| Recent research suggests that yawning functions to cool the brain during periods of mild hyperthermia. Evidence for this hypothesis is largely behavioral, and includes reports of increased yawning during increases in ambient temperature and times of stress as well as an amelioration of yawning upon nasal breathing and forehead cooling. Little research has been published on the physiological mechanisms supporting a brain cooling function, however. The current set of studies explores human and animal physiological parameters in search of evidence of brain cooling during yawning. In humans, heart rate, skin temperature, and skin conductance findings suggest that yawning involved an acute sympathetic nervous system response which increases blood flow. Additionally, mental activity, which may increase metabolic heat production in the brain, was shown to increase yawning frequency. In rats, direct measurement of brain temperatures showed that yawning is preceded by an increase and followed by a decrease in cortical temperature. Rate of change in brain temperature measured from the preoptic area of the hypothalamus also reveals significant decreases following a yawn. Finally, it is demonstrated that hypothalamic temperatures are highly influenced by environmental temperatures, which may account for previous reports of increased yawning during increases in ambient temperatures. The findings reported herein support a thermoregulatory function of yawning. |
