why do we yawn?!
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We need oxygen.
Other Answers:
we yawn when we are tired or lazy to get the blood flowing again
I read somewhere that it helps get more oxygen to the brain, which helps you wake up a little if you're drowsy.
we yawn because we are not getting enough oxygen to the brain. When we are sleepy our circulation is slowed so the brain does not get oxygen as quickly as when we are fully alert. Yawning allows us to get a large amount of air (oxygen) at one time.
I heard it was lack of oxygen to the brain? if true do not know, but I have heard more than once
Causes of yawning
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 affect emotions, mood, appetite and other phenomena. These chemicals include serotonin, dopamine, glutamic acid and nitric oxide. As more of these compounds are activated in the brain, the frequency of yawning increases. Conversely, a greater presence in the brain of opiate neurotransmitters such as endorphins, reduces the frequency of yawning. Patients taking the serotonin reuptake inhibitor Paxil (Paroxetine HCl) have been observed yawning abnormally often.
Another theory is that yawning is similar to stretching. Yawning, like stretching, increases blood pressure and heart rate and also flexes many muscles and joints. Some have observed that if you try to stifle or prevent a yawn by clenching your jaws shut, the yawn is unsatisfying. As such, the stretching of jaw and face muscles seems to be necessary for a good yawn.
Source(s):
http://www.answers.com/topic/yawn?gwp=19
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