Snakes and fish do it. Cats and dogs do it. Fetuses in the womb do it. But despite being such a common phenomenon, the reasons we yawn remain a mystery.

You may be ten minutes into a lecture about Drosophila, or maybe you’ve just woken up, dreary eyed and slumber­-faced, or perhaps you’ve just witnessed someone else do it. A compelling urge seems to manifest from deep within your being and the more you try to suppress it the more it consumes you. And now you’re yawning and so is that person you awkwardly made eye contact with. Before you know it you’ve started an epidemic.

A yawn is a true test of the physical capabilities of your face to stretch without dislocating your jawbones. But really, it’s a coordinated, purposeful (however inexplicable that purpose may be) movement of your thoracic muscles.

In the late 80s, Robert Provine, a leading expert in the not­ so­ lucrative field of yawning, wrote that “[yawning] has the dubious distinction of being the least understood, common human behaviour”. It’s been more than three decades and scientists have yet to reach a consensus as to what purpose yawning serves.

It’s mystified scientists and laymen for almost 2,500 years. The famous Father of Medicine, Hippocrates, theorised that during fevers, yawning releases noxious air. He wrote that “…the accumulated air in the body is violently expelled from the mouth when the body temperature rises”.

In the 18th and 19th Centuries, scientists challenged this theory and proposed that yawning is a mechanism that increases alertness by raising blood pressure, heart rate and oxygen levels.

This logic persists even today – the most accepted explanation of our jaw­widening, mouth ­gaping incidences is that it brings an abundance of oxygen ­rich air to the brain, keeping us alert during periods of boredom or sleepiness.

But that myth has been busted.

Studies have shown that yawning doesn’t increase oxygen levels, and if it did, wouldn’t you expect to be yawning while exercising?

Turns out, yawning has less to do with respiration and more to do with thermoregulation.

First up, Andrew Gallup, a psychological researcher from SUNY College proposed that the stretching of our jaw increases blood flow to the skull, carrying away excess heat, whilst the simultaneous inhalation cools the blood flowing into the brain. That’s why we yawn the way we do.

Furthermore, Steven Platek, a psychology professor at Georgia Gwinnet College, found that in studies of mice, yawns were preceded by increases in brain temperatures and followed by significant cooling.

In a nutshell, yawning prevents the brain from overheating. All the things that we associate with yawns – sleep, stress, boredom – are characterised by fluctuations in brain temperature. The body and brain are at their highest temperatures before we fall asleep and when we wake up, accounting for those typically sleepy yawns.

Finally, Jorg Massen and his team from the University of Vienna conducted an experiment to see if people were more or less likely to yawn at different temperatures.

Participants holding a warm pack to their foreheads were likely to yawn 41 per cent of the time when watching a video of others yawning, compared to nine percent for those holding cold packs. The brain functions more efficiently when it’s working at its optimum temperature, which may explain why we feel more alert after a hearty yawn.

If this is the case, then why is yawning contagious? Some have suggested that it is a positively contagious phenomenon which maintains herd behaviour. Perhaps the subconscious mimicry cues other individuals to regulate their neural processes, or perhaps it serves a social function.

Studies have shown that our susceptibility to contagious yawning correlates to our capacity for empathy. The areas of the brain activated during contagious yawning, the posterior cingulate and precuneus, are involved in processing our own and others’ emotions.

Platek states that 60 to 70 per cent of people yawn contagiously, and this occurs more frequently in individuals who have a high score on measures of empathetic understanding.

Yawning is so contagious that we only need to think about it to want to yawn. By this point, you’re probably stifling more than a few yawns and hopefully it’s not from boredom.

A word of advice: if you ever find yourself in a situation where you cannot appropriately satisfy that yearning to yawn (perhaps during a particularly tedious conversation), breathe through your nose and you’ll be rid of the sensation completely.

The yawn remains a brain­ boggling phenomenon, but the next time you find yourself jaws astretch, know that you are revelling in one of life’s most enduring mysteries.