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. Pandiculation is the term for the act of stretching and yawning. Yawning is a powerful non-verbal message with several possible meanings, depending on the circumstances: An indication of tiredness, stress, over-work or bore ...

Including:

• Yawn - Causes of yawning
• Yawn - Notes

Read more here: » Yawn: Encyclopedia - Yawn

A long-standing hypothesis is that yawning is caused by an excess of carbon dioxide and lack of oxygen in the blood. The brain stem detects this and triggers the yawn reflex. The mouth stretches wide and the lungs inhale deeply, bringing oxygen into the lungs and hence to the bloodstream. It is almost certain however, that this hypothesis is not correct; a more recent hypothesis is that yawning is used for regulation of body temperature. Another hypothesis is that yawns are caused by the same chemicals (neurotransmitters) in the brain that a ...

See also:

Yawn, Yawn - Causes of yawning, Yawn - Notes

Read more here: » Yawn: Encyclopedia II - Yawn - Causes of yawning