Several stimuli cause arousal from sleep,- less intense stimuli cause a shift to a lighter sleep stage without frank arousal. In general, arousal from REM sleep is more difficult than from slow-wave sleep. In humans, hypercapnia is a more potent arousal stimulus than hypoxia, the former requiring a Paco2 of about 55 mm Hg and the latter requiring a Pao2 less than 40 mm Hg. Airway irritation and airway occlusion induce arousal readily in slow-wave sleep but much less readily during REM sleep.
All of these arousal mechanisms probably operate through the activation of a reticular arousal mechanism similar to the wakefulness stimulus. They play an important role in protecting the sleeper from airway obstruction, alveolar hypoventilation of any cause, and the entrance into the airways of irritating substances. Recall that coughing depends on the aroused state and without arousal airway irritation leads to apnea. Obviously, wakefulness altered by other than natural sleep—such as during drug-induced sleep, brain injury, or anesthesia—leaves the individual exposed to risk because arousal from those states is impaired or blocked. From a teleological point of view, the most important role of sensors of the respiratory system may be to cause arousal from sleep.
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