All vertebrates practice yawning, or engage in yawning-like behavior. Until relatively recently, the purpose of yawning was not clear, and it is still a matter of controversy among researchers and scientists. But why do we yawn? Is that useful?
“When I survey the public and ask: ‘Why do you think we yawn?’, most of them point out that it has to do with breathing or the process of breathing, and that it might increase oxygen in the blood in some way,” Andrew Gallup, a professor of behavioral biology at Johns Hopkins University, told The Guardian.
This is self-evident, as most yawning does include a clear respiratory component, which is inhaling the air deeply, but what most people do not realize is that this hypothesis has been subjected to careful testing and shown to be wrong.
To test the idea that yawning brings in more oxygen or expels excess carbon dioxide, studies published in the 1980s manipulated levels of both gases in the air that volunteers inhaled.
She found that although the changes significantly affected other breathing processes, they did not affect the regularity of yawning. There also does not appear to be any systematically measurable difference in the yawning behavior of people suffering from diseases related to breathing and lung function, which is what would be expected if yawning were related to breathing.
The subject interested Gallup and he began to examine the pattern of movement involved in this extended abduction of the jaw accompanied by a deep inhalation of air, followed by a rapid closing of the jaw and a faster exhalation. It occurred to him that this was likely to have important cyclic consequences local to the skull.
Jaw spacing can be thought of as a localized stretch, similar to muscle stretching in other areas of the body, and this appears to be exactly what happens as several studies indicate that yawning increases arterial blood flow to the skull, and then venous return (the rate of blood flow from the head to the heart).
Cranial temperature regulation
“We can think of jaw abduction as a localized stretch, similar to muscle stretching in other areas of the body,” Gallup says. Just as stretching helps stimulate circulation in the extremities, yawning appears to improve circulation in the skull.
Gallup and his colleagues began to develop the idea that yawning helps regulate temperature in and around the skull.
Brain temperature is mainly determined by 3 variables:
- The rate of arterial blood flow to the brain
- The temperature of that blood
- Metabolic heat production that occurs within the brain based on neural activity.
Yawning can, in theory, change the first two variables. When we yawn, you take a deep breath of air that moves across the moist surfaces of the mouth, tongue, and nasal passages.
There are studies that confirm this, as the ambient temperature has a largely predictable effect on the frequency of yawning, as it increases when the weather is very warm and decreases when the weather is colder.
Another theory
But this theory is not the only one. There is evidence to support another theory, which is the “change of arousal” theory, which is simply that yawning helps the brain transition between states from sleep to wakefulness, from boredom to attention, and so on.
Medical historian Dr. Olivier Valusinski, who has authored several research papers on the subject, according to the newspaper, says: “It is possible that yawning helps the brain switch between using the default mode network – the areas associated with daydreaming, recalling memories, and self-reflection – and the attention network, which is responsible for preparing the body for activity.”
“One proposed mechanism for this is that it helps circulate cerebrospinal fluid, which is the fluid that surrounds and supports your brain and spinal cord,” he adds.