Original ArticleU-shaped associations between time in bed and the physical and mental functioning of Japanese civil servants: the roles of work, family, behavioral and sleep quality characteristics

- Subjects with short time in bed (TIB) had poor subjective sleep quality and daytime dysfunction.
- Subjects with long TIB had long sleep latency, poor sleep efficiency, and sleep disturbances.
- The prevalence of poor sleep quality was lowest among subjects spending 8 h in bed.
- Subjects with short TIB had more work and family stresses whereas those with long TIB were likely to be daily drinkers.
- Subjects with short and long TIB had poor sleep quality, which mediated U-shaped associations between TIB and health.

ObjectivesThis study aimed to evaluate (i) whether work, family, behavioral and sleep quality characteristics differ among individuals with different time in bed (TIB), and (ii) whether and how much the U-shaped associations between TIB and health can be explained by these characteristics.MethodsParticipants were 3510 employees (2371 males and 1139 females) aged 20-65 years working in local government in Japan. They completed a questionnaire regarding work, family, and behavioral characteristics. Sleep quality and physical and mental functioning were evaluated using the Pittsburgh Sleep Quality Index and the Short Form 36.ResultsHigh job demands, long work hours, and high work-family conflict were more prevalent among those with short TIB. Those with long TIB had daily drinking habits. Whereas those with short TIB had poor sleep, mainly due to poor subjective sleep quality and daytime dysfunction, those with long TIB had poor sleep, mainly due to long sleep latency, poor sleep efficiency and sleep disturbances. The U-shaped associations between TIB and poor physical and mental health, with the best health observed in those spending ~8 h in bed, weakened considerably after adjustment for sleep quality, followed by work and family characteristics. After adjusting for behavioral characteristics and long-standing illnesses, the associations hardly changed.ConclusionsThe U-shaped associations between TIB and health may be explained by U-shaped associations between TIB and poor sleep and psychosocial stress in work and family life.