The purpose of this study was to examine the relationships among nocturnal jaw 
muscle activities, decreased esophageal pH, and sleep positions. Twelve adult 
volunteers, including 4 bruxism patients, participated in this study. Portable 
pH monitoring, electromyography of the temporal muscle, and audio-video 
recordings were conducted during the night in the subjects' homes. Rhythmic 
masticatory muscle activity (RMMA) episodes were observed most frequently, with 
single short-burst episodes the second most frequent. The frequencies of RMMA, 
single short-burst, and clenching episodes were significantly higher during 
decreased esophageal pH episodes than those during other times. Both the 
electromyography and the decreased esophageal pH episodes were most frequently 
observed in the supine position. These results suggest that most jaw muscle 
activities, ie, RMMA, single short-burst, and clenching episodes, occur in 
relation to gastroesophageal reflux mainly in the supine position.