Not much is known about why we yawn, or if it even serves any useful function. In fact, very little research has been done on the subject because for most of us, yawning is not a problematic behavior. Although not yet fully understood, yawning involves complex interactions between unconscious parts of the brain and the body (1).
Although everyone seems to know WHAT a yawn is, scientists are still debating WHY we yawn. So why is the simple act of yawning so laden with controversy? The unsatisfying truth is that although humans have probably been yawning for as long as we have existed, we still cannot come to a consensus on why we do it. Although there are several theories to explain why we yawn, here are the most common:
~The physiological theory~
The physiological theory encompasses a myriad of biological and brain mechanisms. These theorists believe that our bodies use the act of yawning to either drawn in more oxygen, or remove a build-up of carbon dioxide. Could this be the reason we tend to yawn in groups? Larger groups produce more carbon dioxide, which means our bodies would act to draw in more oxygen and get rid of the excess carbon dioxide (2).
But if we yawn expressly to oxygenate our blood, why aren’t we yawning continuously while huffing and puffing on mile two on the treadmill? Robert Provine, a neuroscientist from the University of Maryland who is a leading expert on yawning, tested this theory. He discovered that neither supplementing additional oxygen nor decreasing the amount of carbon dioxide in a participant’s environment significantly decreased the amount of yawning observed (3).
But the brain itself may provide further insights to the physiological theory of yawning. In the brain, an area called the hypothalamus plays an important role in the physiological mechanisms that may result in a yawn. In particular, the paraventricular nucleus (PVN) of the hypothalamus seems to be the “yawning center” of the brain.
Low oxygen levels in the PVN can induce yawning (1). The PVN also contains several chemical messengers that may induce yawning. For example, research shows that yawning occurs when neurotransmitters like dopamine, nitric oxide, glycine, and oxytocin are injected into the hypothalamus of animals (1).
~The evolution theory~
Some evolutionary biologists believe that our early ancestors may have yawned to intimidate others by showing our teeth (2). The evolutionary theory also entices us to accept that yawns are contagious because at one time in evolutionary history, the yawn served to coordinate the social behavior of a group of animals. For example, when one member of the group yawned to signal an event, all the other members of the group also yawned. Could yawning have been evolutionarily selected for as a signaling behavior for us to change activities?
This idea suggests that yawning could have been a form of nonverbal communication to communicate changes in environmental or internal body conditions, perhaps as a way to synchronize behavior (1). We already learned that seeing, hearing, or even thinking about yawning can send us directly into pandiculation. Today, yawning is probably a vestigial behavior (an evolutionary leftover) that is no longer a valuable behavioral response (3). Unfortunately, evolutionary theories such as these are difficult to test under experimental conditions because not only can we not go back and observe our ancestors directly, but cannot easily reproduce the environments of our ancient human ancestors in the present.
~The boredom theory~
We’ve all yawned repeatedly the day following a night of fitful sleep, during a less than spectacular lecture, or following heavy exercise. Also, our heart rate sometimes increases by 30 percent of its pre-yawn pace (2). Could we be yawning to snap out of a sleepy bored funk? The boredom theory seems to fit nicely with physiological theories: when we are bored or tired, our respiratory rate decreases.
As our breathing rate decreases, less oxygen is taken into the lungs. As oxygen levels decrease in the blood, the PVN detects the deficiency and a yawn may be produced (1, 3). Although we do tend to yawn when bored or tired, this theory cannot fully explain the phenomenon of yawning. For example, with the world watching, Olympic trainers have observed their athletes yawning immediately before competing in their event!
Studying complex behaviors like yawning, which involve interactions between the brain and behavior, not only aid us in our understanding of the brain, but help further our quest in understanding human nature. This quest has lead us to believe that the act of yawning is much more complex (and much less boring) than we previously believed.
Although research supports each of these most common theories, often the findings contradict one another. Yawning may not only be a sign of tiredness or boredom, but is also a much more general sign of conditions within the body changing (1), and is probably context-dependent. Therefore, although each theory explaining why we yawn may be unsupported in some contexts, currently the explanations provided by all three theories combined best explain the comprehensive phenomenon of yawning.
1). Andrews, M.A. (2006). Ask the Brains: Why do we yawn? Scientific American Mind: Oct/Nov, pg 82.
2). How stuff works: What makes us yawn? Retrieved from: http://www.howstuffworks.com/question572.htm
3). Chudler, Eric: Neuroscience for kids: Yawning and why yawns are contagious. Retrieved from http://faculty.washington.edu/chudler/yawning.html