A common act with an uncertain job
A yawn is hard to mistake: a slow, deep breath, a stretched jaw, a long exhale. It is also hard to explain. Almost all vertebrates yawn, including fish, reptiles, birds and mammals, which suggests an old behavior. Yet no single explanation has won clear agreement among researchers. That is a good place to start, because most popular answers to “why do we yawn?” sound more settled than they are.
Idea one: cooling the brain
The thermoregulatory theory, developed by psychologist Andrew Gallup and colleagues, proposes that yawning helps regulate brain temperature. The deep inhale may bring in cooler air and change blood flow in the head. A 2013 review by Gallup and Omar Eldakar summarized the supporting evidence:
- In rats, yawns followed rapid brain-temperature rises and were accompanied by cooling.
- In two women with chronic yawning, mouth temperature dropped slightly after each bout.
- Nasal breathing and cooling the forehead reduced contagious yawning in a human experiment.
- Yawning rates in several species, humans included, appeared to be highest at moderate temperatures and to fall when it was near or above body temperature.
The review also admitted weaknesses. Its human evidence rested partly on tiny case series and an observational survey of passers-by. In rats, temperature did not rise before yawns in every part of the brain. And, importantly, the authors are the theory’s proponents, so the review is part advocacy. Critics have argued that yawning cannot cool the body much without sweating, that sleepiness could explain the temperature link, and that social explanations fit as well.
- Brain warms slightly. Sleepiness or other states may raise local temperature.
- A deep inhale. Cooler air passes through the nose and mouth, and jaw movement affects blood flow.
- Possible cooling. Proponents suggest this lowers local temperature a little.
- Alertness may follow. Yawning is linked to transitions such as waking or shifting tasks, though why is not settled.
Idea two: arousal and alertness
Another family of ideas treats a yawn as helping the body shift state, perhaps by increasing alertness. An often-repeated anecdote describes paratroopers yawning before a jump, but anecdotes are not experiments. Other proposals involve blood gases, stretching of throat muscles or hormone changes. In reality these may not compete: a yawn could serve more than one purpose, or different purposes in different species. Encyclopedic summaries still say there is no clear consensus.
Why yawns spread
Contagious yawning, where seeing or hearing a yawn prompts your own, is a separate puzzle. It has been reported in people, chimpanzees, some monkeys, dogs and wolves. Several studies find that contagion is stronger between people who know each other well, which led to a popular claim: contagious yawning reflects empathy.
Here the evidence is mixed. Studies of children on the autism spectrum have often found lower contagious yawning, which fits an empathy link, though that link is debated. But in 2017, Jorg Massen and Andrew Gallup reviewed studies across many populations and concluded that the results connecting contagious yawning and empathy are “inconsistent and inconclusive.” They highlighted confounds such as how much people were looking at the yawner and social inhibition, and asked for more objective measures. Findings in budgerigars that contagion did not depend on social closeness complicate the empathy story further.
What this means for your yawns
A yawn does not reliably mean you are bored, tired or short of oxygen, and a person who does not catch yours is not necessarily less empathic. Context helps: yawns tend to turn up around waking, going to sleep, stress and anticipation, but these are patterns, not diagnoses. If you yawn excessively and unusually, and it comes with other symptoms, talk to a doctor; this article is educational, not medical advice. For more on the body’s rhythms, see why humans sleep and what scientists are still discovering.
