Nocturnal myoclonus and nocturnal myoclonic activity in the normal population.

The presence of nocturnal myoclonus (NM) and nocturnal myoclonic activity (NMA) was assessed in the sleep laboratory in 100 subjects without any complaint of sleep disturbance. Subjects were included only if they were physically and mentally healthy and not using any medication. The subject sample was selected to have a representative proportion of women and men and a wide age distribution. Six percent of the subjects met the clinical criteria for NM, and five percent had NMA. Thus, either NM or NMA was noted in 11 percent of the total sample. The prevalence of NM or NMA was slightly higher in men than in women and positively and significantly correlated with increasing age. No relationship was found between the presence of NM or NMA and weight, smoking, or the consumption of caffeinated beverages. When the amount of each sleep stage was controlled for, myoclonic activity occurred most frequently during stage 2 sleep. The degree of sleep disturbance related to myoclonic activity was minimal, with the mean total duration of both types of myoclonic activity per subject-night being only about six minutes. In addition, only about 10 percent of the events were related to EEG arousals.