Yawning warning: why do we yawn?

Warning: you may find yourself yawning as you read the following blog contribution, because one of the many triggers of yawning is reading or thinking about it. I certainly yawned frequently while researching this piece.

Like sneezing, about which I recently wrote, yawning is a physiological phenomenon that is far from fully figured out. Its scientific study has recently acquired a name, chasmology, which has yet to find its way into dictionaries.

A yawn consists of the simultaneous inhalation of air and the stretching of the eardrums, followed by an exhalation of breath. It often occurs before and after sleep, but is also associated with stress, boredom and even hunger. It may be accompanied by a general stretching of the muscles, in an act known as the stretch-yawn syndrome or pandiculation.

Yawning is also contagious, as Erasmus noted 500 years ago when he wrote, “One man’s yawning makes another yawn.” And contagious yawning may occur across species. We may yawn after seeing pets or zoo animals yawning, and they may yawn after watching us.

So what is the purpose of yawning? At least 20 physiological reasons have been proposed, with little agreement. Many mysteries surround the phenomenon. Why, for instance, does it frequently occur before we go to bed and after we rise but not while we are lying awake in bed?

Since the 19th century, yawning has been claimed to be a reaction to hypoxia and/or hypercapnia, triggering a rush of oxygen into the blood and flushing out carbon dioxide. But studies have disproved this theory, showing that yawning has no effect on the body’s oxygen or carbon dioxide levels.

The current physiological theory is that yawning may be a thermoregulatory mechanism. The idea is that when the brain becomes too hot, yawning helps cool it down, increasing the heart rate and blood flow while delivering a big gulp of air that directly cools blood in the head. Sleep deprivation and exhaustion are both known to increase brain temperature, so it may be that yawning when we are tired simply helps to cool our brains rather than help us to stay awake.

The thermoregulation theory may also explain contagious yawning, which could be a herd behaviour that helps keep every member of the group alert.

Studies by researchers from the University of Vienna found that contagious yawning was most frequent when the air temperature was such that the yawned air would cool the body down. It was less common when the air was so cold or so hot that rapid inhalation might be harmful.

One of the latest theories is that yawning somehow resets the brain, stopping current activity and allowing the “cleaned” brain to focus on the next task. This may explain why, for instance, athletes may yawn before a race and musicians before a concert.

That’s all: you can stop yawning now.