It can signal tiredness, boredom or even rudeness, and everyone does it. But, now, a new science may explain why people yawn, and even crack one of the world's most mysterious phenomena the contagious yawn.
Some people think yawning stems from a need to stretch or change. Until recently, scientists thought people yawned to replenish oxygen supplies to their blood.
But University at Albany psychology professor Gordon Gallup said, as people yawn, they cool off their brains.
"Brains are like computers," he said. "They only operate efficiently and effectively when they're cool. And, therefore, there are some very intricate cooling mechanisms that serve to regulate the temperature of the brain."
He said many things associated with yawning, like being tired, actually make the brain hot, and yawning can alleviate the heat.
"Extended periods of sleep deprivation raise brain temperature and do produce excessive yawning," Gallup said.
In a sense, the yawn is like the brain's air conditioner, which may make some rethink societal assumptions.
"Many people have the impression when talking to someone that if they yawn, it's an insult," Gallup said. "But according to our hypothesis, it's really a compliment, because it suggests that brain mechanisms have kicked in to reinstate and optimize mental processing."
Gallup and his son tested their idea by creating an experiment where students were asked to watch a film of other people yawning.
When they held a cool compress to their head, or breathed through their nose a natural brain coolant they didn't yawn at all. But, if they breathed normally or held a warm compress to their heads, they were far more likely to yawn.
Gallup believes the yawn is more than just a way to cool the brain.
"During human evolutionary history, when we were subject to attacks by a group, then the ability to detect danger would have had a critical relationship to your ability to respond to it on a moment to moment basis," Gallup said.
"If everyone yawns in response to seeing someone yawn, it reinstates an optimal level of vigilance on the part of people in the group."
But, for those looking to avoid yawning, one of the best ways is to breathe through the nose.