Yawning is a telltale sign of sleepiness. Research has confirmed what most people would suspect, the incidence of yawning increases when we are sleepy.
But yawns also accompany feelings of boredom and hunger. There are several theories about the purpose of yawning. The oft-repeated theory that we yawn because we have too much carbon dioxide, and a shortage of oxygen in the blood or brain has been disproven. A newer theory suggests that yawning is a way to regulate the temperature of the brain. Indeed, yawning occurs less frequently in colder temperatures.
But the thermoregulatory theory of yawning cannot explain another curious aspect of the phenomenon — social animals like humans, chimpanzees, and dogs yawn more when they see others yawn. Robert Provine RP Robert Provine believes that contagious yawning has a separate and relatively recent evolutionary origin. Most animals with a backbone yawn spontaneously, including fish, reptiles, and birds. But contagious yawning is rare. Although human fetuses yawn in the womb, contagious yawning is not observed in children until they are several years old, suggesting that contagious yawning is learned social behavior.
Provine believes that yawning may be a method the body uses to facilitate the transition between two behavioral states, such as wakefulness to sleep or boredom to vigilance.