Both animals and humans yawn. Why do we yawn and why is it contagious?
It's not because we need more oxygen to the brain.
When the urge to yawn arises it is difficult to resist. Your jaw opens wide and air is drawn down into your lungs.
If we see others yawning, we often have to yawn ourselves. Seeing a picture of someone yawning, or just reading about it, can be enough.
Yawning often occurs when you are tired or bored. This has led researchers to wonder whether yawns have a social function.
Maybe it is meant to signal how you feel to others. It has also been suggested that ‘answering’ a yawn with a yawn is linked to empathy (link in Norwegian), however, studies have found this not to be the case.
Others believe that the reflex has a physiological basis.
Over 20 different reasons (link in Norwegian) for our yawning have been suggested.
In a new study in the journal Animal Behaviour, evolutionary biologist Andrew Gallup reviews research on yawning.
He is a researcher at the State University of New York Polytechnic Institute in the United States and has studied the subject for many years.
Gallup believes that yawning must have had a purely physical function in the beginning. He believes yawning goes back a long way in evolutionary history.
There are in fact many different animals that yawn, even animals that live mostly alone.
In a study from 2021, researchers found a connection between the size of the brain and the duration of a yawn in 100 bird and animal species. This suggests that yawning has a physical effect on the brain.
Yawning may later have developed an additional social role in some species.
Not to do with oxygen
You may have heard that we yawn to get more oxygen into our blood. That's not true, according to Gallup.
“Despite continued belief, research has explicitly tested that hypothesis and the results have concluded that breathing and yawning are controlled by different mechanisms,” Andrew Gallup says in an interview with the journal Science.
You do not yawn more or less even if the levels of oxygen or CO2 in the air change.
“There are some really interesting cases of yawning in marine mammals, where the yawning occurs while the animal is submerged under water, and therefore does not breathe,” Gallup says.
Does it cool our brains down?
Researchers are not entirely sure why we yawn or why it is contagious. But there have been several studies that support the belief that yawning can help regulate the temperature in the brain.
Gallup was the one who came up with the idea. The hypothesis was first presented in the journal Evolutionary Psychology in 2007.
According to the theory, yawning works by cooling down the brain. It should improve the ability to pay attention and have an invigorating effect.
The basis of this hypothesis is that the brain works best within a narrow temperature range and that you can become tired if your brain does not have optimal conditions, according to a previous article on forskning.no (link in Norwegian).
Several experiments have been conducted
Laboratory studies of humans, rats and birds have shown that higher temperatures in the head trigger yawning. After the yawn, the temperature drops, Gallup writes.
In a study from 2014, researchers tested whether the outdoor temperature affected how often people yawned. It turned out that people yawned more when it was hot. At the same time, they yawned less when the temperature was around 37 degrees Celsius, which is a normal body temperature. At that point it was no longer an effective way to cool down the brain.
People yawn most at night when the temperature in the brain is at its peak and after getting up, when the temperature starts to rise again after being at its lowest point, according to a previous review study on the theory from 2012.
The hypothesis that yawning regulates brain temperature now has the most scientific support, according to Norwegian Health Informatics (NHI).
The website explains that when we draw a lot of air down into our lungs in a yawn, it cools down the blood that flows to the brain. In addition, the wide opening creates pressure that pushes blood out of the brain. The new, cooled blood flows in quickly.
In an article published in the journal Søvn (Sleep) in 2019, sleep researcher Ståle Pallesen also writes that it seems that the thermoregulatory theory has the most support, in combination with theories of social causes.
The hypothesis has also been criticised. In a review from 2010, researchers considered the evidence to be unconvincing.
The criticism has been, for example, that it is difficult to know if people yawn more when it is hot because you simply get more tired, or because the brain needs to be cooled down.
Some have argued that the theory cannot explain why fetuses yawn (link in Norwegian). They do not need to cool their brain as their mother controls the thermoregulation. Another point is that the theory cannot explain yawn-like mouth gaping in cold-blooded animals.
Stress and activity shifts
You’re more likely to yawn just before and after sleep. This applies to both animals and humans.
Gallup writes that yawning is linked to shifts in activity, either from sleep to wakefulness, or from inactivity to movement.
According to the article, animals and humans can also start yawning when a stressful situation builds up.
It can be before a musician goes on stage or before athletes have to perform in a competition. Stress can increase body temperature, according to Gallup.
Yawning may thus help to regulate the temperature in the brain. But why are we often triggered to yawn when we see others doing it?
Contagious yawning has been documented in budgies, chimpanzees, wolves, pigs, and lions.
"Contagious yawns are elicited by seeing or hearing yawns in others, and [they] have only been documented in highly social species, humans included," Gallup told the journal Science.
Does it increase the attention of the group?
Gallup points out two possible reasons why yawning is contagious, which he believes fits with previous research.
"Contagious yawning may have evolved to synchronise group behavior – yawns often cluster during particular times of day that coincide with transitions and activity," he tells Science.
In the article, Gallup mentions a study of two lion prides, where it was observed that contagious yawning preceded shifts in activity in the group, from rest to movement, or vice versa.
“Yawning may also have evolved to increase alertness in a group,” Gallup says.
The idea behind this is that a yawn can show the others in the group that you are sleepy. An animal that is falling asleep is less aware of dangers.
If the yawn spreads to the others in the group, it can have an invigorating effect that increases the vigilance of others. It can, according to the hypothesis, compensate for those who are tired.
“The spread of contagious yawns can increase the vigilance of the whole group,” Gallup says.
Gallup writes that more research is needed to understand what kind of social role yawning plays.
Researchers can, for example, test whether seeing others yawn makes you more aware of dangers.
Translated by Alette Bjordal Gjellesvik.
Read the Norwegian version of this article on forskning.no
Andrew C. Gallup The causes and consequences of yawning in animal groups, Animal Behaviour, 187, 2022. DOI: 10.1016/j.anbehav.2022.03.011
Massen et al. Brain size and neuron numbers drive differences in yawn duration across mammals and birds, Communications Biology, 4, 2021.
Andrew C. Gallup and Gordon G. Gallup jr. Yawning as a Brain Cooling Mechanism: Nasal Breathing and Forehead Cooling Diminish the Incidence of Contagious Yawning, Evolutionary Psychology, 2007. Doi.org/10.1177/147470490700500109
Ståle Pallesen Hvorfor gjesper vi? (Why do we yawn?),Søvn, 2, 2019.
Guggisberg et al. ‘Why do we yawn?’, Neuroscience & Biobehavioral Reviews, 2010. Abstract.
Andrew C. Gallup and Omar T. Eldakar The thermoregulatory theory of yawning: what we know from over 5 years of research, Frontiers in Neuroscience, 2012. DOI: 10.1016/j.anbehav.2022.03.01110.3389/fnins.2012.00188
Tess Joosse Why yawns are contagious — in all kinds of animals, Science, 2022.