We yawn when we’re sleepy or bored or if we see someone else yawn. Some people also yawn when they’re anticipating something important and when they’re stressed. Many think it’s because the brain lacks oxygen, but according to a new study, the thing most of these instances have in common is thermoregulation. Yawning helps cool the brain.
Sleep cycles, cortical arousal, and stress are all associated with fluctuations in brain temperature. In previous studies on rats and humans, yawns are preceded by rises in brain temperatures, and they're followed by equivalent decreases immediately afterwards. If yawning does keep brain temperatures balanced, then to maintain homeostasis, we should only be yawning within an optimal range of temperatures.
To find this “thermal window,” a team led by Jorg Massen from University of Vienna and Andrew Gallup of SUNY Oneonta measured the frequency of contagious yawning of 120 random pedestrians walking outside in Vienna, Austria, during the winter (average temp 1.4 degrees Celsius) and summer (19.4 oC). The recruits looked at a series of 18 people yawning and then took a survey on if and how often they had yawned during the experiment. There were also questions about sex, age, how long they’ve been outdoors, and how much they slept; some of these factors have been correlated to yawning in previous studies.
The difference between self-reported yawning in the two seasons was noticeable: 18.3 percent of participants yawned in the winter, while 41.7 percent reported yawning in the summer. However, when the team compared these results with an identical study conducted in Tucson, Arizona, they realized they got opposite results. In this drier clime, people yawned more in the winter (22 oC) than in summer months (37 oC).
Turns out, it wasn’t the seasons themselves, nor the amount of daylight hours experienced. Rather, contagious yawning was constrained to an optimal thermal zone: a range of ambient temperatures around 20 degrees Celsius. The yawning diminished when temperatures were relatively high and relatively low.
Maybe cooling the brain improves mental efficiency. If it is for brain cooling, contagious and spontaneous yawning isn’t functional when ambient temperatures are as hot as the body, Massen explains in a news release. And it probably isn’t necessary (or may even be harmful) when it’s freezing outside.
The work was published in Physiology & Behavior this week.
[Via University of Vienna]