Background: Epidemiologic studies have shown that habitual sleeping patterns are associated with all-cause mortality risk. However, sleep duration may be affected by physical, mental, or social conditions, and its impact on health may differ depending on the time or place.
Objectives: To examine the effects of sleep duration on all-cause mortality after adjusting for several covariates, mental condition in particular.
Methods: A total of 104,010 subjects (43,852 men and 60,158 women), aged 40 to 79 years, who enrolled in the JACC Study (Japan Collaborative Cohort Study on Evaluation of Cancer Risk Sponsored by Monbusho) from 1988 to 1990 and were followed for an average of 9.9 years. Average sleep duration on weekdays and covariates, including perceived mental stress and depressive symptoms, were used in the analyses. Relative risks were calculated by Cox's proportional hazard model separately by sex.
Results: Men tended to sleep longer than women, and the elderly slept longer than younger subjects. Mean sleep duration was 7.5 hours for men and 7.1 hours for women; mode durations were 8 hours (range, 7.5-8.4 hours) and 7 hours (6.5-7.4), respectively. Sleep duration of shorter or longer than 7 hours was associated with a significantly elevated risk of all-cause mortality. However, the significant association with short sleep disappears when adjusted for some covariates among men.
Conclusions: Sleep duration at night of 7 hours was found to show the lowest mortality risk.