Patients with obstructive sleep apnea have ≥ 5 episodes of apnea or hypopnea (with each episode lasting at least 10 seconds) per hour during sleep (apnea / hypopnea index [AHI]) plus ≥ 1 of the following: Daytime sleepiness, unintentional sleep episodes, unrefreshing sleep, fatigue, or insomnia Awakening with breath holding, gasping, or choking Reports by a bed partner of loud snoring, breathing interruptions, or both during the patient's sleep Obstructive sleep apnea can be categorized by severity: Mild: 5 to 15 episodes / hour Moderate: 16 to 30 episodes / hour Severe: > 30 episodes / hour Upper airway resistance syndrome (airflow limitation secondary to increased upper airway resistance or respiratory effort-related arousals [RERAs]) can cause excessive daytime sleepiness or other manifestations suggesting obstructive sleep apnea but with few episodes of apnea / hypopnea or oxygen desaturation and thus does not meet full criteria for obstructive sleep apnea.