Objective:
Sleep bruxism, a major sleep disorder that causes serious harm to oral health, is considered a multifactorial disease. Sleep bruxism can be induced by secondhand smoke (SHS), which adversely affects sleep quality. The objective of present study was to clarify the associations between sleep bruxism, sleep quality, and SHS exposure.

Methods:
To assess the prevalence of sleep bruxism, sleep quality, and SHS exposure, we conducted oral examinations and self-report questionnaires on university students in Japan. The inclusion criteria were age 18 or 19 years, non-smokers and non-alcohol drinkers. The exclusion criteria were failing to complete the questionnaire in full. To assess the associations between sleep bruxism, sleep quality, and SHS exposure, we used chi-squared test and structural equation modeling.

Results:
We analyzed a total of 1,781 Japanese young adults. Females who had been exposed to SHS had worse sleep quality (p = 0.019) than those who had not. Females with worse sleep quality showed a higher prevalence of sleep bruxism (p = 0.034) than those with better sleep quality. Using structural equation modelling, direct associations were identified between SHS exposure and poor sleep quality (standardized coefficients, 0.153; p = 0.008) and between sleep bruxism and poor sleep quality (standardized coefficients, 0.187; p = 0.022) in females. However, no association was found among males.

Conclusions:
These results suggest that SHS exposure is associated with poor sleep quality and sleep bruxism in Japanese young adult females.