Rhythmic Masticatory Muscle Activity (RMMA) has been reported in 60% of normal sleepers and in patients presenting tooth-grinding during sleep, a condition known as sleep bruxism (Lavigne et al., 2001). Objectives: The present study was conducted to assess the distribution of RMMA episodes over sleep stages and to characterize sleep stage shifts occurring in close temporal association with RMMA. Methods: In this study, 20 moderate to severe bruxers were selected. Polysomnographic recordings were performed on two consecutive nights in a sleep laboratory. RMMA episodes were scored as previously described (Lavigne et al., 2001). Standard and event-based sleep stage scorings were performed. For the latter, the sleep stage during the 20 sec before (B20) the beginning of the episode was scored. Furthermore, the sleep stage during the 20 sec after (A20) the end of the episode, and during the following 20 to 40 sec (A40) and 40 to 60 sec (A60) were scored. RMMA episodes separated by less than 100 sec were considered as being part of the same global episode (cluster) and sleep stages before and after the cluster were compared. Results: The distribution of RMMA episodes (n=1107), using standard sleep staging, was 27.0% in stage 1, 58.0% in 2, 4.9% in 3+4 and 9.1% in REM. A sleep stage shift occurred in 53.6% of episodes between B20 and A20. However, this decreases to 39.0% and 31.2% after 40 and 60 sec respectively. Most RMMA episodes occurred in cluster (75.8%). Episodes in cluster were associated with sleep stage shift in 56.8% of cases compared to 50.4% in isolated episodes (p=0.14). Conclusions: These results show that approximately half of the RMMA episodes are associated with a transient sleep stage shift immediately after the episode. The sleep stage then returns to its previous value in most episodes (69%).

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