Why are you yawning right now?

Posted by Susan Lubejko on August 1, 2019 in Neuroscience | Leave a comment

As a graduate student, spending an early morning or late night in the lab is not uncommon. During those hours, it’s also not uncommon to catch me in the midst of a yawn (or many). This makes sense, though; I’m tired from little sleep or a long day. However, you may be surprised to learn that for something as common as a yawn, there is very little scientific consensus as to why we yawn at all. What, if anything, does yawning contribute to our body and brain? Can yawning, spontaneously or in response to someone else’s yawn, tell us anything about the wiring of our brains?

Yawning is a common reflexive movement in humans and throughout the animal kingdom, including our cats and dogs, but even occurring in some birds, fish, and reptiles [1]. The stereotyped behavior consists of a long inhale, a brief pause, then a rapid exhale, and is usually accompanied by facial muscle movement, including the jaw stretching open and the eyes closing [2]. There are two main types of normal yawns: spontaneous and contagious. Spontaneous yawns often happen when we are bored or tired and may serve a variety of different functions, such as improving respiration. Contagious yawning describes how we are often prompted to yawn when we hear, see, or even think about another person yawning (Come to think of it, are you yawning right now?). The propensity to catch yawns from others varies by age and even from individual to individual. In the animal kingdom, contagious yawning seems to be restricted to humans, certain primates, and dogs [3].

While both spontaneous and contagious yawns are exceedingly familiar occurrences to most, controlled laboratory research has only begun to answer questions about the long-observed phenomenon. Importantly, yawning in a research setting would typically be categorized as contagious yawning because study participants are aware of what is being studied and are often provided with a yawn-inducing visual stimulus, but a lot of the results are thought to be applicable to both types of yawns. Yawning can be examined through two major lenses: physiological (what yawning might be doing for our bodies) and social (how we might use yawning for interpersonal communication).

Physiological Theories for Yawning 

Hypotheses regarding the physiological benefit of yawning have a varied history. The oldest hypothesis, dating back to the 1700s, is known as the respiratory hypothesis. This theory holds that yawning occurs when the oxygen levels are decreased and carbon dioxide levels are increased in the blood as detected by special receptors in the brain and blood vessels of the head and neck. Yawning, then, forces the body to take a deep breath of oxygen-rich air to restore the balance of these gasses [2]. When we are tired, we tend to take shallow breaths, which could cause a buildup of carbon dioxide in the blood. However, despite the continued popularity of this hypothesis among some clinicians, there isn’t much evidence that this is the reason for frequent yawns. The most popular argument against this idea lies in a research article from 1987 where healthy volunteers breathed pressurized gasses containing different mixtures of oxygen and carbon dioxide to change the amount of these gasses in their blood. Yawning frequency did not change when the amount of carbon dioxide was increased [4].

A second hypothesis for yawning is known as the arousal hypothesis. In this case, arousal simply refers to a state in which an organism is actively awake and paying attention to the stimuli that surround it. Yawning often happens at the transitions between sleep to awake states and during boring periods, where a boost in arousal would be most beneficial. Based on physical movement levels after yawning and the neurochemicals that are released in the brain during a yawn, many scientists have concluded that yawning has an “anti-sleep effect” and increases arousal [2]. Interestingly, it has been proposed that the stretching movements of the body that often accompany yawning could increase the circulation of cerebrospinal fluid (the fluid that surrounds and cushions the brain and spinal cord) to flush out neurochemicals that promote sleep [5]. However, the arousal hypothesis is also imperfect, as actual measurements of arousal in human subjects after a yawn, such as measurements of brain waves using an electroencephalogram (EEG) and skin conductance, have provided mixed results [2].

More recently, some yawn researchers have subscribed to the brain-cooling hypothesis. The deep breath of cool air along with the sharp jaw movements associated with yawning may cause cerebral cooling, i.e. a decrease in the temperature of the brain. As body and brain temperatures increase, we tend to feel sleepy. Yawning may then serve to combat this sleepiness by cooling the brain [6]. In one study, pedestrians in Vienna, Austria and Tucson, Arizona reported more yawns in the summer than in the winter. The authors concluded that the changes in ambient temperature that correspond to each season affect the drive to yawn [7]. This hypothesis too, however, has met opposition from other yawning experts who argue that while yawning and body temperature are probably controlled by a similar part of the brain, this does not necessarily mean that the two are directly related. Additionally, drops in brain temperature hypothesized by proponents of the idea might not actually be possible by yawning alone [2].

Social Theories for Yawning

While there are contested opinions about what yawning may be doing for our brains and bodies, another quite different idea has been proposed. What if yawning isn’t actually doing anything for us at all physiologically? Proponents of the communication hypothesis argue that yawning is simply used to communicate nonverbally within a group. In humans, yawning could be used to signal boredom to a larger group. In certain species of animals, yawning signals stress or a threat [2]. Support for this claim is provided by the fact that yawns are incredibly contagious. In the laboratory, 40-60% of people yawn in response to a yawn stimulus (a video or image of a person yawning) [8]. In many people, simply thinking about yawning spurs the difficult-to-control action.

