Yawning may increase alertness

Blood vessels in the nasal cavity send cool blood to the brain, so breathing through the nose or cooling the forehead cools the brain and eliminates the need to yawn, says Gordon Gallup. He argues that brains operate more efficiently when cool, and that yawning enhances brain function. "According to our hypothesis, rather than promoting sleep, yawning should antagonise sleep," he says.

"Paratroopers report yawning before they jump," says Robert Provine of the University of Maryland, Baltimore County. "Yawning signals a transition between the behavioural states of wakefulness and sleepiness, and boredom to alertness."