Sleep Disordered Breathing includes everything from non-apneic snoring to obstructive sleep apnea and hypopnea. Obstructive sleep apnea is caused by the total collapse of the upper airway resulting in a temporary cessation of breathing. Hypopnea is when the upper airway only collapses partially, restricting airflow. The collapse of these tissues is the result of muscle relaxation at the onset of sleep. Structural abnormalities that further narrow the airway cause increased resistance to airflow. Loud and frequent snoring is a common symptom of obstructive sleep apnea, a sleep illness that often causes excessive daytime sleepiness.