The proposed reasons for why yawning is contagious and why the phenomenon varies among individuals have evolved over time. A strong contender that has been used to explain contagious yawning is empathy. Contagious yawning typically starts at ages when children are able to understand the emotions of others, and the occurrence of contagious yawning increases with increasing social bond (i.e. we are more likely to catch the yawns of family members or romantic partners compared to those of acquaintances) [9]. Also, contagious yawning is impaired in individuals with psychiatric illnesses such as schizophrenia that impair social and empathetic interactions [10]. However, a more recent study that included a large group of over 300 participants in the laboratory setting and at home found that although individuals are very consistent in their own propensity for contagious yawning over the testing period, variation between individuals could not be explained by careful scoring of empathy [8]. This result implies that the brain mechanisms that regulate empathy may not be the same that control contagious yawning. Instead, those authors found that a better predictor of contagious yawning is actually age. 82% of people under 25 exhibited contagious yawning, while only 41% of people over 50 did [11].

In an effort to further explain the contagious yawning phenomenon, scientists have increasingly looked toward concrete neural mechanisms. A reasonable first hypothesis in this regard was that the brain’s “mirror neuron system” (MNS) is responsible for contagious yawning. The MNS contains neurons that are active when observing another person complete an action and facilitates our imitation of that action. While this is a tempting explanation for contagious yawning, human neuroimaging experiments did not find that people had more neural activity in the MNS during contagious yawning [12,13]. In other words, contagious yawning isn’t exactly an imitation of the original yawner.

Instead, contagious yawning has been categorized as an “echophenomenon,” which is an automatic motor movement that can be triggered by observing another individual and are more commonly observed in pathological conditions, such as Tourette syndrome. So, instead of a calculated imitation of a complex action that has to be processed by our MNS, contagious yawning is so speedy because we already have a model in our brain of how we should yawn; observing another individual yawn simply unlocks it. A 2017 study found that contagious yawning is increased in people that have more excitable neurons in their motor cortex, a brain region crucial for coordinating movement. People with more active motor cortices are more likely to unlock the stereotyped movement of yawning. This result not only tells us how contagious yawning might be encoded in the brain, but also why different people show different amounts of contagious yawning [3].

Why do we yawn and what can we learn?

For an action that almost all of us experience regularly, there is an awful lot more to learn about yawning. Both spontaneous and contagious yawns when studied in the lab can give clues about potential physiological and social effects of yawning. Why do we yawn? What, if anything, is yawning contributing to our bodies and brains? How can yawning be used for communication among our very social species? Most interestingly, as we continue to study the neural mechanisms for contagious yawning in particular, some researchers believe that our understanding of this phenomenon will furnish our understanding of some difficult-to-treat conditions with similar neural and behavioral signatures, such as Tourette syndrome and epilepsy [3].

So, if you feel that you’ve been yawning more than usual while reading this article, you’re probably not alone, but why are you doing it?

References

1. Baenninger R (1997) On yawning and its functions. Psychonomic Bulletin & Review, 4(2):198-207
2. Krestel H, Bassetti CL, Walusinski O (2018) Yawning – Its anatomy, chemistry, role, and pathological considerations. Progress in Neurobiology, 161:61-78
3. Brown BJ, Kim S, Saunders H, Bachmann C, Thompson J, Ropar D, Jackson SR, Jackson GM (2017) A neural basis for contagious yawning. Current Biology, 27:2713-2717
4. Provine RR, Tate BC, Geldmacher LL (1987) Yawning: no effect of 3-5% CO2, 100% O2, and exercise. Behavioral and Neural Biology, 48:382-393
5. Walusinski O (2014) How yawning switches the default-mode network to the attentional network by activating the cerebrospinal fluid flow. Clinical Anatomy, 27:201-209
6. Rabin RC (2019) Why do we yawn? The New York Times. February 22, 2019
7. Massen JJM, Dusch K, Eldakar OT, Gallup AC (2014) A thermal window for yawning in humans: Yawning as a brain cooling mechanism. Physiology & Behavior, 130: 145-148
8. Bartholomew AJ, Cirulli ET (2014) Individual variation in contagious yawning susceptibility is highly stable and largely unexplained by empathy or other known factors. PLoS ONE, 9(3):e91773
9. Norscia I, Palagi E (2011) Yawn contagion and empathy in homo sapiens. PLoS ONE, 6(12):e28472
10. Haker H, Rossler W (2009) Empathy in schizophrenia: impaired resonance. European Archives of Psychiatry and Clinical Neuroscience, 259:352-361
11. Samuelson K (2017) Why is yawning contagious? Time. June 8, 2017
12. Platek SM, Mohamed FB, Gallup GG (2005) Contagious yawning and the brain. Cognitive Brain Research, 23: 448-452
13. Schurmann M, Hesse MD, Stephan KE, Saarela M, Zilles K, Hari R, Fink GF (2005) Yearning to yawn: the neural basis of contagious yawning. Neuroimage, 24:1260-1264

Image sources:

1. https://www.maxpixel.net/
2. https://www.flickr.com/photos/tambako/24087030771
3. https://en.wikipedia.org/wiki/Joseph_Ducreux