The earliest theory of yawning, laid forth by Hippocrates, suggested that the action sucks in “good” air while pushing out bad air. That isn’t too far off from the theory that dominated the last century, which is that yawning delivers an extra hit of energy-boosting oxygen while pushing carbon monoxide out. But although commonly accepted, that theory has been contradicted by research, which shows that those who need more oxygen — including exercisers and people with oxygen-depriving medical conditions — don’t yawn more than the average person.
Yawning remains a bit of a mystery, perhaps because — pitted against deadly diseases — it is a low priority for medical research funding, suggests Steven M. Platek, Ph.D., a professor of psychology at Georgia Gwinnett College in Lawrenceville, Ga., who studies contagious yawning.
While the medical community has not yet settled on a definitive explanation, the most commonly agreed upon and promising theory (and by promising, we mean the hypothesis has been confirmed in all studies and contradicted by none, but research is not yet conclusive) is that a yawn is triggered by a rise in brain temperature.
“Brains are metabolically costly,” explains Platek. “They’re the size of a grapefruit but they consume 40 percent of our metabolic energy. And the brain runs hot.”
Researchers have found that yawning has a cooling effect on the brain, preventing it from getting overheated, which can diminish alertness. Core brain temperatures rise when we’re tired, when we’re unstimulated (read: bored), and among other circumstances such as hot ambient temperatures or infections. And a quick cooling can help us regain alertness.
“Brain temperatures are determined by three variables: rate of arterial blood flow, the temperature of the blood and the metabolic heat production within the brain,” explains Andrew C. Gallup, Ph.D., an assistant professor of psychology at SUNY College at Oneonta and a lead author on several studies about thermoregulation and yawning. “So yawning may function in altering the first two variables: increasing arterial blood flow and allowing the flow of cooler blood to the brain.”
To answer the question of what happens in the body is fairly straightforward: When you yawn, your mouth gapes open and you inhale deeply, finishing with a short exhalation. During this time, the muscles around your skull contract and stretch and you take in ambient air. New, cooler blood is pushed toward the skull as warmer venous blood is pushed out.
“That action increases cerebral blood flow to the brain and to the skull and, at same time, it forces the warmer venous blood away from the skull,” explains Gallup. “The muscle stretching increases circulation to that area.”
Secondary behaviors, like stretching out your arms or throwing your head back as you yawn, also function as cooling techniques as ambient air hits the under-arm area, points out Platek. What’s more, these full-body stretches prep your muscles for quick action, contributing to the overall push toward alertness that comes from the cooler brain temperature.
In a research review of thermoregulation and yawning published in Frontiers in Neuroscience, Gallup and his colleague Omar T. Eldakar found that rises in brain temperature preceded yawns in both humans and rats, and that brain temperatures went down following a yawn. They also found research that demonstrated how yawns are more frequent when ambient temperatures are high (contributing to high brain temperatures), but actually decrease when they are so high that they exceed internal brain temperatures, thus rendering the ambient air useless in the service of cooling the brain.
Brain thermoregulation has been an important component in the study of human evolution. Platek points to the work of anthropologist Dean Falk, who specializes in paleoneurology and whose radiator hypothesis suggests that our ancestors’ brains began to grow to their current powerhouse size after they developed cranial veins that help cool the brain, allowing for the larger, more complex and metabolically costly brains we now possess.
But the thermoregulation hypothesis for yawning only explains the root cause of what’s known as “spontaneous yawning.” Much of the research done on the subject actually focuses on contagious yawning — a phenomenon in which we yawn in response to watching someone else yawn, or even hearing mention or thinking briefly about the action. What’s more, we are even more likely to yawn when we watch a close friend or family member do so.
Most of the research on contagious yawning (though a recent paper questioned this connection) has focused on the role of empathy. But we’re not talking about compassion or even cognitive empathy — we’re talking about a really unconscious, low-level impetus to relate to others. Think, Platek suggests, of a televised sporting event: If you watch a football player get a terrible sports injury on TV, you might flinch, develop a sympathy pain, or react physiologically in some immediate way. This is the type of empathy researchers are referring to when they discuss its role in yawns. Interestingly, people with autism or schizotypal personality disorder — neurological conditions characterized by a lack of even low-level empathy — do not catch yawns as frequently.
And from an evolutionary perspective, this makes sense: If you are in a circumstance in which your brain is heating up or you require some greater alertness, chances are the other people in your group may be experiencing the same thing. Initiating a yawn contributes to the group’s communal alertness. “If one member of a group yawns, it’s indicative of something that you should be doing,” theorizes Platek. Since contagious yawns are physiologically identical to spontaneous ones, they both serve the same purposes.
So why do you yawn? It could be to cool down your hard-working brain or improve alertness — or it could simply be because you watched someone else do it. Or maybe you’re yawning right now as you read this.