To examine the nature of sleep disturbance in patients with a variant form of late infantile neuronal ceroid lipofuscinosis (CLN5), we studied 12 patients (age range 7-32 years). We used a sleep questionnaire to assess sleep and its disturbances quantitatively. To identify the periodicity in the diurnal rest-activity rhythms, the motor activity level was recorded by activity monitors continuously for a 1-week period with concomitant sleep logs. In addition, whole-night polysomnographic recordings were performed. The patients under 20 years of age had an excess of nocturnal sleep (the mean of the usual duration of nighttime sleep was 10.0 hours) and frequent daytime naps. Frequent shifts of the longest sleep period into the daytime hours and fragmented diurnal rest-activity patterns with no distinct rhythm occurred in the older patients. The progressive disease may damage the internal circadian timing system and also impair the ability of patients with variant late infantile neuronal ceroid lipofuscinosis to use external time cues for synchronization of their sleep and environmental time.