Upper airway occlusion during inspiration

In normal NREM sleep phase, the basal tension of the upper airway muscles decreases, the caliber of the upper airway decreases, and the airway resistance increases, but the discharge phase of the upper airway muscles and the rhythmic contraction of the intercostal muscles remain intact. During REM sleep, the underlying tension of the upper airway muscles, intercostal muscles, and most skeletal muscles is further suppressed. Reduced pharyngeal muscle tension can cause occlusion of the upper airway during inspiration.