Publishing in the recently resurrected journal Medical Hypotheses, researchers from Yale University and the University of Maryland propose that yawning, which opens the sinuses located to the left and right of the nose, acts to cool the brain when it gets too hot.
Excessive yawning, argue the researchers, appears to be a symptom of conditions that increase the brain or core temperature, such as damage to the central nervous. In addition, fits of yawning often precede epileptic seizures and migraines. Therefore, the authors say, understanding the physiological purpose of the reflex could have medical relevance.
Earlier work by the authors showed that the brains of mice increased in temperature just before a yawn and decreased directly after. The authors propose that the mucus within the sinus constantly evaporates and, like a refrigerator, cools the surrounding blood vessels and cerebrospinal fluid. A yawn, they suggest, would amplify this process by stretching the jaw, which flexes the walls of the sinus bringing new air into it rapidly cooling an overheated brain.