First night effect on polysomnographic sleep bruxism diagnosis varies among young subjects with different degrees of rhythmic masticatory muscle activity.

OBJECTIVE:This study investigated the first night effect on the polysomnographic diagnosis of sleep bruxism (SB). METHODS:Polysomnographic recordings were performed for two consecutive nights in forty-three subjects (mean age 23.7 ± 0.32 years [range: 20.0-33.0]). Sleep variables and rhythmic masticatory muscle activity (RMMA) were scored for two nights. The diagnosis of SB was graded by the frequency of RMMA with cut-off values of two and four times per hour of sleep. RESULTS:Participants were classified into control (n = 15), low (n = 13) and moderate-high (n = 15) groups. Among the three groups, the concordance of the SB diagnosis was compared between the two nights. Sleep variables showed a significant first-night effect with lower sleep efficiency, longer sleep latency and higher frequency of arousals. The frequency of RMMA significantly increased from the first to the second night in the moderate-high SB group only. The concordance rate of the severity between the two nights was 93.3% (14/15) in the control group, 76.9% (10/13) in the low SB group and 60% (9/15) in the moderate-high SB group. When the severity was determined on the first night, it remained the same on the second night in 77.8% (14/18) of the control group, 66.7% (10/15) of the low SB group and 90.0% (9/10) of the moderate-high SB group. CONCLUSION:The results showed that the first night effect on the occurrence of RMMA differed among the different degrees of the RMMA frequency, and suggest that, due to the first night effect, single-night polysomnography may underestimate the moderate-high level of SB but differentiate the low level of SB from controls.