It is estimated that more than 12 million individuals in the United States have sleep apnea, in which breathing stops for short periods during sleep. Most persons with sleep apnea do not even know they have this serious medical problem. Sleep apnea is more than just snoring, although snoring is one of the signs of sleep apnea. Obstructive sleep apnea is caused by intermittent closure of the airway. Much less commonly, central sleep apnea can happen when abnormal messages sent from the brain make breathing stop inappropriately. If left undetected and untreated, sleep apnea may cause poor quality of sleep, excessive daytime sleepiness, weight gain, hypertension (high blood pressure), heart failure, stroke, and death. Hypoxemia (decreased oxygen concentration in the blood) and hypercapnia (increased carbon dioxide concentration in the blood) are common in persons who have sleep apnea. Both hypoxemia and hypercapnia are serious problems and can have many ill effects on the organs of the body.