Sleep apnea is when someone involuntarily does not breath during sleep.

During normal breathing, air passes through the throat on its way to the  lungs. The air travels past the soft palate, uvula, tonsils, and tongue. When a  person is awake, the muscles in the back of the throat tighten to hold these  structures in place preventing them from collapsing into the airway

Sleep apnea must be diagnosed by a sleep study. A sleep study measures the  heart rate, breathing patterns, and brain waves of the patient while they sleep.  It is absolutely essential that this study be done on anyone suspected of having  sleep apnea.