Full-night polysomnography (PSG) will be performed in the sleep laboratory according to protocol of sleep medicine department after US assessment. Apnea will be defined as the absence of airflow ≥10 sec and hypopnea will be defined as >50% decrease in airflow ≥10 sec associated with reduced arterial oxygen saturation by 4%, or an arousal. These two parameters are collectively defined as apnea-hypopnea index (AHI). An arousal will be recorded if there is a 3 second or longer abrupt shift in electroencephalogram frequency to alpha or theta or above 16 Hertz, following at least 10 seconds of sleep, and, if arising in rapid eye movement sleep, there should be a rise in electromyographic tone. An independent technician blinded to the US results will analyze the sleep studies.