A yawn is a natural human reflex. It involves the inhalation of air, stretching of the eardrums, and is followed by a large exhalation of breath. Yawning is associated with tiredness, stress, lack of stimulation, and boredom. It is one of the most bizarre human functions.
Yawning can be infectious and is triggered by seeing others yawn, hearing a yawn, or talking on the phone with someone who is yawning.
A number of theories have been proposed that attempt to explain why animals yawn, but nothing has been substantiated.
One theory states that yawning occurs when a person's blood contains an increased amount of carbon dioxide and needs oxygen. However,other studies show that yawning decreases oxygen intake.
Another theory states that yawning is a person's way of controlling brain temperature. It has been suggested that a yawn can reduce or increase blood pressure in the brain. This theory has been supported by the fact that people get a desire to yawn while making a drastic changein elevation.
Another speculated reason for yawning is the desire to stretch one's muscles. Contagious yawning may be a way of keeping a group of animals alert. It may be an instinctual reaction from one member of theherd to the next, reminding everyone to stay alert.
Anecdotal evidence suggests that yawning helps increase the state of alertness
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