Traditionally associated with boredom, very little research has been done to uncover the biological function of yawning. Now, Princeton researchers propose that yawning is triggered by increases in brain temperature and its function is to cool the brain.
Their study, published in Frontiers in Evolutionary Neuroscience, is the first to show that yawning frequency in humans varies with the season and that people are less likely to yawn when the heat outdoors exceeds body temperature. Study authors Andrew Gallup and Omar Eldakar suggest that this seasonal disparity indicates that yawning could serve as a method for regulating brain temperature.
Documenting yawning frequency in winter and summer in Tucson, Arizona, Gallup and Eldakar found that participants were more likely to yawn in the winter, as opposed to the summer when ambient temperatures were equal to, or exceeded, body temperature. The researchers concluded that warmer temperatures provide no relief for overheated brains, which, according to the thermoregulatory theory of yawning, stay cool via a heat exchange with the air drawn in during a yawn.
Eldakar adds that there was a higher incidence of yawning with lower temperatures even after statistically controlling for other factors such as humidity, time spent outside and the amount of sleep the night before. He points out that nearly half of the people in the winter session yawned, as opposed to less than a quarter of summer participants.
The applications for this research are intriguing, not only in terms of basic physiological knowledge, but also for better understanding diseases such as multiple sclerosis and epilepsy, which are typically accompanied by frequent yawning and thermoregulatory dysfunction.