The throat and tongue muscles relax during sleep and can sometimes block the airway, making it difficult to breathe. Air that has already been used by the body has no exit route, and air the body needs cannot enter. In order for the body to get rid of excess carbon dioxide and breathe in a healthy dose of oxygen, the brain sends out an emergency alert for breath, causing the person to wake up.
Obstruction usually occurs in the pharynx.
During sleep, the muscles that normally support the pharynx relax. Compared to other parts of the head and neck, the pharynx lacks supporting cartilage and bone. This makes collapse more likely during sleep. Breathing pauses until high carbon dioxide levels and low oxygen levels cause the brain to force breathing, causing the person to wake up momentarily. Diseases and conditions that can damage the central nervous system, such as stroke, polio, and cancer, can cause poor respiratory control that results in central sleep apnea.