Determination of sleep latency in polysomnographic evaluations of daytime somnolence in patients with sleep apnea and patients with narcolepsy.

Sleep onset during the multiple sleep latency test was scored by three criteria for 21 patients with narcolepsy and 21 patients with obstructive sleep apnea: a single epoch of stage 1, three consecutive epochs of stage 1, and a single epoch of stage 2 or REM. Mean sleep latency for both groups was predictably shortest using a single epoch of stage 1 and longest using a single epoch of stage 2 or REM. All estimates of sleep latency were highly correlated. It was concluded that a single epoch of any stage of sleep is an appropriate measure of sleep latency for patients with narcolepsy, although a modified scoring system should be developed for patients with sleep apnea. The obstructive apneic episodes prevented or delayed sleep onset on 4.8% to 33.3% of trials depending on the specific criteria used to determine sleep latency. Such apnea-related interruptions to sleep should be taken into account when assessing daytime somnolence in patients with sleep apnea.