The most common form of sleep apnea is obstructive sleep apnea, which is caused by a blockage of the breathing passage or a collapsed airway. With a blocked airway, it is difficult to breathe-keeping air from traveling freely.
When you're awake, throat muscles help keep your airway stiff and open so air can flow into your lungs. When you sleep, these muscles relax, which narrows your throat. Normally, this narrowing of your throat doesn't prevent air from flowing into and out of your lungs.