WHEN I give talks about my work, I know that before too long the audience will start to yawn. Just thinking about yawning makes us do it, but the mechanisms behind this everyday behaviour are a mystery.
Yawning is often the first thing we do when we wake up, but we also do it when we are bored, tired or anxious. It is a primitive reflex that we share with other animals, from fish to mammals. We first start yawning in the womb, at around 20 weeks gestation, and as we age, we yawn less frequently. It happens most in the morning, but despite yawning being universal, the reason we do it is still unclear.
Boredom leads to yawning, so it has long been assumed that the action is an arousal reflex that makes us feel more awake by stretching the muscles surrounding the lungs, or by bringing more oxygen to the blood and the brain. Yet in tests, breathing air with elevated carbon dioxide levels or even pure oxygen .
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Another theory, proposed by the French physician Olivier Walusinski, is that yawning improves the circulation of the fluid surrounding the brain, which switches on our attentional systems and helps us focus on a task. This might explain why parachutists are encouraged to yawn before they leap, and why elite athletes often yawn naturally before they compete.
But the strangest aspect of yawning is its infectiousness. When we see another person – or even an animal – yawn, it makes us do it too. How can we make sense of these different types of yawning?
In 2007 a new perspective emerged: yawning is a . Experiments lead by Gordon Gallup at the University at Albany, State University of New York, showed that people yawned far less when their head was cooled by a cold pack, compared with if they held a heated pack to their forehead. In fact, this view is not so new. In 400 BC, Hippocrates wrote about yawning in his Treatise on Wind, “…the accumulated air in the body, like steam escaping from hot cauldrons is violently expelled when the body temperature rises”.
The temperature of the brain rises when we are fatigued, so it is possible that yawning is a mechanism to protect the brain from overheating due to tiredness. This fits with my observations of people with multiple sclerosis (MS), who often yawn a great deal when they are fatigued. Common symptoms of MS, a disease in which the body’s immune system attacks the myelin sheath that coats nerve cells, are fatigue and rises in brain temperature. Yawning, fatigue and rises in brain temperature appear to be linked. How?
Temperature regulation is controlled by a brain region called the hypothalamus, and this is intimately linked to two other structures: the pituitary gland – also in the brain – and the adrenal glands, which secrete adrenaline as well as the hormone cortisol. This is produced in response to stress and is important for increasing alertness.
My research has revealed that , and I suspect that it is the rise in cortisol that triggers a yawn (Medical Hypotheses, vol 83, p 494). Yawning seems to raise cortisol levels further, and in doing so protects the body by stimulating the production of adrenaline to make us more alert. It also triggers the hypothalamus to cool down the brain. This might also explain why we yawn when we are anxious, as we often start to sweat and our brains possibly become slightly hotter – though this idea has not been tested.
I came to work on yawning through a different avenue of research. I was fascinated by an observation I made that the temperature at the top of your head makes a difference to the way you feel after exercise. For example, if your head is hotter, you are more likely to feel tired. This led me to investigate fatigue, and in particular why it makes us yawn. Maybe the hormones that underlie the natural response to stress and fatigue were also responsible for yawning?
I am currently exploring the link between cortisol levels, yawning and fatigue in conjunction with a team of neuroscientists at various French universities. We are investigating whether the frequent yawning by people with MS is indeed a protective response, using fMRI brain scanning and cortisol testing. We think that excessive yawning and abnormally high cortisol levels may even be early warning signs of MS.
Other neurological conditions can give us clues about yawning. For example, in 1923 the British neurologist Francis Walshe found that patients who were paralysed as a result of lesions in the brainstem region could raise their arm when they yawned. The yawn somehow overruled the paralysis. This ability is also seen in people who have had a stroke, and indicates that the trigger site of the yawn might be in the brainstem. I plan to investigate yawning in other conditions, such as Parkinson’s disease, too.
“Patients were able to raise their paralysed arm when they yawned”
Infectious yawning
How does contagious yawning fit into this picture? None of these observations explain it. It’s possible that contagious yawning is something different, and might instead have a social purpose. Steven Palek and his colleagues at the University at Albany propose that contagious yawning is part of a more general phenomenon to do with the ability to infer the mental states of others. They found that people’s susceptibility to contagious yawning is linked to their ability to infer or empathise with what others want or intend to do, and that seeing another person yawn taps into the part of our brain responsible for self-awareness.
The response is especially strong with members of our own group, and is influenced by other social factors. For example, chimpanzees are more likely to yawn when they see a dominant male do it, compared with a female. And the likelihood of someone “catching” a yawn depends on their ability to perceive facial expressions – hence – according to research by Atsushi Senju, now at Birkbeck, University of London.
I suspect that the social drivers of yawning can override the physiological ones, so that we yawn when others do irrespective of the more “primitive” drivers such as brain temperature or cortisol levels.
This simple act turns out to be surprisingly complex. Have I made you yawn yet?
This article appeared in print under the headline “This will make you yawn”
