Sleep apnea

Apnea means "absence of breath." In sleep apnea, the sleeper stops breathing.  The throat collapses and carbon dioxide builds up in the bloodstream.  The brain awakens the sleeper to breathe, but usually the sleeper falls right back asleep and never realizes what happened.  The cycle can repeat hundreds of times in one night.  Because the normal sleep cycle is interrupted all night long, the victim is unable to reduce his sleep debt.  The most common symptom of sleep apnea is daytime fatigue.  Sleep apnea is also very hard on the cardiovascular system because blood pressure rises each time the brain has to awaken the sleeper to breathe. 

Source:

Dement, The Promise of Sleep