Numerous theories as to why we yawn have been advanced, but yawning appears to be a complex phenomenon that is difficult to study. The physiology of yawning with its sharp chest inhalation, along with its contagious nature, can be explained by an arousal theory, that is, yawning promotes vigilance and alertness within a group that overrides a drive to sleep. Other theories of yawning, such as brain thermoregulation, are not necessarily excluded by the arousal theory but may be the results of yawning rather than its cause.