Why Do We Yawn?

Everyone yawns. Babies yawn before they are born. Many animals yawn too — dogs, cats, birds, even fish. But scientists are still not sure why we do it.

For a long time, people believed that we yawn because we need more oxygen, or because there is too much carbon dioxide in our blood. Scientists tested this idea. They gave people extra oxygen, but they yawned the same amount. So this old idea is probably wrong.

A newer idea is that yawning cools the brain. When you yawn, you take a deep breath of cooler air and more blood flows to your head. This may lower the temperature of the brain a little and help you think better.

There is also "contagious" yawning. When you see someone yawn — or hear it, or even read about it — you often yawn too. This is stronger between people who know each other well. Some scientists think it is connected to empathy: feeling what other people feel.

Yawning is universal among humans, appears in the womb, and occurs across a wide range of vertebrates, yet its function is still not settled — an unusual situation for a behaviour this common and this easy to observe.

The intuitive explanation, that yawning corrects a shortage of oxygen or a build-up of carbon dioxide, has largely been abandoned. In controlled experiments, altering the oxygen and carbon-dioxide content of the air people breathe does not reliably change how often they yawn.

The leading current hypothesis is thermoregulation: yawning may help cool the brain. A yawn combines a deep inhalation of ambient air with a strong stretch of the jaw that increases blood flow to the head, and studies find that yawning is more frequent when brain or ambient temperature is elevated and less frequent when the body is cooled. On this view, a yawn is a small reset that supports alertness, which fits the everyday observation that we yawn around transitions — waking, tiring, becoming bored.

Contagious yawning is a separate puzzle. Seeing, hearing or even reading about a yawn can trigger one, and the effect is stronger between people who are emotionally close. It has been linked to social bonding and to the neural systems involved in empathy, and versions of it have been observed in chimpanzees and dogs, suggesting deep evolutionary roots.

Few behaviours are as democratic as the yawn — it crosses age, culture and species, emerges in the foetus, and requires no learning — and few remain as poorly explained. That combination of ubiquity and obscurity makes it a useful case study in how hard it is to establish function even for something entirely available to observation.

The folk theory, and for a long time the textbook one, held that yawning served respiratory homeostasis: a corrective for low blood oxygen or elevated carbon dioxide. It has not survived scrutiny. Manipulating inspired gas concentrations, which reliably alters breathing rate and depth, does not produce the predicted change in yawn frequency, and the theory struggles to explain why yawning clusters around state transitions rather than tracking metabolic demand.

The hypothesis with the most current support is thermoregulatory. The physical act — a maximal jaw stretch that boosts cerebral blood flow, paired with a deep draw of ambient air — is well suited to shedding heat from the brain, and the empirical pattern fits: yawning rises with brain and ambient temperature up to a point, falls with cooling of the head or body, and is suppressed when ambient air is already near body temperature and therefore useless as a coolant. On this account a yawn is a brief thermal and arousal adjustment, deployed when the system is shifting gear.

Contagious yawning is almost certainly a distinct phenomenon layered on top. Its triggering by visual, auditory and even textual cues, its sensitivity to social closeness, and its presence in other social mammals point to a link with the perception-action coupling that underlies imitation and, more speculatively, empathy. Whether it does any work — synchronising vigilance in a group, signalling state — or is simply a by-product of having brains that automatically mirror what they perceive, is not resolved.