I intended, when I began thinking about the topic for this entry, to write up a single entry on yawning, since I’ve been marginally interested in it for years.  However, when I began to look at the literature, I realized yawning is a much bigger and more interesting phenomenon than I previously thought.  To that end, I have decided to spread the topic out across the next several entries by addressing the following questions in turn:

What is a yawn?
What is the neurophysiology of yawning?
Why do we yawn?

The last may end up broken into at least two sections, one on hypotheses regarding the physiological function of yawning, including a recent one that seems to be gaining a lot of traction, and hypotheses on the communicative function of yawning.  There may also be an entry into yawning as a symptom, since it turns out excessive yawning is seen in a number of ailments, including migraines, strokes, and adverse responses to medication.

What is a yawn?

Questions about yawning are quite old, dating back to the ancient Greek philosophers, such as Hippocrates and Galen (both should be familiar to you from last week’s lecture material and the textbook).  According to Walusinski (2010), Hippocrates wrote the first treatise describing and attempting to explain yawning, published in 400 B.C.  Hippocrates linked yawning to the elemental humour of air (the others being fire, water, and earth), and proposed that it was the body’s way of reducing its temperature during a fever.  This idea has not entirely left us, as you shall see in a later blog entry on the function of yawning.

A yawn is a highly stereotypical and involuntary behavior that includes three distinct phases.  The first phase is inspiratory, a slow, involuntary intake of breath.  This is immediately followed by the acme or peak of the yawn in the second phase, during which the eyes are closed and the yawner may stretch.  The yawn concludes with the third phase, a rapid expulsion of breath (Barbizet, 1958).  One interesting feature of yawning is that the airways open significantly, which has led theorists to conclude that yawning serves some regulatory function.  Gallup and Gallup (2005) point out several interesting aspects of yawning culled from a review of the literature, namely

(1) Yawning is observed in 20-week humans in utero, and continues across the lifespan.

(2) Most people close their eyes at the peak or acme of the yawn.

(3) Yawns can last up to 10 seconds, but typically are four to seven seconds in duration.

(4) Yawns are often accompanied by stretching.

(5) People yawn most frequently before or after sleeping.

(6) Yawning is associated with boring activities.

(7) Yawning is found in an enormous number of species.

Guggisberg (2010) also notes that yawning is very old, and is found in most vertebrate species.  Since it is so wide-spread among species, and appears to have evolved long ago, many researchers assume yawning serves some adaptive function.  Several theories about yawning have been proposed, though until recently there was little in the way of empirical work on the subject.  An early theory suggests that yawning is communication, while another theory explores the idea that yawning serves to regulate our physiology in some way.  Within each of these theories are several hypotheses that have recently been tested.  To date, however, there is little about our understanding of yawning that is definitive.  It remains an interesting enigma as a behavior, and surprisingly complex.

Finally, there are several types of behaviors that are tedrmed “yawns,” though it is unclear how they are related to the yawning behavior I have outlined above.  For example, male primates will perform a “yawn display” in which their mouths open very wide to show their canines while their eyes remain open.  Fish and birds also routinely open their mouths, in what look like yawns.

On a personal note, I hate yawning, and it always seems to crop up on me when I least expect it, or am likely to be most embarrassed by it.  I find if I have appointments with students late in the afternoon I often have to apologize for yawning while we’re talking, no matter how hard I try to suppress it.  Around 4pm is my low-energy point of the day, and I notice an increase in yawning, accompanied by difficulty concentrating.  One thing I have inadvertently discovered, however, is that if I breathe through my nose, I yawn less often.  And, it turns out, there’s research that has been done on this, so look for more details on that in an upcoming blog entry.

Finally, we all also know that yawns are contagious.  I used to sit in church as a young girl and watch the yawns travel through the people in the pews.  It was actually more interesting than the church service to me.  We now know more about why yawns are contagious, but answering that question opened up another mystery about yawning, which I will talk about when we discuss the physiology of yawning.  As I watched people around the church yawn, I would often wonder if there were any differences in susceptibility to contagious yawning.  Some people rarely yawned, I noticed, but others spent the entire church service yawning.  I remember wondering if you could make all other things equal (amount of sleept, etc) if you’d find differences in people’s susceptibility to yawning, and what that would tell you.  I was a budding scientist, even then, it seems.

References

Barbizet, J. (1958). Yawning. Journal Of Neurology, Neurosurgery & Psychiatry, 21203-209. doi:10.1136/jnnp.21.3.203

Gallup, A. C., & Gallup, G. R. (2007). Yawning as a brain cooling mechanism: Nasal breathing and forehead cooling diminish the incidence of contagious yawning. Evolutionary Psychology, 5(1), 92-101.

Guggisberg, A. G., Mathis, J., Schnider, A., & Hess, C. W. (2010). Why do we yawn? Neuroscience And Biobehavioral Reviews, 34(8), 1267-1276. doi:10.1016/j.neubiorev.2010.03.008

Walusinski, O., & S. Karger (Firm). (2010). The mystery of yawning in physiology and disease. Basel [Switzerland: Karger.

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