Diurnal variations and intermittent arousals modulate jaw-opening and -closing muscle activity level during sleep in rats

Increased jaw muscle activity during sleep, which is associated with conditions such as sleep bruxism and obstructive sleep apnea, is a significant clinical concern in dentistry. The present study aimed to investigate the factor influencing natural variations on jaw muscle activity during sleep throughout 24 h. In ten male SD rats, electrophysiological recording was conducted to monitor electroencephalography (EEG), electromyographic (EMG) activity of neck, jaw-closing (masseter), and jaw-opening (digastric) muscles while freely behaving throughout 24 h. Jaw muscle activity level during sleep exhibit a diurnal variation, with lower level in the light phase than in the early dark phase. The jaw muscle activity level was significantly higher during intermittent arousals than during quiet sleep period. Jaw muscle activity levels during intermittent arousals were significantly higher in dark phase within arousals during NREM sleep. Within intermittent arousals, jaw muscle activity level was negatively correlated with EEG delta power in NREM sleep or EEG theta power in REM sleep. Clustering analysis further revealed that multiple muscles including neck and jaw muscles are often activated within intermittent arousals. These findings revealed the occurrence of intermittent arousals under diurnal influences underlie the variations of jaw muscle activity during sleep in 24 h.