The yawn: a mystery in plain sight.
Not a great deal of research has gone into studying yawning; it has not necessarily captured scientists' imagination en masse. But, once you start to peel back the layers, there is much to talk about.
The process of yawning seems relatively simple: the mouth opens, there is a swift intake of air, a brief pause in breathing, a longer expiration, and sometimes an accompanying stretch.
But that is far from the full list of events. A yawn is a relatively complicated behavior, spread over an average of 6 seconds.
Facial muscles stretch and the head tilts back. The eyes narrow or close and can often water. Saliva is produced, and the Eustachian tubes of the middle ear are opened. Beyond this, there are probably cardiovascular, neuromuscular, and respiratory changes that we are yet to define.
It is common knowledge that we yawn when we are bored or tired, and studies have demonstrated this to be scientific fact. However, there are also reports of paratroopers yawning before a skydive, musicians yawning before hitting the stage, and dogs yawning before they attack - all of which mean that there is obviously more to the yawn than sleepiness.
The sexuality of a yawn
Interestingly, the yawn shares a number of components with other behaviors. For instance, there are some similarities between the "yawn face" and the "orgasm face." Some researchers believe that there may be common origins, and although at first glance that may seem a little absurd, there is some evidence of a link. For a start, yawning is triggered by androgens (male sex hormones) and oxytocin. Furthermore, most drugs that produce stretching and yawning in rats also cause penile erection.
Another intriguing link between sex and yawning concerns antidepressants. Drugs including clomipramine and fluoxetine tend to depress sexual desire, and in some people, they also produce an odd side effect: their yawns trigger orgasm.
We have all been yawning since we were in the womb, and a whole host of animals - from flies to snakes, and from bears to badgers - all indulge. It is clearly an ancient behavior, and the fact that it has been conserved throughout evolution means that it must do something useful.
The program that controls yawning appears to rest in an ancient part of the brain, too. As an example, patients with amyotrophic lateral sclerosis who are "locked in" and who have virtually no control over their muscles can yawn normally. Neurologists believe that this means the yawn coordinates are kept in the brainstem, an evolutionarily old part of the brain, along with the centers responsible for controlling breathing and vascular tone.
Yawn contagion and primal empathy
Bonobos "catch" yawns more easily from those they are closest to.
One of the most interesting things about yawns are their contagious prowess. As you read this article, many of you will be yawning as a result of simply thinking about it.
According to Robert R. Provine, the most prolific yawn researcher, he himself has become a "yawn stimuli." Because his friends are all aware of his work, he simply has to enter a room and people begin to yawn.
Human-to-human yawn infection is well documented. However, it is not contagious in children until they reach around 5 years of age.
Catching yawns in non-human animals is much less common, but it has been observed in chimpanzees and some other primates.
A study looking at yawning in gelada baboons found that yawns were more contagious among individuals that shared a closer relationship, as measured by levels of grooming. A similar observational study on bonobos also found that a yawn is more likely to be contagious when it is spread between individuals that are more closely bonded.
Aside from primates, it has been demonstrated that dogs more easily catch yawns from their owner than from a stranger, which provides more evidence that being close knit enhances the yawn bond.
To some researchers, these findings mean that yawning might offer an intriguing insight into the human mind - specifically, an insight into primal empathy.
Researcher Steven Platek and his colleagues conducted a study in which participants received MRI scans while watching other people yawning. Activity was measured in the posterior cingulate and precuneus brain regions. These areas are involved in self-referencing, theory of mind, and autobiographical memory. The authors conclude:
"Our findings provide further support for the hypothesis that contagious yawning may be part of a neural network involved in empathy."
This potential measure of empathy could prove useful in relation to certain mental conditions. For instance, in disorders such as autism and schizophrenia, an individual's ability to assess other people's emotions and motivations are substantially reduced.
As an example, one study published in 2009 found that individuals with schizophrenia were less susceptible to both yawn and laughter contagion.
Not everyone believes that yawning is linked with empathy, however. An experiment published in PLOS One found only one significant predictor of yawn contagion: age. Older people were significantly less likely to yawn when watching a video of people yawning than younger people were. The researchers also found that an individual's level of empathy (as assessed by questionnaires), time of day, and intelligence were not significant predictors of one's likelihood to catch a yawn.
