The Science of Yawning: It Is Contagious, It Cools the Brain, and It Is Still Not Fully Understood

You are probably going to yawn while reading this. Not because you are tired or bored, but because I have now planted the idea of yawning in your brain and your brain, annoyingly, does not fully distinguish between seeing something, reading about something, and doing it. The fact that you might yawn while reading an article about yawning is itself one of the stranger things about how the mechanism works. So, here is what scientists actually know, what they suspect, and what they still cannot explain.

What Yawning Actually Is

A yawn is a specific motor pattern: a large gaping of the mouth with a deep inhalation, a brief peak period where the muscles are fully contracted, and then a passive closure of the jaw with a shorter exhalation. The whole thing takes about six seconds. It appears across almost every vertebrate class, meaning fish, reptiles, birds, and mammals all do versions of it. Something this conserved across so many species almost certainly has a biological function. The question is what.

For a long time, the standard answer was oxygen. The theory was that yawning brought more oxygen into the blood and expelled carbon dioxide. This made intuitive sense. You yawn when you are tired, your oxygen use drops, and yawning corrects the imbalance. Clean and logical. Researchers tested it in the 1980s by having subjects breathe air with elevated carbon dioxide or elevated oxygen and measuring whether yawn frequency changed. It did not. The oxygen theory was wrong.

The Brain Cooling Theory

The leading current explanation is considerably stranger and more interesting. Yawning cools the brain. The mechanism works through several pathways simultaneously. The deep jaw stretch during a yawn increases blood flow through the head, neck, and face. A network of veins running through the jaw muscles acts as a pump, moving blood through when those muscles contract. The deep inhalation pulls cooler ambient air across nasal and oral blood vessels, chilling the venous blood before it reaches the brain. The jaw movement also flexes the walls of the maxillary sinuses, ventilating them and adding another layer of cooling.

The evidence supporting this theory is specific. In animal studies, yawns were consistently triggered by rapid increases in brain temperature of just 0.11 degrees Celsius. After a yawn, brain temperature dropped by as much as 0.5 degrees Celsius within one minute. A human case study found a similar pattern: yawning began during mild elevated body temperature and produced an average temperature drop of 0.4 degrees Celsius afterward.

Furthermore, people yawn more frequently in warmer environments and less in cooler ones, which fits the theory precisely. However, there is a limit in both directions. When ambient temperature exceeds body temperature, yawning becomes less frequent too, because inhaling hotter-than-body air would actually warm the brain rather than cool it. Researchers studied pedestrians outdoors in winter and summer and found dramatically different yawn rates: 18 percent in winter cold versus 42 percent in summer warmth. Temperature was the only significant predictor.

If yawning cools the brain, it may function as a way to maintain optimal cognitive performance during periods of rising temperature or fatigue.

Why It Is Contagious

Contagious yawning is a separate phenomenon and possibly a separate function. Seeing someone yawn makes you yawn. Reading about yawning can make you yawn. Just thinking about it can make you yawn. This is bizarre behavior for a biological mechanism and requires explanation.

The current best explanation connects contagious yawning to social cognition and empathy. Research has consistently found that people who score higher on empathy measures are more susceptible to contagious yawning. People yawn more contagiously with close friends and family than with strangers. People with autism spectrum conditions, which typically involve differences in social cognition, show reduced contagious yawning.

Notably, contagious yawning is not something humans are born with. Infants do not catch yawns. The response develops around age five and gradually increases through childhood, reaching adult levels around age eleven or twelve. This timeline closely tracks the development of theory of mind, the ability to understand that other people have mental states different from your own.

One hypothesis holds that contagious yawning evolved as a group synchronization mechanism. If yawning cools the brain and helps maintain alertness, then contagious yawning could serve to keep an entire group’s cognitive function synchronized. One person’s brain getting warm triggers a yawn, which spreads through the group, cooling everyone simultaneously. In a social species where group vigilance matters for survival, this would be genuinely useful.

Dogs, chimpanzees, and other social mammals show contagious yawning, particularly across species lines. Dogs catch yawns from humans. This cross-species contagion has led some researchers to suggest the mechanism is older than the social species themselves.

What Still Does Not Fit

The brain cooling theory is currently the most supported explanation, but it does not fully account for everything. People yawn when waking up from sleep, when their brain temperature should be at its lowest. People yawn when transitioning between states of alertness generally, not just when they are warm. Yawning appears during extreme stress responses in some animals. Yawning shows up before seizures in temporal lobe epilepsy.

These findings suggest yawning may do multiple things, or that the brain cooling function intersects with something about state transitions and cortical arousal that researchers have not fully characterized. Some scientists propose it functions as a reset mechanism, helping the brain shift from one state to another. This is not incompatible with the thermal theory but does not reduce to it either.

Additionally, excessive yawning can indicate neurological problems. Multiple sclerosis, stroke, brain tumors, and epilepsy can all produce increased yawning. This means yawning connects to brain function in ways that go beyond simple temperature regulation, though the exact mechanisms are not fully understood.