Why Do We Yawn?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Nobody is entirely sure why we yawn, which is remarkable for something so universal. The explanation most people were taught, that yawning brings in more oxygen, has been tested and does not hold up. The leading current theory involves cooling the brain.
Not for oxygen
The oxygen theory was tested directly in the 1980s. Participants breathing air with raised carbon dioxide or reduced oxygen did not yawn more, 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 brain cooling theory
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.
When yawning happens
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
Why it is contagious
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.
When it is worth mentioning to a doctor
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 in other animals
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.
The takeaway
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.