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Why do we yawn?

You might have just done it by reading the title of this article. Yet, why do humans yawn and why are they contagious?

Let’s play a game. I dare you to read this entire article without yawning. This task may seem simple, but you will quickly realize it is nearly impossible. Seeing images and even reading about yawning behavior, causes humans to mimic it. In fact, yawning is such an ingrained motor function that in 1923 a British neurologist, Sir Francis Walshe, noticed his paralyzed patients regained motion when they yawned (8). After noticing this, he wrote that yawning is as primary and fundamental as breathing. His viewpoint would later be supported with research performed by Dr. Johanna de Vries, a professor of obstetrics at Vrije University Amsterdam, that showed babies start yawning as early as their first trimester in the womb (8). Furthermore, author and neuroscientist Robert Provine states, “You don’t decide to yawn. You just do it. You’re playing out a biological program.” (8) Thus, humans can not control their yawning patterns, and even if they have a specific goal in mind to not yawn, it can still happen spontaneously as seen earlier. Although it is a foundational trait, why is yawning necessary at all?

The average human being will yawn 240,000 times in their life with each yawn lasting around six seconds (7). Despite this, there is very little conclusive research on the reasoning behind why people yawn. Guinea pigs do it to display anger and snakes yawn to realign their jaws after eating (1). However, scientists have no clear reasoning on why humans must yawn. The first theory proposed by Hippocrates in 400 B.C.E. stated that we do it to expel toxins from our bodies during fever (8). Although this theory was later disproven, it was a popular concept for generations.

One new theory suggests people may yawn as a way to physiologically cool the brain (6). The brain works best at in a particular temperature range and must be cooled often as one-third of your daily caloric intake is used in this region (1). Yawning is associated with things such as increased heart rate, more use of the face muscles, and blood flow which are essential to cooling the brain (1). Furthermore, humans yawn more in states of exhaustion and sleep deprivation- both of which are linked to higher brain temperatures (1). A research study demonstrated that people yawn 41% of the time when watching other people yawn when they place a hot pack on their head as opposed to 9% of the time with a cold pack (1). In addition, a study where fevers where induced in subjects resulted in the warmer individuals yawning more (4). This is also consistent with research done by Gallup on thermoregulation disorders and yawning (5). Yet, there is no affirmative research that shows yawning is biologically responsible for cooling (4). There are only a variety of sources that show a correlation between rising body temperatures and the prevalence of yawning.

In addition to this theory, there are others which suggest yawning may occur as a way to demonstrate feelings of boredom, fatigue, and hunger. This is supported by work done by neurologist Dr. Provine who noticed that people yawn more before waking up and falling asleep (8). He also noted that people put into induced states of boredom tended to yawn more than those who were being entertained (8). Despite this, it is more likely that yawning occurs when humans are transitioning between states. As Provine points out, “You yawn when you’re obviously not bored. Olympic athletes sometimes yawn before their events; concert violinists may yawn before playing a concerto.” (8) This shows that people use yawning not only to communicate boredom but as a method to switch between behavioral states: asleep to awake, boredom to focused, celebrative to calm.

However, this is not the entire picture because this explanation does not explain why yawning is contagious. Recently, it was discovered that contagious yawning does not begin until children are 4-5 years old (1). This is the same time period that they are constructing empathetic emotions. Plus, a fMRI research study performed by the National Institute of Health found that yawning induces activity in the ventromedial prefrontal cortex of the brain which is associated with decision making and empathy (2). It has also been discovered that children and adults with developmental issues that affect how they respond to emotions such as Autism react less to the contagion (1). Therefore, it is very possible that contagious yawning is directly linked to empathetic emotions. However, the notion that empathy is the root caused is challenged by a study conducted at Stanford University. Psychologists Jennifer Yoon and Claudio Tennie lead the research and found little evidence that contagion and empathy were in any way causally related (8).

It is more plausible that yawning is a window into the past and the way humans used to communicate before speech. When people yawn, it is in response to hunger, fatigue, boredom, and moments of high stress. Thus, yawning could be an ancient signal for help (8). In addition, it is also possible that mirror neurons play a role. Mirror neurons fire when people view others doing actions or even hear others talk about a particular action. They are important for learning, self-awareness, and relating to others. Thus, when humans see someone else yawn, they copy the behavior and yawn as well (1). Whatever the ultimate reason is that humans yawn, the research that has been conducted is truly a testament to the ancient past and modern research fusing together. Maybe one day people will understand the complexity of the yawn, but until then, let us ponder its mystery and appreciate its connection to the ancient past.

References:

  1. https://www.youtube.com/watch?v=I0dQx4SNSwE
  2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4041699/
  3. https://www.sciencedirect.com/science/article/pii/S037663571100252X?via%3Dihub
  4. https://www.ncbi.nlm.nih.gov/pubmed/28939427
  5. https://link.springer.com/article/10.1007%2Fs11325-009-0287-x
  6. https://www.sciencedirect.com/science/article/pii/S0031938414001784
  7. https://sleepacademy.org/2012/09/14/behind-the-yawn/
  8. https://www.newyorker.com/science/maria-konnikova/the-surprising-science-of-yawning

My name is Ellie Hummel and I'm a junior at DuPont Manual High School in Louisville, Ky. I am a contributor for The Student Scientist. I love biology research and computer programming and hope to spread these passions to others through my writing. Over the course of the past three years, I have researched spinal cord injuries and axon growth. I also work for STEMY, a local non-profit dedicated to providing STEM resources to youth and lead my school's SNHS and HOSA chapters. In my free time, I play with my dog, Harry, and coach swim lessons.

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