Yawning may appear the height of rudeness, but in fact your body is desperately trying to keep you awake, according to research from the US.
Psychologists who studied 44 students concluded that yawning sent cooler air to the brain, helping it to stay alert.
Yawning therefore delays sleep rather than promotes it, the study in Evolutionary Psychology suggested.
The desire to yawn when others do so may also be a mechanism to help a group stay alert in the face of danger.
Chill out
The common wisdom is that people yawn because they need oxygen, but the researchers at the University of Albany in New York said their experiments showed that raising or lowering oxygen and carbon dioxide levels in the blood did not produce that reaction.
Their evidence suggested instead that drawing in air helps cool the brain and helps it work more effectively.
In a study of the 44 students, researchers found that those who breathed through the nose rather than the mouth were less likely to yawn when watching a video of other people yawning.
This was because vessels in the nasal cavity sent cool blood to the brain, they said.
The same effect was found among those who held a cool pack to their forehead, while those who held a warm or room-temperature pack yawned when watching the video.
"Since yawning occurs when brain temperature rises, sending cool blood to the brain serves to maintain optimal levels of mental efficiency," the authors wrote.
"So the next time you are telling a story and a listener yawns there is no need to be offended - yawning, a physiological mechanism designed to maintain attention, turns out to be a compliment."
http://news.bbc.co.uk/1/hi/health/6268428.stm