According to Psychologist Andrew Gallup of SUNY College at Oneonta, USA, yawning frequencies of people vary with temperature of the season and it subsequently functions to keep the brain temperature balanced and in optimal homeostasis.
Sleep cycles, cortical arousal and stress are all associated with fluctuations in brain temperature and it can also be easily manipulated by ambient temperature variation, since exchange with cool ambient air temperature may facilitate lowering brain temperature.
Results showed that in Vienna people yawned more in summer than in winter, whereas in Arizona people yawned more in winter than in summer.
Lead author Jorg Massen explains that where yawning functions to cool the brain, yawning is not functional when ambient temperatures are as hot as the body, and may not be necessary or may even have harmful consequences when it is freezing outside.
The study is published in Physiology and Behavior.