A yawn is a reflex of deep inhalation and exhalation associated with being tired, with a need to sleep, or from boredom. The word "yawn" has evolved from the Middle English word yanen, an alteration of yonen, or yenen, which in turn comes from the Old English geonian. Yawning is a powerful non-verbal message with several possible meanings, depending on the circumstances:

  1. It's a not-always-so-subtle cue to spouses, co-workers, and bosses for attention, sympathy and a respite due to tiredness, stress, over-work or boredom.
  2. An action indicating psychological decompression after a state of high alert. "I've observed that people on the spot, those who are the focus of tough questions never yawn. But afterward, there's a physiological and psychological letdown. They ratchet down, and a yawn is the first step to going 'off duty,' of entering the 'vegging state.'"
  3. A means of expressing powerful emotions like anger and rejection. "Often, for whatever reason, people are not comfortable expressly verbalizing anger, boredom, disagreement or rejection. Thus, the yawn states for them, 'I'm rejecting you. I'm not interested in what you have to say. I'm not interested in you as a person.' It can serve as a passive-aggressive means to express hostility, anger or rejection when an individual isn't able to articulate those verbally. For instance, I've seen marriages where one spouse is giving such non-verbal cues, and the other isn't picking up on them, which further heightens the negative emotions."

A yawn can express strong anti-social messages, and so in some cultures people try to mute or mask them by placing a concealing hand over the yawning mouth.

Causes of yawning

A long-standing theory behind yawning is that there was too much carbon dioxide and not enough oxygen in the blood. The brain stem was assumed to detect this and would trigger the yawn reflex. The mouth stretches wide and the lungs inhaled deeply, causing oxygen into the lungs and thence to the bloodstream. This is not certain however: a more recent theory is that it is a form of bodily temperature regulation. Another theory is that yawns "seem to be caused by the same chemical compounds (neurotransmitters) in the brain that effect emotions, mood, appetite and more - serotonin, dopamine, glutamic acid and nitric oxide. The more of these compounds activated in the brain, the greater the frequency of yawns. Conversely, a greater presence in the brain of opiate neurotransmitters such as endorphins, the less the frequency of yawns."

The yawn reflex is often described as "contagious": if one person yawns, this will cause another person to "sympathetically" yawn. The reasons for this are unclear, possibly due to the "power of suggestion". Other theories include that "the yawn serves to synchronize mood behavior among gregarious animals, like the howling of the wolf pack during a full moon. It signals tiredness to other members of the group in order to synchronize sleeping patterns and periods of activity. Or, it can serve as a warning in displaying large, canine teeth and thus, proclaim "don't attack while I sleep."

Adelie penguins employ yawning as part of their courtship ritual. Penguin couples face off and the males engage in what is described as an "ecstatic display," their beaks open wide and their faces pointed skyward.

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