20.11 THIS IS HOW WE DO IT: Why do we yawn?

The purpose of yawning has long vexed scientists. It’s an almost universal behavior among vertebrates, yet its purpose has never been understood. Thanks to some recent creative hypothesis testing, that may be changing.

Speculation about the function of yawning has been wide-ranging. Two thousand years ago, Hippocrates proposed that it served to rid the body of illness-causing “bad air” that accumulates inside us. This idea morphed into a widely held belief:

• Yawning is triggered by insufficient oxygen in the brain.

Widely held, but wrong.

The evidence against the “insufficient oxygen” hypothesis is clear.

1. People don’t yawn more when they exercise, although they require more oxygen.

2. When they are exposed to a gas mixture high in carbon dioxide and low in oxygen, people aren’t more likely to yawn.

3. Breathing air with a higher-than-typical concentration of oxygen doesn’t reduce the rate of yawning.

On top of these findings, there is no evidence that yawning actually increases brain oxygenation. Moreover, an organism can increase the supply of oxygen to its brain by simply breathing more rapidly or deeply).

Some other persistent but unsupported hypotheses include:

• Yawning reduces ear pressure. This does, in fact, occur. But there is no evidence that yawning rates increase in response to rapidly changing air pressure. And many other behaviors, including swallowing and chewing, have the same pressure-reducing effect.

• Yawning counteracts drowsiness, increasing arousal. Although we do yawn more frequently when we are tired, further observations contradict this hypothesis. For example, measurements of brain activity after yawning show the opposite response: increased drowsiness and reduced vigilance.

Recently, a new idea has generated some interesting experiments and promising results—simultaneously illustrating how researchers approach a question. The idea is this:

• Yawning is a mechanism for cooling the brain.

Several observations prompted this line of thinking. (1) Yawning—at least in humans and rats—is commonly preceded by a brief increase in brain temperature; (2) following a yawn, brain temperature usually decreases; and (3) yawning is associated with drowsiness, a physiological state correlated with increases in body temperature.

Researchers proposed that yawning increases cerebral blood flow and—as the sinus walls expand and contract like a bellows—pumps air onto the brain, lowering its temperature. They predicted that, if yawning serves as a mechanism for cooling the brain, the effectiveness of yawning will depend on ambient air temperature. In other words, when the air temperature is high, there will be little benefit to yawning—the air pumped onto the brain won’t provide cooling. And when the ambient air temperature drops below some critical point, cooling should be unnecessary. The researchers used three extremely simple but powerful experiments to test their predictions.

Experiment 1

What they measured: Yawning frequency in mild versus hot ambient temperatures. During winter (February) and summer (June), the researchers approached pedestrians in outdoor public areas in Arizona and asked them to participate in the study. Each participant (80 individuals participated during each of the two data-collection periods) was shown 20 images of people yawning and then asked to fill out a survey, reporting (among other things) whether they had yawned while looking at the images. (They also reported how long they had slept the previous night and how long they had been outdoors before participating in the study.) While the participant was filling out the survey, the researchers measured the temperature. During the winter data collection, the average temperature was 22° C (71° F). During the summer, it was 37° C (98° F).

Prediction: The researchers predicted that a higher percentage of individuals would yawn during the winter portion of the study than the summer portion.

Results: The findings provided strong support for the hypothesis. During the winter data collection, 45% of the participants reported yawning, while during the summer—when ambient temperature equaled or exceeded body temperature—only 24% reported yawning.

Experiment 2

What they measured: Yawning frequency in mild versus cold ambient temperatures. Using the same methods as in Experiment 1, during winter and summer the researchers approached pedestrians in outdoor public areas and asked them to participate in the study. This time, however, the study was conducted in Vienna, Austria. During the winter data collection, the average temperature was 1° C (33° F). During the summer, it was 19° C (66° F). As in the first experiment, participants were shown 20 images of people yawning and then asked to fill out a survey, which included reporting whether they had yawned while looking at the images.

Prediction: The researchers predicted that a higher percentage of individuals would yawn during the summer portion of the study than the winter portion.

Results: Again, the researchers found strong support for the hypothesis. During the winter data collection, just 18% of the participants reported yawning, while during the summer, 42% reported yawning.

Experiment 3

What they measured: Forehead cooling and yawning frequency. Participants held a warm pack (46° C/114° F) or a cold pack (4° C/39° F) to their forehead while watching videotapes of people yawning, and the researchers observed whether they yawned.

Prediction: The researchers predicted that holding a cold pack to the forehead would reduce brain temperature, thereby reducing the need for the thermoregulatory benefit of yawning.

Results: The results strongly supported the hypothesis. Among the participants that held a warm pack to their forehead, 41% yawned while watching the videotape. Among those holding a cold pack to their forehead, just 9% yawned.

Taken together, the results of these experiments provide compelling support for the idea that yawning’s function is to cool the brain. Further, the researchers found that temperature was the only significant predictor of yawning; other factors, including sex, age, humidity, and hours of sleep on the previous night, did not have a significant effect on the incidence of yawning.

At this point, the issue is not completely resolved. A variety of observations suggest that other factors may also influence yawning. The contagious nature of yawning, in particular, suggests a possible link to empathy. Researchers have found, for example, that the closer two people are genetically or emotionally, the more likely they are to “catch” each other’s yawns. They’ve also observed that yawning rates are reduced among people with schizophrenia and autism. As additional evidence accumulates, a fuller picture will emerge. And, unlike over the past two thousand years, progress is likely to be much more rapid given the results presented here.