During inspiration a negative intrapharyngeal pressure develops but airway collapse is prevented by the action of the pharyngeal abductor and dilator muscles. These muscles are activated rhythmically during daytime respiration but, they become hypotonic during sleep, and airway stability becomes dependent upon pharyngeal size and pharyngeal tissue compliance. Apnea occurs when the throat muscles and tongue relax during sleep and partially block the opening of the airway. As yet, little is known about the compliance of the pharyngeal tissues. However, conditions that reduce airway dimensions result in OSA.