The phenomenon of contagious yawning has been analyzed by researchers for many years. Though many believe yawning increases oxygen supply to the brain, researchers publishing in Physiology & Behavior have concluded that the purpose of yawning is to cool the brain.

The team, led by Andrew Gallup of SUNY College at Oneonta, NY, notes that previous research into the subject has not been able to produce a link between yawning and blood oxygen levels.

They say that changes in brain temperature are linked with sleep cycles, cortical arousal and stress. As such, they theorized that yawning could also be affected by surrounding variations in temperature and that exchanging cool air via yawns could lower brain temperature, achieving "optimal homeostasis."

In detail, they predicted that yawning would only occur within a "thermal window" - that is, within an optimal span of temperatures.

Though previous research has shown that frequency of yawning decreases as ambient temperatures increase and approach body temperature, the team says "a lower bound to the thermal window has not been demonstrated."

To investigate further, Jorg Massen and Kim Dusch, of the University of Vienna in Austria, gauged the frequency of contagious yawning in pedestrians walking around outside in Vienna during both winter and summer months.

They then compared their results with a corresponding study conducted in the dry climate of Arizona.

Exposure to 'optimal thermal zone' affects contagious yawning

The team asked pedestrians to look at an image series of people yawning, and the subjects then self-reported their own yawning behavior.

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Contagious yawning may work to cool the brain, thus improving mental efficiency, researchers say.

Overall results revealed that subjects in Vienna yawned more in summer than in winter. In Arizona, however, the people yawned more in winter than in summer.

Interestingly, the researchers found that it was neither the seasons nor the amount of daylight hours the subjects experienced, but rather, the exposure to an optimal thermal zone of around 20° C that affected contagious yawning.

They also found that contagious yawning decreased when temperatures were high - around 37° C in Arizona's summer - and low - around freezing in Vienna's winter.

Jorg Massen says this may be because yawning is not useful when outside temperatures are as warm as the body, so it may not be required, or it may be harmful when the outside temperatures are freezing.

He and his team say cooling the brain works to improve "arousal and mental efficiency." They suggest the spreading of this behavior through contagious yawning could improve group alertness.

Speaking with Medical News Today, Massen said:

"As yawning cools the brain, it gets the brain in optimal homeostasis, which is of course nice when you want to be alert, for example in a threatening situation.

In turn, when you see someone in your group yawning (and thus securing a cold brain that allows them to be alert), it might be adaptive to also be alert, as your group member might have seen something threatening, and consequently, you also want to cool your brain for optimal homeostasis, and thus you yawn, too."

He explained to us that this is "a completely subconscious mechanism."

The team says their findings add to building research that suggests the mechanism for yawning has to do with regulating the temperature of the brain.

In 2013, Medical News Today reported on a study that suggested yawns from owners are contagious to dogs, and that they are a result of empathy rather than stress.