A yawn (from the Middle English yanen, an alteration of yonen or yenen, which in turn comes from the Old English geonian), is a reflex of simultaneous inhalation of air and stretching of the eardrums, followed by exhalation of breath. Pandiculation is the term for the act of stretching and yawning simultaneously. Yawning is associated with tiredness, stress, overwork, lack of stimulation, and boredom.

Yawning can also be a powerful non-verbal message with several possible meanings, depending on the circumstances. In humans, yawning has an infectious quality (i.e., seeing a person yawning, talking to someone on the phone who is yawning, or just thinking of yawning can trigger yawning.) which is a typical example of positive feedback. Infectious yawning has also been noted in chimpanzees.

There are a number of theories that attempt to explain why animals yawn. It is likely that there are a number of triggers, not just one for the behavior. However, there are a few select theories that attempt to explain the primary evolutionary reason for the yawn.

None of them has been empirically substantiated. The first states that yawning occurs when one's blood contains increased amounts of carbon dioxide and, therefore, becomes in need of the influx of oxygen (or expulsion of carbon dioxide) that a yawn can provide. Researchers believed this theory to be true at least since Hippocrates, but studies have since shown it to be either incorrect or, at the very best, flawed.

Yawning may, in fact, reduce oxygen intake compared to normal respiration, not increase it. The second notion states that yawning is the body's way of controlling brain temperature. The process is thought to cool off the brain, much like a fan cools the inside of a computer.

Another speculated reason for yawning is the desire to stretch one's muscles. Nervousness has also been suggested as a possible reason. There have been studies that suggest that yawning, especially for psychological reasons "contagious" yawning, may have developed as a way of keeping a group of animals alert.

Anecdotal evidence suggests that yawning helps increase the state of alertness of a person, paratroopers have been noted to yawn in the moments before they exit the aircraft.

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