Sleep duration and risk of breast cancer: The JACC Study.
The evidence on beneficial or adverse effects of sleep duration on risk of breast cancer remains controversial and limited, especially in Asia. A prospective study of 34,350 women aged 40-79 years in whom sleep duration, and menstrual and reproductive histories were determined by a self-administered questionnaire. The follow-up period was from 1988 to 2009, and hazard ratios (HRs) with 95% confidence intervals (CIs) of breast cancer incidence were calculated for shorter sleep duration in reference to sleep duration of ≥ 8 h/day by Cox proportional hazard models. During 19.2-year median follow-up (236 cases), we found a significant inverse association between sleep duration and risk of breast cancer, especially among postmenopausal women and women with low parity (nulliparous and women with < 3 children); the multivariable HRs (95% CIs) among postmenopausal women who reported 7 h/day and ≤ 6 h/day of sleep in reference to ≥ 8 h/day were 1.49 (0.81-2.76) and 1.98 (1.08-3.70) (P for trend = 0.028), respectively, and the corresponding values among women with low parity were 1.50 (0.96-2.35) and 1.76 (1.01-2.79) (P for trend = 0.018). Short sleep duration was associated with increased risk of incident breast cancer, especially among postmenopausal women and women with low parity.