AAll vertebrates yawn or exhibit behavior that is at least recognizable as yawning. Sociable baboons yawn, but so do semi-solitary orangutans. Parrots, penguins and crocodiles yawn – and so did history's first jawed fish. Until recently, the purpose of yawning was not clear and is still disputed by researchers and scientists. But this commonality provides a clue to what's really going on—and it's probably not what you expect.
“When I poll an audience and ask, 'Why? You Do you think we yawn?', most people assume it's related to respiration or respiration and may somehow increase oxygen in the blood,” says Andrew Gallup, a professor of behavioral biology at Johns Hopkins University. “And this makes intuitive sense because most yawns do have this distinct respiratory component, this deep inhalation of air. What most people don't realize, however, is that this hypothesis has been explicitly tested and found to be false.”
To test the idea that we yawn to breathe in more oxygen or remove excess carbon dioxide, studies published in the 1980s manipulated the levels of both gases in the air inhaled by volunteers – and found that although changes did significantly affect other respiratory processes, but do not affect the regularity of yawning. There also does not appear to be any systematically measurable difference in yawning behavior in people with diseases related to breathing and lung function—which is what would be expected if yawning were related to breathing.
This is where, more or less, Gallup approached the topic. “When I was working on my thesis, my supervisor at the time said: why not study yawning, because no one knows why we do it?” he says. “It was intriguing – we knew it must have some basic physiological function. So I started studying the pattern of motor action that it involves – an extended opening of the jaw, accompanied by a deep inhalation of air, followed by a rapid closure of the jaw and a faster exhalation. And it occurred to me that this probably has important consequences for blood circulation, localized in the skull.”
In fact, that's what appears to be happening: Some 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 jaw opening as a local stretch, similar to muscle stretches in other parts of the body,” says Gallup. “Just as stretching helps circulation in the extremities, yawning appears to do the same for the skull.”
From there, Gallup and his fellow researchers began to develop the idea that yawning helps regulate temperature in and around the skull. Your brain temperature is primarily determined by three variables: the speed of arterial blood flowing into the brain, the temperature of that blood, and the metabolic heat production that occurs in the brain based on neuronal activity. Theoretically, yawning can change the first two. When you yawn, you take a deep breath of air that moves across the moist surfaces of your mouth, tongue, and nasal passages, much like air flowing through a car's radiator, carrying away heat through evaporation and convection.
Research seems to confirm this: Ambient temperature has a fairly predictable effect on the frequency of yawns, which increases when it is just A little too toasty (when it gets very hot, the air temperature is too high for the radiator effect to kick in, so other cooling mechanisms like sweating and yawning are quieted down again) and cooled down when it gets colder.
This also seems to explain why some diseases are associated with excessive yawning: either the diseases themselves or the drugs used to treat them cause an increase in brain or body temperature. The “neural activity” explanation is also supported by animal studies: mammals and birds, which have more neurons in their brains, yawn longer, regardless of their actual brain size.
This does not mean that other hypotheses are completely rejected. The best supported theory is “arousal shifting”: essentially, yawning helps the brain transition from one state to another – from sleep to wakefulness, from boredom to alertness, and so on. “One possibility is that yawning helps the brain switch between using the 'default mode network' – areas associated with daydreaming, recalling memories and self-reflection – and the attention network responsible for preparing the body for action,” says medical historian Dr Olivier Walusinski, author of several papers on the topic. “One proposed mechanism for this may be that it helps the circulation of cerebrospinal fluid, the fluid that surrounds and cushions your brain and spinal cord.”
In fact, it is possible that this function evolved first, with the thermoregulatory effect being a beneficial consequence: we will have stronger evidence for this as studies are carried out on a wider range of animals. It's also possible that the two explanations are directly related: these changes in condition likely mean changes in brain activity and temperature, which means more blood flow is needed. And neural cooling. This explains why you yawn when you're bored: your brain's activity level may increase as it begins to think about how to move you into a more stimulating situation, as well as its circulatory needs.
Wait, though: what about infectious are you yawning? We all know about the phenomenon where one person in a room – or even on a TV screen – takes a breath of air, and everyone else does the same. Some researchers suggest that this kind of infectious behavior brings groups together, perhaps because it is difficult to fake a signal of sleepiness, boredom or alertness – although this is unlikely to be the main purpose of yawning, since many solitary animals yawn regularly.
“It is possible that contagious yawning has no function and is merely a byproduct of evolved social cognitive mechanisms in highly social species,” Gallup says.
Simply put, many animals, including humans, have various ways of improving their empathy, including “mirror neurons,” which are activated when a person performs an action and when they see someone else performing a similar action. Perhaps then, when you see someone yawning, your mirror neurons simply fire, prompting you to yawn yourself. But a contagious yawn Maybe also play a role in group coordination through mechanisms associated with arousal shift theory: helping each animal in the group switch states from relaxed to active.
One A 2021 study tested this effect on lions. found that another behavior could be contagious among yawners: if one lying lion yawns and then gets up to walk, other onlookers follow.
Contagious yawning may also promote group vigilance: if one baboon in a troop causes others to yawn, they may all become more vigilant. It can also work the other way around – helping to reduce arousal levels before bed.
In other words: yawning is probably good for you and probably helps your brain function better. Oh, and if you've been noticeably yawning to get your five-year-old to sleep, don't stop—there's a chance it actually works.
