Why Do We Yawn?

The oxygen theory was tested directly in the 1980s. Participants breathing air with raised carbon dioxide did not yawn more, and pure oxygen did not reduce yawning, which is the opposite of what the theory predicts.

Yawning also does not increase noticeably during exercise, when oxygen demand is highest and breathing rate rises sharply. If yawning were about oxygen intake, exercise would be when it happened most, and it is not.

The leading current explanation is thermoregulation. A yawn involves a deep inhalation of cool air and a powerful stretch of the jaw, which increases blood flow through the head and may help cool the brain.

Supporting evidence includes findings that people yawn less when holding something cool to their forehead or breathing through the nose, and that yawning frequency varies with ambient temperature, peaking in a moderate range and dropping when the surrounding air is as warm as the body.

The circumstances are informative. Yawning clusters around transitions:

• •Waking up and preparing for sleep
• •Boredom or low stimulation
• •Before stressful or high focus events, including sport and public speaking
• •During changes in alertness rather than during steady states

Contagious yawning is a separate phenomenon from spontaneous yawning and appears to be social. Seeing, hearing or even reading about yawning can trigger it, as many readers will have noticed by now.

It is more likely between people who are close, with the strongest effect among family and friends and the weakest among strangers. This has led researchers to link it to empathy and social bonding, although the evidence is mixed and the connection is debated. Contagious yawning also appears in chimpanzees, dogs and some other social species.

Excessive yawning, well beyond normal, is occasionally associated with medical conditions including certain neurological disorders, sleep disorders and side effects of some medications.

It is also worth noting that developmental evidence shows fetuses yawn in the womb, well before there is any question of tiredness or boredom. Whatever yawning is for, it starts extremely early and is deeply built in.

Yawning is not remotely unique to humans. It has been observed in fish, reptiles, birds and mammals, which suggests a very old and basic function rather than anything to do with boredom or social life.

Contagious yawning is much rarer and appears mainly in social species, including chimpanzees, wolves, elephants and domestic dogs. Dogs have been shown to yawn in response to human yawning, more readily with familiar people. That distribution supports the view that spontaneous yawning is physiological and ancient, while contagious yawning is a separate and much later social overlay.

Some things yawning does are real but probably not its purpose. A wide yawn opens the Eustachian tubes that connect the middle ear to the back of the throat, which is why yawning can relieve the blocked feeling in your ears on a descending plane. A yawn also stretches the jaw and face muscles, and it is often accompanied by a whole-body stretch. These effects are useful to know about, but researchers treat them as by-products rather than the reason the reflex exists.

Comparisons between species offer a clue that fits the cooling idea. Researchers who compared yawns across many mammals found that species with larger brains, and more brain cells, tend to produce longer yawns. Humans have some of the longest yawns measured. If yawning helps regulate brain temperature, a bigger brain would plausibly need a bigger yawn. This is a correlation rather than proof, and it does not rule out other explanations, but it is the kind of pattern a good theory should be able to predict.

A yawn follows a reliable sequence. It begins with a slow, deep breath in as the jaw drops wide, reaches a brief peak where the jaw, face and often the neck are stretched hard, and ends with a quicker breath out as the muscles relax. The psychologist Robert Provine, who studied yawning for years, measured a typical yawn at around six seconds. He described it as a fixed action pattern, a behaviour that tends to run to completion once triggered, which is why trying to stop a yawn halfway feels so unsatisfying.

Provine also asked volunteers to try yawning with their teeth clenched, and many reported that it felt incomplete, as if the yawn had been cut off. The movement seems to be controlled by circuits deep in the brainstem rather than by deliberate choice. One striking clue comes from some people who have lost voluntary movement in an arm after a stroke. Doctors have recorded cases in which the paralysed arm lifts or stretches involuntarily during a yawn, suggesting that the yawn uses pathways that bypass the damaged areas used for ordinary movement.

Babies yawn spontaneously from birth, but catching a yawn from someone else develops much later. Several studies have found that children under about four or five rarely yawn in response to seeing others yawn, and that susceptibility increases through the early school years. That timing roughly matches the age at which children become much better at understanding what other people think and feel. It is one reason researchers suspect contagious yawning draws on social and emotional skills, while spontaneous yawning, which appears even before birth, runs on older and more basic circuitry.

Experiments have also probed what exactly triggers the contagion. Robert Provine found that videos of yawning faces with the mouth hidden still set viewers off, suggesting that the eyes, squint and whole facial expression carry the signal rather than the open mouth alone. Some early studies reported that autistic children caught fewer yawns, which was taken as support for the empathy idea. Later work found that the difference shrank in some experiments when children were encouraged to look at the eyes of the person yawning. Attention to faces, not just empathy, may therefore shape who catches a yawn, and the question remains open.

A good theory should make predictions that could turn out wrong. The brain cooling idea predicts that yawning should be most common when the air is cooler than the body but not freezing, and should fall off in heat. Researchers led by Andrew Gallup tested this on pedestrians in Tucson, Arizona, showing them pictures of people yawning. In winter, people were roughly twice as likely to yawn as in the scorching summer. A later study in Vienna, where summers are mild and winters cold, found the reverse seasonal pattern, with more yawning in summer, just as a 'thermal window' would predict.

Other researchers measured brain temperature directly in rats, using fine probes placed in the brain's frontal region. The temperature tended to rise in the moments before a yawn and then drop afterwards, a pattern consistent with a yawn bringing cooler blood to the brain. This is still correlation rather than proof, since the rise might trigger the yawn or simply accompany it. But it moves the debate beyond human questionnaires and observations. The study was small, and other researchers argue that yawning mainly boosts alertness at moments of transition, so the two explanations may not be mutually exclusive.

Yawning is not about oxygen, with brain cooling the leading current explanation, and contagious yawning appears to be a separate social phenomenon that is stronger between people who know each other well.