Sleep apnea is characterized by repetitive cessations of respiration during sleep with consequent decreases in arterial oxygen saturation. Sleep apnea can be caused by upper airway obstruction or by central nervous system failure to sufficiently excite the diaphragm and accessory respiratory muscles. However, in most cases, both processes are involved. Hypopneas, in which airflow is significantly compromised without complete cessations of respiratory flow, and snoring are often found in association with apneas. Both apneas and hypopneas cause repetitive disruptions of sleep, consequent daytime somnolence, and complex cardio-respiratory disturbances.