Obstructive sleep apnea (OSA) -- also called obstructive sleep apnea syndrome -- occurs when there are repeated episodes of complete or partial blockage of the upper airway during sleep. During a sleep apnea episode, the diaphragm and chest muscles work harder trying to open the airway. Breathing usually resumes with a loud gasp, snort, or body jerk. These episodes can interfere with sound sleep. They can also reduce the flow of oxygen to vital organs and cause irregularities in heart rhythm. WebMD