Ten patients with myotonic dystrophy (MyD) and excessive daytime sleepiness (EDS) were studied. Daytime sleepiness was assessed by means of a subjective alertness rating scale, multiple sleep latency tests and auditory event-related potentials. In addition, the diurnal pattern of daytime sleepiness and ultradian rhythm characteristics of nocturnal sleep were assessed. The multiple sleep latency tests and the P300 component of the auditory event-related potentials gave evidence of EDS, whereas the subjective alertness ratings did not. However, the alertness ratings showed a significantly reduced circadian periodicity as compared with controls. Consistent with this, no time-of-day effect was observed for the multiple sleep latency test outcomes. The ultradian rhythm characteristics of nocturnal sleep indicated a prolonged mean cycle duration and decreased stability of NREM/REM cycle. Moreover, the temporal structure of REM sleep showed a pattern similar to that of subjects who develop free-running rhythms when living in temporal isolation. These findings suggest that EDS in MyD reflects a complex and wide-spread malfunction of the circadian and ultradian timing system.