Spontaneous rhythmic masticatory muscle activity (RMMA) during sleep occurs in relation to transient activation in the cerebral and autonomic nervous systems of normal subjects and in patients with sleep bruxism (SB). In this study, we made a quantitative assessment of the sequential changes in cortical electroencephalographic (EEG) and autonomic-cardiac activities associated with micro-arousals preceding RMMA episodes. We matched 10 SB patients with 10 normal subjects. The onset of RMMA episodes was defined in terms of the onset of activation in the suprahyoid muscles. In SB patients, an increase in cortical EEG activity was observed 4 seconds before the onset of suprahyoid activity in 79% of episodes. A significant acceleration in heart rate was initiated one cardiac cycle before RMMA onset. A clear sequence of cortical to autonomic-cardiac activation precedes jaw motor activity in SB patients. This suggests that SB is a powerful oromotor manifestation secondary to micro-arousal.