Either way, there certainly is a social aspect to yawns - but that fails to answer the question of why we yawn, specifically. It could just as easily be sneezing or hiccups that are contagious, so why is it this long, deep breath?
If yawning has been retained throughout evolution, it must do something physiologically important. There are a few theories, but, to date, no single notion offers a water-tight explanation. Below are some of the most popular.
Too much or too little gas
This is the most commonly known theory surrounding yawns. The idea is that either too little carbon dioxide (CO2) or too much oxygen (O2) produce a yawn to restore the balance.
If we were to travel back to around 400 BC and ask Hippocrates, he would have told us that yawning was to remove "bad air" from the lungs. But, if history has taught us anything, it is that an old theory is not necessarily a good theory.
However, in the only experiment to test this theory, both variants were soundly put to bed. The authors concluded:
"The CO2/O2 hypothesis was rejected because breathing neither pure O2 nor gases high in CO2 had a significant effect on yawning although both increased breathing rate."
From a common sense stance, yawning as a way of altering gas levels in the lungs seems unlikely. Increased breathing rate is the normal way for the body to increase oxygen levels in the blood, and there is no evidence that yawning can do any better.
If readers would like to test this theory themselves, simply hold your breath for as long as you can - you'll find that it will not produce a yawn.
Another theory is that the act of yawning helps to increase oxygenation in the tissues. Again, the theory has holes; although yawns do trigger the autonomic nervous system, thereby increasing vasodilation and heart rate, it does not affect these systems any more than simple body movements or deep breaths.
Yawns improve arousal
Could a yawn help us to stay alert?
Another theory is that, by some mechanism, yawning stimulates and refreshes a tired brain. It is certainly true that yawning tends to occur just before and after sleep, as well as at times of day when the urge to sleep is more intense.
However, studies have shown that if arousal state - measured using electroencephalogram - is taken before and after a yawn, there is no significant lasting difference.
Similarly, participants' desire to sleep, or sleep pressure, remained the same with or without a yawn. Therefore, this theory does not appear to hold any water, either.
A relatively recent theory of yawning is the thermoregulatory theory. The idea goes that the process of yawning helps to lower the temperature of the brain if it gets too warm. In a thorough description of the theory, one paper published in Physiology and Behavior concluded:
"Excessive yawning appears to be symptomatic of conditions that increase brain and/or core temperature, such as central nervous system damage, sleep deprivation, and specific serotonin reuptake inhibitors."
One study, discussed in the paper, induced yawns by asking participants to watch videos of other people yawning. Some of the participants held ice packs against their foreheads, while others held warm packs to their heads.
The ice pack cohort yawned less frequently, in agreement with the thermoregulation theory. However, other researchers have pointed out that ice on your forehead is likely to promote wakefulness, whereas a warmed forehead might induce sleepy feelings, therefore increasing the likelihood of yawning. Because of these factors, the results may have nothing to do with the temperature of the brain.
The biggest challenge to the thermoregulatory theory is, "How does yawning cool the brain?" Yawning interrupts nasal breathing, which is much more effective at cooling the brain.
The ear pressure hypothesis
You may have noticed that yawning evens out air pressure in the middle ear. This is particularly obvious when in an aircraft. The yawn manages this satisfying ear pop by contracting and relaxing the tensor tympani and stapedius muscles, which opens the Eustachian tubes and aerates the tympanic cavity.
This, it is argued, may be a defense mechanism to prevent ear damage. To date, however, there are no studies that demonstrate that yawning rates increase at altitude. Swallowing and chewing can do the same job, so it does not seem to impart a specific evolutionary advantage. It also does not explain why yawns are more frequent when tired - even at sea level.
These are the most common theories, but they are not the only ideas floating about. Some people believe that yawns help to prevent the lungs from partially collapsing, renew films of surfactant in the lungs, or aerate the tonsils. None of those theories have, so far, been tested.
It seems a shame to end this article without making some conclusions about why we all yawn, but, unfortunately, the yawn is yet to be conquered by science. It may well be that the answer encompasses some or all of the theories above. For now, though, we will have to keep on guessing.