Why we yarn wen we fell very sleepy........?
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Answer:
Yawning is a stereotypical reflex characterized by a single deep inhalation (with the mouth open) and stretching of muscles of the jaw and trunk. It occurs in many animals, including humans, and involves interactions between the unconscious brain and the body, though the mechanism remains unclear. As for the etiology of yawning, for many years it was thought that yawns served to bring in more air because low oxygen levels were sensed in the lungs. We now know, however, that the lungs do not necessarily sense oxygen levels. Moreover, fetuses yawn in utero even though their lungs arent yet ventilated. In addition, different regions of the brain control yawning and breathing. Still, low oxygen levels in the paraventricular nucleus (PVN) of the hypothalamus of the brain can induce yawning. Another hypothesis is that we yawn because we are tired or bored. But this, too, is probably not the case because the PVN also plays a role in penile erection, which is not typically an event associated with boredom.