So far, the common notion has been that yawning is a precursor to sleep, but a new study has found that the yawn is in fact designed to prevent sleep.
Researchers at the State University of New York have demonstrated that a yawn is the body's way of making us more attentive and alert.
According to the study, the vessels in the nasal cavity send cool blood to the brain during a yawn, making it operate more efficiently. So rather than promoting sleep, yawning may help keep us awake.
The researchers have also explained why yawns are so contagious. The New York researchers say that contagious yawning is an evolved protective mechanism in our brain that makes a group more alert.
The researchers spent months observing 44 students, investigating why they yawned and what was the outcome of yawning.
They found that the physical mechanism of yawning cooled the brain, and made it function more efficiently.
"According to our hypothesis, rather than promoting sleep, yawning should antagonise sleep. We think contagious yawning is triggered by empathic mechanisms which function to maintain group vigilance," the Daily Mail quoted lead researcher, Dr Gordon Gallup, as saying.
For the study, the researchers recruited 44 college students to watch, individually, films of people yawning and recorded the number of contagious yawns each volunteer made. While watching the films, students were told to inhale and exhale in one of four ways: strictly orally; strictly nasally; orally while wearing a nose plug; or just breathe normally.
The results showed that 50 per cent of people told to breathe normally or through their mouths yawned while watching other people yawn, while none of those told to breathe through their noses yawned. The researchers also found that subjects who held a cold pack to their forehead did not catch yawns from the film, while those who held a warm or room-temperature pack yawned normally.
Researcher Robert Provine, from the University of Maryland in Baltimore, said yawning could mark the body's readiness to become alert.
"Yawning signals a transition between the behavioural states of wakefulness and sleepiness, and boredom to alertness," Provine said.