Objective: Recently, it was reported that nocturnal rhythmic masticatory muscle activity (RMMA) and gastroesophageal reflux frequently occurred in the supine position and were closely associated with body movements. The purpose of this study was to examine the relationship of sleep position and its changes to nocturnal jaw muscle activity and decreased esophageal pH. Methods: Ten healthy young adult volunteers participated in this study. We measured the masticatory muscle activity (EMG), orofacial movements, sleep position, changes in sleep position and intraesophageal pH by using conventional methods. We scored RMMA, short burst, and clenching episodes according to the research criteria of Lavigne et al. We then scored decreased esophageal pH episodes. For comparisons within subjects, the paired t-test or Wilcoxon rank sum test was used according to the data distribution. Results: All EMG episodes and decreased esophageal pH episodes occurred most frequently in the supine position when compared to other positions. Approximately 20 % of decreased esophageal pH episodes and approximately 10 % of RMMA episodes were closely associated with changes in sleep position. Other EMG episodes were not associated with the changes in sleep position at all. Approximately 40 % of the changes in sleep position were associated with the decreased esophageal pH episodes, and approximately 30 % of the changes in sleep position were associated with RMMA. Conclusion: These results suggest that the majority of jaw muscle activities and gastroesophageal reflux occur in the supine position and that RMMA and reflux are closely associated with changes in sleep position.

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