Yawning is an involuntary action that everyone does. We start before we are born and most creatures on the planet do it - even snakes and fish.
New research suggests rather than being a precursor to sleep, the purpose of yawning is to cool the brain so it operates more efficiently and keeps you awake.
|The average yawn lasts six seconds.
The theory could explain a puzzling question about subconscious human behaviour - why many of us yawn when we see or hear another person doing it, or even read about it or even just think about it?
The brain cooling theory says that when we contagiously yawn we are participating in an ancient, hardwired ritual that evolved to help groups stay alert and detect danger.
It's not copying another person's sleepiness, say scientists at the University of Albany in New York, who are behind the latest research.
"We think contagious yawning is triggered by empathic mechanisms which function to maintain group vigilance," says Dr Gordon Gallup, a leading researcher at the university.
The belief is further supported by the observation of University of Maryland's Robert Provine that paratroopers report yawning before jumping.
But there are other theories. It's been suggested contagious yawning could be a result of an unconscious herding behaviour - a subtle way to communicate to those around us, similar to when flocks of birds take flight at the same time.
Another theory suggests contagious yawning might have helped early humans communicate their alertness levels and co-ordinate sleeping times.
Basically, if one decided it was time to sleep they would tell the others by yawning and they would do it in return to show they agreed.
Chimpanzees also suffer from contagious yawning, according to researchers at Kyoto University in Japan. They are thought to be the only other creatures, apart from humans, who do so.
The rest of the animal kingdom - including birds, snakes and hippos - yawn for other reasons. Dogs yawn to stay calm in certain situations, says Turid Rugaas, author of On talking Terms with Dogs.
Anyone who gets to the end of this article without yawning may wish to think of themselves as a medical apparition. In fact, only about half of adult humans are prone to contagious yawning.