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Humans and monkeys do it, dogs and cats do it, and birds and fish yawn too. If something is so widespread, it must have a purpose. Or rather, purposes, because in addition to sleep, yawning has a variety of functions: it aids breathing, empathy, and alertness.
“Yawning is a silent cry for coffee” is a phrase that can be read on many a cafeteria mug. It has some merit, but it doesn’t quite satisfy the requirements of a scientific explanation—in this case, the field responsible is chasmology, as the study of the causes of yawning is officially called. It has developed all kinds of hypotheses, some of them curious, as to why we regularly open our mouths wide to… achieve what? The most critical hypotheses are listed below.
Oxygen has no influence
First, there is the assumption that yawning serves to enrich the blood with oxygen or counteract a lack of oxygen caused by fatigue. This hypothesis is by far the best known—and the only one that has been clearly refuted.
In the 1980s, American psychologist Robert Provine, a pioneer in “yawnetics,” had test subjects breathe air with varying oxygen content, and one group even breathed pure oxygen, and noted the frequency of yawning. The result: although the measure changed the oxygen content in the blood, it did not change the frequency of yawning. The experiment was repeated in variations, but the result remained the same: oxygen plays no role.
Cooling effect and muscle tone
However, there is evidence to support the cooling hypothesis. The basic idea is that after the body temperature drops slightly during sleep, yawning could prepare the head and body for cooling by inhaling cold air. Strictly speaking, it is about reaching an ideal temperature, as a research team at the University of Vienna discovered a few years ago. People yawn less in particularly hot or cold air—and most at around 20 degrees Celsius. This indicates that heat exchange does indeed play a role.
The following idea is also plausible: yawning could serve to stabilize breathing. “We have muscles around the trachea that keep the airway open,” German sleep researcher Dieter Riemann told ORF Wissen. “When we get tired, these muscles are no longer as toned. Most people have short pauses in breathing when they fall asleep. The idea is that yawning really stimulates the airways again.” In other words, it is a gymnastic exercise for the respiratory muscles, or, to put it more strongly, a prophylactic measure against breathing interruptions.
Increased alertness
A new hypothesis has recently been added that focuses on brain activity. The “arousal hypothesis” states that yawning is a kind of switch that mediates between mental rest and increased alertness. According to this hypothesis, the causal chain is as follows: yawning increases the heart rate and activates the carotid artery, which leads to the release of messenger substances such as adenosine and catecholamines. These messenger substances help the brain ramp up its activity.
The interpersonal aspect should not be forgotten either. Yawning is contagious, which may strengthen group cohesion. It is also conceivable that we use it to demonstrate our capacity for empathy. This would be supported by the fact that mirror neurons are active when we are infected by the yawning of others, sometimes even across species boundaries.
Yawning is contagious, even for monkeys
A few years ago, the journal Biology Letters reported what dog owners have always known: dogs are also infected by yawning humans. More surprising were the findings of British researchers this summer: when chimpanzees see a humanoid robot imitating the movement pattern of yawning, they themselves begin to yawn heartily.
“Because we are tired”
So which hypothesis is correct? It is very likely that several of the causes currently being discussed are valid. Sleep researcher Dieter Riemann assumes that yawning developed over the course of natural history for physiological reasons and that social signals were then superimposed on it, so to speak.
In any case, the pivotal point of the arguments is fatigue: “We yawn because we are tired. If we’re not tired, we don’t yawn.” This is what yawn therapy takes advantage of. It’s a non-drug intervention developed by Scottish sleep researcher Colin Espie: when his patients suffer from sleep problems, he shows them videos of people yawning.
source: Robert Czepel, ORF Wissen/picture: Bild von Kathrin Pienaar auf Pixabay
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