Yawning seems to serve no purpose—it‘s just a big exhalation of air when we‘re tired, bored, or next to someone else who‘s yawning. So why do we yawn? The short answer is that no one really knows. But there are few interesting theories.
The reason for yawning is not to get rid of extra carbon dioxide.
The belief that we yawn to get rid of carbon dioxide and take in more oxygen persists despite research disproving it. According to this theory, people breathe more slowly when they‘re bored or sleepy and less oxygen gets to the lungs. As CO2 builds up in the blood, the brain reflexively prompts a deep, oxygen-rich breath.
The problem with this theory is a 1987 study by the late Dr. Robert Provine, a professor at the University of Maryland Baltimore County, who was regarded as the world‘s foremost yawn expert. Provine set up an experiment in which volunteers breathed one of four gases that contained varying ratios of carbon dioxide to oxygen for 30 minutes. Normal air contains 20.95 percent oxygen and 0.03 percent carbon dioxide, but neither of the gases in the experiment with higher concentrations of CO2 caused the research subjects to yawn more.
Scientists have suggested that yawning cools the brain.
In 2007, two researchers at the University of Albany proposed that the purpose of yawning is to cool the brain. They conducted an experiment similar to Provine‘s and again found that raising or lowering oxygen and carbon dioxide levels in the blood did not change the number or length of yawns.
Subsequent experiments focused on two well-established brain cooling mechanisms: nasal breathing and forehead cooling. When you breathe through your nose, it cools the blood vessels in the nasal cavity and sends that cooler blood to the brain. Likewise, when you cool your forehead, the veins there, some of which are directly connected to the brain, deliver cooler blood. The researchers found that their test subjects with warm or room temperature towels pressed against their heads yawned more than those with cold towels. Subjects who breathed through their noses during the experiment did not yawn at all.
The researchers said their evidence suggests that taking in a big gulp of air with a yawn cools the brain and maintains mental efficiency.
Contagious yawns suggest that yawning might have more to do with sociology than physiology.
Reading about or even thinking about yawning can cause you to yawn. Almost all vertebrates yawn spontaneously, but only humans, chimps, and macaques yawn as a result of watching another individual do it.
Given that these are social creatures that live in groups, the contagious yawn may have evolved as a way to coordinate behavior and maintain group vigilance. When one individual yawned, the group took that as evidence that their brain temperature was up and their mental efficiency was down. If all members of the group then yawned, the overall level of vigilance in the group was enhanced. In humans, who have developed other ways to signal how vigilant they should be, contagious yawns may remain as a vestigial response.
A version of this story ran in 2008; it has been updated for 2023.