and saying if lack of sleep is to blame, are the most polite responses. Being on the receiving end of a yawn, meanwhile, particularly if you are mid-monologue, should give you pause for thought. “You’d do well to change the subject or ask some questions and refocus the conversation,” says Wyse.

“I’m not going to be in any way insulted if people yawn in my presence, it’s a natural response.”

Studiers of the yawn are unsurprisingly blase about its implications in a social setting. “I’m not going to be in any way insulted if people yawn in my presence, it’s a natural response,” Gallup says. “Just let it go to full action.” A cautionary tale of opening too wide, though: a yawn during a politics lesson dislocated British teen Holly Thompson’s jaw, putting her in hospital, according to a BBC series about strange emergencies.

Still, in some classrooms, yawning has a role. Melbourne voice teacher Anna McCrossin-Owen instructs classes to “stretch and yawn” as a warm-up exercise. She finds some students will feign a yawn and segue into a real one. “Something has been triggered,” she says. McCrossin-Owen says the action creates space in the vocal tract, making the breath more full and preparing the body for expression. “There are many benefits of the yawn.”

For the sleep association’s Cunnington, concealing our yawns is part of sleep being a private activity – he finds people hesitant to discuss sleepwalking, sleep talking and snoring too. “Very long ago, [groups] would be sleeping together … it was a shared experience,” he says. “You get into the 20th century, and [more] people start to go into their individual bedrooms, and it becomes this thing that’s not talked about.”

The patient he was unable to stop from frequently yawning had a degenerative condition and even as their opportunities for socialising were narrowing, their yawns were getting in the way. “There was this social thing that made it uncomfortable to be in public,” Cunnington says. “Given how little we actually understand about yawning, we should be cautious about attaching such clear labels about what it means.”

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