Why Do We Yawn?

What happens in the body during a yawn?

One common definition describes a yawn as a wide gape of the jaws, a deep breath in, a shorter breath out and a closing of the jaws.^[3] Another description adds a short pause in breathing before the breath out.^[1] In laboratory measurements, heart rate, lung volume and eye muscle tension went up during or just after a yawn.^[1]

Spontaneous yawns come more often when people are bored or tired.^[5] Yawning is most common just before falling asleep and just after waking up.^[2] According to a 2021 news release from Utrecht University, people yawn about 5 to 10 times a day.^[7] Scientists have not agreed on what yawning is for.^[5]

Do we yawn to get more oxygen?

Perhaps the most common belief about yawning is that it raises oxygen levels in the blood.^[2] A study published in 1987 by Provine and colleagues tested this idea.^[2] Breathing extra oxygen or extra carbon dioxide did not change how often people yawned, although it made them breathe faster.^[1][2] In the same study, exercise hard enough to double the breathing rate had no effect on yawning either.^[1][2]

Reviews of the evidence conclude that yawning does not serve a breathing function and is controlled separately from breathing.^[2]

Does yawning cool the brain?

The brain-cooling hypothesis, first proposed in 2007 and studied especially by the lab of Andrew Gallup, holds that yawning works as a cooling mechanism for the brain.^[2][7] The stretch of the jaw increases blood flow in the neck, head and face, and the deep breath is thought to exchange heat with cooler outside air.^[2]

Several studies have tested the idea:

• People who breathed through the nose or had their forehead cooled, both known to lower brain temperature, showed less contagious yawning.^[2]
• In a 2010 study using temperature probes in rats' brains, yawns came while brain temperature was rising, and temperature returned to baseline afterwards.^[8] The rapid rise before a yawn was about 0.11°C, followed by a drop of the same size.^[2]
• Pedestrians in Tucson, Arizona, shown photos of people yawning were nearly twice as likely to yawn in winter, at an average of 22°C, as in early summer, at 37°C, about human body temperature.^[2] The hypothesis predicts this: a yawn should lose its cooling effect when the air is as warm as the body.^[2]
• A 2021 study of more than 1,250 yawns from 55 mammal species and 46 bird species found that, regardless of body size, species with bigger brains and more neurons yawn for longer.^[7] According to Utrecht University, this supports brain cooling, because a larger brain needs more cooling.^[7]

Not everyone accepts the hypothesis.^[2] A 2010 review by Guggisberg and colleagues concluded that the link between yawning and temperature was not causal, arguing for example that a warm pack on the forehead might increase yawning simply by making people sleepy.^[2] Another critic, Elo, argued that yawning cannot lower temperature much without sweating.^[2] In a 2013 review, Gallup and a co-author called these objections untenable and wrote that no study had found evidence contrary to the findings that support the theory.^[2]

Does yawning help the brain switch states?

Another line of research holds that yawns stimulate arousal, meaning alertness, when the body changes state, as between sleeping and waking.^[2][6] Across vertebrates, yawning also tends to happen before important events and during changes from one activity to another.^[2] However, studies that measured arousal around yawns have produced conflicting results.^[2]

The ideas overlap: Gallup has proposed that brain cooling is how yawning boosts alertness.^[2] A third proposal, from Guggisberg and colleagues, is that the main purpose of yawning is social communication, an idea drawn from the fact that yawns are contagious.^[1]

Why is yawning contagious?

Hearing, seeing, reading or even thinking about yawning can set off a contagious yawn.^[5] In controlled studies, about 40–60% of healthy volunteers yawn when shown someone yawning.^[5] Children do not seem to catch yawns until about the age of four or five.^[3][4]

A popular view links contagious yawning to empathy.^[2] In a study published in 2011, Norscia and Palagi observed 109 adults in everyday settings for a year and found that only the social bond between two people predicted whether a yawn spread.^[4] Yawns spread most often between relatives, then friends, then acquaintances, and least between strangers.^[4]

Other studies cast doubt on the empathy link.^[5][6] In a 2014 Duke University study, 222 of 328 people yawned at least once while watching a video of yawning.^[5] Once age was taken into account, empathy, time of day and intelligence made no difference, and age itself explained only 8% of the variation.^[5] In a 2022 study, 69% of participants reported catching yawns from pictures of animals ranging from fish to apes, and neither the species' closeness to humans nor being a pet changed the response.^[6] Its authors said this does not support a link with empathy, and that it remains unclear whether people catch more yawns from friends and family because of closeness or because they pay more attention to them.^[6]

Do animals and unborn babies yawn?

Yawning has been recorded in all five classes of vertebrates.^[1] Mammals appear to yawn for longer than birds, which the authors of the 2021 study link to birds' higher core body temperature letting their blood cool faster.^[7]

Contagious yawning is common in chimpanzees, bonobos and orangutans, but studies of gorillas show no sign of it.^[6] Studies also suggest or report it in wolves, pigs, African lions, African elephants, sheep and budgerigars.^[6] A study of young ravens found no evidence of it.^[6] Dogs have been reported to catch yawns from humans but not from other dogs.^[6]

Yawning in the womb has been reported from the end of the first trimester, about 11 weeks of pregnancy according to one review.^[2][3] Some researchers have argued that these movements are simple mouth openings, not yawns.^[3] A 2012 study that scanned 15 healthy fetuses at 24, 28, 32 and 36 weeks found that yawns could be reliably told apart from other mouth openings by how long the mouth took to open fully.^[3] The number of yawns fell from 28 weeks onward, and the authors note that contagion cannot be the reason fetuses yawn.^[3]