Physiological hypnic myoclonus (PHM) was quantified during wakefulness and sleep in 7 normal subjects. PHM was evident during relaxed wakefulness and increased during stage 1 and especially REM sleep. In some muscles, however, (e.g., soleus) it showed no increase during sleep. PHM resembled simple fasciculation potentials, or, when repeated or in clusters, the fragmentary myoclonus in NREM sleep. Both 'hypnic' and 'myoclonus' are terms inappropriate to the description of this physiological spontaneous motor activity.