Anatomy And Physiology

Why do People Yawn



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Everyone knows that yawns are contagious. A person will often yawn after seeing someone else do so. Even reading the word y-a-w-n can trigger the behavior. And just try stopping in mid-yawn! You can't.

Once a person begins to yawn, this instinctive, hard-wired fixed action pattern (FAP) must run its course, from beginning to end. Although fixed action patterns are most common in lower animals, with simpler brains, humans also exhibit instinctive FAPS. Yawning is a great example. The typical yawn lasts about six seconds and, like all fixed action patterns, is nearly impossible to stop once started.

But why do people (and animals for the matter) yawn? Since the behavior is instinctive, you might think that the answer would be simple. Not so. Here's what science currently reveals about the possible causes of yawning.

* Reduced Oxygen Levels *

It's often assumed that yawning occurs when a person is tired or bored. Scientists do agree that opening of the mouth, followed by deep inhalation and slow exhalation, regulates the carbon dioxide and oxygen levels in the blood. This may be related to fatigue and boredom, because when we experience either, our breathing tends to become shallow, and we take in less oxygen to support our cells.

During the act of yawning, our alertness is also heightened. The sudden intake of oxygen increases heart rate, rids the body of toxic carbon dioxide buildup, forces oxygen through blood vessels in the brain, and restores normal breathing.

* But The Oxygen Hypothesis Doesn't Explain It All *

There are, unfortunately several problems with the explanation given above:

* Fetuses yawn in the womb, although they do not take oxygen into their lungs until birth.
* Breathing more oxygen does not decrease yawning.
* Breathing more carbon dioxide does not increase yawning.

The oxygen hypothesis also doesn't address why yawns are so contagious, or why individuals with a high concentration of oxygen in their blood streams still yawn.

* General Sign of Changing Alertness *

Since people often yawn when awakening, or during other times when level of alertness is changing, perhaps yawning is a much more general sign of changing conditions within the body. A variation on this theory is that yawning stretches the lungs and lung tissue. Stretching and yawning may be a way to flex muscles and joints, increase heart rate, and feel more awake.

* Contagiousness of Yawning *

There is evidence suggesting that yawning is a means of communication, a way of conveying changing environmental or internal body conditions to others. This would somewhat explain the yawn's contagious nature, as a form of communication within groups of animals, perhaps a way of synchronizing behavior. If this is true, yawning in humans is most likely a "left over" ancestral mechanism that is no longer useful, having lost its evolutionary significance over time.

* The Brain's Paraventricular Nucleus *

Low oxygen levels in the paraventricular nucleus (PVN) of the hypothalamus can induce yawning, and research suggests that the PVN is, among other things, the "yawning center" of the brain. This area of the hypothalamus contains several chemical messengers that can induce yawns, including dopamine, glycine, oxytocin and adrenocorticotropic hormone (ACTH). ACTH increases at night and just before awakening, inducing yawning and stretching behavior in humans.

* A Means of Cooling the Brain *

A hypothesis recently proposed by Andrew and Gordon Gallup states that yawning may be a way to keep the brain cool. Endotherms are animals (including humans, mammals and birds) that regulate their internal body temperature, and are not dependent on the environmental temperature for warming and cooling their bodies. And it is important for endotherms not to get too hot, since our brains works best when cool.

These researchers showed their subjects videos of other people yawning. When the study participants viewed the videos while holding heat packs up to their foreheads they yawned often. But when they held cold packs up to their foreheads or breathed through their noses (another means of brain cooling), they did not yawn at all.

In summary, there is no shortage of theories, but obviously much more to be learned about the common, yet mysterious behavior of yawning.

* Sources *

Gordon G. Gallup. 2007. Good Morning America, "The Science of Yawning."

Gallup AC & Gallup GG Jr (2007). "Yawning as a brain cooling mechanism: Nasal breathing and forehead cooling diminish the incidence of contagious yawning." Evolutionary Psychology, 5 (1).

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