Why Do We Yawn?

Yawns are involuntary reactions to a variety of stimuli including being tired and seeing someone else yawn. But it doesn’t just happen after a long day or when you’re so bored you want to go to sleep. Yawning can happen even when you wake up after a good night’s rest. Despite the yawns reputation as something that happens when you’re feeling relatively inactive, it has been known to happen paratroopers before their first parachute jump – hardly a dull experience! Experts and laypeople alike consider yawning to be “contagious” because one person yawns someone else in the room is likely to follow suit. In fact, cutting edge yawn research has shown that yawning or a still image of a person in mid yawn. Even reading about yawning can cause you to start yawning ( you may have noticed this happening to you as you read this answer).

Yawning is more than simply opening your mouth wide and breathing in deeply, it’s a complex action involving stretching your facial muscles, narrowing or closing your eyes, tearing and salivating. The physiological effects of yawning are equally varied. Yawning opens the Eustachian tubes connecting your ears to your oral cavity, as well as briefly (less than 20 seconds) increasing your alertness among other things. Researchers have a hard time pinning down the exact meaning or purpose of yawning due to its multifaceted nature. Some people believe that yawning’s primary purpose is about oxygenating the blood. However, research has shown that yawning does not occur more frequently when blood oxygen levels become low. Experts seem to think that the true meaning of yawning may have to do with it’s social role which may be related to empathy; yawns seem to be connected to minor displeasure like feeling sleepy, mildly stressed or bored.

Not everyone yawns the same. Most of us yawn and stretch when we wake up or go to bed, yawn when we bored or when we need to pop our ears after traveling in high altitudes and even yawn just because we saw someone else yawn. The act of yawning occurs in almost all vertebrates even birds and fish exhibit a form of mouth gaping similar to yawning. In humans, yawning can start as early as 20 weeks after conception.

A yawn is a coordinated movement of the thoracic muscles in the chest, diaphragm, larynx in the throat and plate in the mouth. By yawning we help distribute surfactant to coat the alveoli in the lungs. We can not yawn on command. It is theorized that yawning is a semi-voluntary action and partly a reflex controlled by neurotransmitters in the hypothalamus of the brain. It is also associated with increased levels of neurotransmitters, neuropeptide proteins and certain hormones.

Why do we yawn?

There are numerous theories on why we yawn and scientist have yet to come to any consensus. One of the first theories for yawning can be traced back to Hippocrates the father of medicine, who hypothesized that yawning precedes a fever and is a way to remove bad air from the lungs. Based on modern evidence, however, it seems unlikely that yawning serves as a function of the respiratory system.

A yawn comprises an initial deep intake of breath which is then held for several seconds, after which there is a slower passive expiration. Over two or three millennia this behavior has intrigued and puzzled most people. A recent publication provides a good summary of current research and opinions on yawning.

However, in spite of much research over many years, there is no agreement as to the immediate stimulus triggering a yawn. This is partly because a yawn is commonly associated with a variety of conditions such as tiredness, boredom, sleepiness, loss of attention, fatigue, hunger, malaise and most surprisingly observing other people yawning. Some other occasions when yawning occurs will be mentioned in the discussion. This suggests that there may be several triggering stimuli. In spite of such views, I have made an assumption that there is a common trigger for the yawn in most of these conditions in spite of their apparent differences. A particular problem in the study of yawning is that, unlike many other reflexes a yawn is not easily and reliably triggered. An exception to this statement is contagious yawning and also yawning by thinking about it and taking a deep breath. However it is not clear that such “copy” yawns are identical with the naturally occur ring yawn. One way around this difficulty is to choose a period of the day when yawns occur reliably. Two such periods are the period of about an hour just before sleep and the hour just after waking. I have found the latter period to be the more reliable and all my experiments have been based on this condition.

Methods 

Yawning is often accompanied by stretching of limbs and other parts of the body. I have not made any detailed observations in this area, primarily because of the difficulty of quantifying this activity. With regard to yawns, it is suggested that an induced yawn should be used to alleviate atelectasis in patients unable to take deep breaths because of post-operative pain. There is reason to think that this procedure would be helpful and therefore the relationship between atelectasis and yawning might be a casual one. One way to test this idea is to study the effect of deep breaths on yawning.

What is the usefulness of yawning?

It follows from the argument put forward in the above paragraphs that yawning should relieve atelectasis. The mild atelactasis I am proposing as a trigger for yawning is probably not a cause of serious changes in oxygen or carbon dioxide levels but could be if not relieved by yawning or deep breathing. Furthermore yawning or deep breathing is more efficient at preventing atelectasis than in re-opening collapsed alveoli. Also, it has been shown previously that installation of surfactant to reduce atelectasis attenuates bacterial growth and translocation and prevents pneumonia. So it would seem that yawning could act similarly. Therefore yawning must be seen as an important reflex. Clinical atelectasis occurs when the patient has difficulty breathing for example after a chest or abdominal operation and in other conditions such as in premature infants who may have inadequate surfactant. Surfactant reduces surface tension and enables the alveoli to remain patent. Hyperinflation therapy is widely used in such states and is very effective. There is an important connection between hyperinflation and surfactant because it is well established that stretching the alveoli releases surfactant. It has also been shown that surfactant is released from its celluar source, the alveolar epithelial type II (AEII) cells by direct stretch of these cells. Furthermore it has been shown that a variable stretch pattern is more effective in causing release of surfactant than a monotonous pattern. Thus the irregular pattern of yawning may very well have an unanticipated benefit in surfactant release.

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