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Humans develop sleep apnea and obstructive sleep disorders as they age and become obese, or secondary to healing after chronic infections. Sleep apnea has not been described in non-human primates. Obstructive sleep apnea occurs because of anatomic differences in the human upper respiratory tract. The upper respiratory tract changes necessary for speech required a supra vocal cord tract, in which the ratio is 1:1, the ratio of the horizontal distance between the incisor tooth and the pharynx, with the vertical distance between the pharynx and the larynx is about equal. In the chimpanzee and other mammals this ratio is not maintained, because the upper respiratory tract is designed just for feeding and respiration. To acheive the 1:1 ratio, there was laryngeal descent in humans, shortening of the mandible, posterior rotation of the facial skeleton and loss of the epiglottic-soft palate lock-up. These changes also allowed the tongue to become larger and thus it became partially housed in the elongated oral pharynx. When the mass of the tongue increases, it can occlude respiration during sleep. Thus the obstruction occurs mainly at night when the oropharynx is relaxed.
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