Sleep duration and risk of breast cancer: The JACC Study

PurposeThe evidence on beneficial or adverse effects of sleep duration on risk of breast cancer remains controversial and limited, especially in Asia.MethodsA prospective study of 34,350 women aged 40-79years 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 8h/day by Cox proportional hazard models.ResultsDuring 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 7h/day and 6h/day of sleep in reference to 8h/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).ConclusionsShort sleep duration was associated with increased risk of incident breast cancer, especially among postmenopausal women and women with low parity.