People who have sleep apnea repeatedly stop and start breathing when they sleep. There are two main types of sleep apnea. The more common form is obstructive sleep apnea, or OSA. It happens when muscles in the back of the throat relax, narrowing the airway and making it hard to take in enough air. Central sleep apnea is less common. It happens when the brain doesn't send the proper signals to the muscles that control breathing.