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Why do we yawn? Dogs do it, lions do it, even babies in the womb do it – but nobody really knows why. Theories abound. We open wide when we are tired, bored, or hungry. Some have suggested that a sudden drop in blood oxygen, or a surge of carbon dioxide pumped out by a tired body, sparks it off – but no, breathing air rich in that gas, or with extra oxygen, makes no difference.
It happens on hot days more than on cold, which leads to speculation that the action cools the brain. On the other hand, someone running a fever indulges in such acts less than normal, while uncontrollable repeats may be a symptom of diabetes or a stroke. Needless to say, a certain part of the brain lights up when the jaw is so engaged, but what that proves is hard to tell.
Involuntary gaping peaks just before bed-time but, oddly enough, disappears when we are lying, still awake, between the sheets. It is also common just after we get up – when, presumably, we are not tired at all.
Perhaps, then, an open mouth is more a sign of some impending change of state rather than a statement of tiredness itself; awake to asleep or vice versa. It may even be a general preparation for some new mental experience. Certainly, people expecting something novel to happen indulge in yawning quite frequently; parachutists about to jump tend do it, as did (allegedly) Neville Chamberlain in the moment when the disastrous failure of his Munich discussions with Hitler became clear and war seemed inevitable.
Yawning is catching, too, but as anyone who has sat through a lengthy schlock-horror movie can attest, so is laughter, vomiting, or (for men at least) having a pee if in company with other gents so occupied. A yawn is more contagious than any of these. People rarely giggle, puke or urinate when reading a witty, repulsive or aquatic novel; but I bet that readers of this column – and those who may be watching them – will drop their jaws even more often than usual as they plough through it.
That may be a hint that the action evolved as a social cue – “time for us all to go to bed” – although as usual when evolution and human behaviour meet, the tie is speculative at best.
However yawns arise, and whatever they signify, such a spontaneous copying response to a second person’s signal of mood is an unmistakable sign of empathy; of an ability to understand and to react to someone else’s state of mind. People with autism or with schizophrenia find it hard to do that – and they respond less to yawns than do most of us.
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