Many theories have instead focussed on thestrange, contagious nature of yawning – a fact that I know only too well frommy conversation with Provine. “Around 50% of people who observe a yawn willyawn in response,” he says. “It is so contagious that anything associated withit will trigger one… seeing or hearing another person, or even reading aboutyawning.” For this reason, some researchers have wondered if yawning might be aprimitive form of communication – if so, what information is it transmitting? We often feel tired when we yawn, so one idea is that it helps set everyone’sbiological clocks to the same rhythm. “In my view the most likely signallingrole of yawning is to help to synchronize the behaviour of a social group – tomake them go to sleep more or less at the same time,”saysChristian Hess, at the University of Bern in Switzerland. With the sameroutine, a group can then work together more efficiently throughout the day.

Yet we also yawn during times of stress:Olympic athletes often do it before a race, while musicians sometimes succumbbefore a concert. So some researchers, including Provine, believe that thestrenuous movements might have a more general role in rebooting the brain –when you are sleepy they make you more alert, or when you are distracted theymake you more focussed. Spreading through a group, contagious yawns could thenhelp everyone reach the same level of attention, making them more vigilant to athreat, for instance. The mechanism is somewhat hazy –though oneFrench researcher, Olivier Walusinski, proposes that yawning helps to pumpcerebrospinal fluid around the brain, which could trigger a shift in neuralactivity.

With so many competing and contradictoryideas, a grand unifying theory of yawning may seem like a distant speck on thehorizon. But over the last few years, one underlying mechanism has emerged thatcould, potentially, appease all these apparent paradoxes in one fell swoop.Andrew Gallup, now at the State University of New York at Oneonta, was firstinspired with the idea during his undergraduate degree, when he realised thatyawning might help to chill the brain and stop it overheating. The violentmovement of the jaws moves blood flow around the skull, he argued, helping tocarry away excess heat, while the deep inhalation brings cool air into thesinus cavities and around the carotid artery leading back into the brain.What’s more, the strenuous movements could also flex the membranes of sinuses –fanning asoft breeze through the cavities that should cause our mucus to evaporate,which should chill the head like air conditioning.

The most obvious test was to see if peopleare more or less likely to yawn in different temperatures. In normalconditions, Gallup found that around 48% felt the urge to yawn, but when heasked them to hold a cold compress to their foreheads, just 9% succumbed.Breathing through the nose, which could also cool the brain, was even moreeffective, completely dampening his subjects’urge to yawn –potentiallysuggesting a handy trick for anyone facing embarrassment during a tediousconversation.