One variation of sleep apnea is called upper airway resistance syndrome (UARS).  Unlike sleep apnea where the upper airway is obstructing the flow of air, during UARS the airway gradually narrows during sleep. During these periods of  airway narrowing, the brain frequently arouses from sleep, causing fragmented sleep patterns, though the oxygen levels in the blood do not decrease.

UARS sufferers often have the same symptoms as those with apnea, including daytime fatigue, loud snoring and morning headaches.