Advance Access publication April 30, 2008 Sleep duration, melatonin and breast cancer among Chinese women in Singapore

Background: Sleep duration has been hypothesized to be inversely associated with breast cancer risk, possibly due to greater overall melatonin production in longer sleepers. However, data are in-conclusive from the three studies conducted in Western popula-tions on sleep duration and breast cancer risk. Methods: We investigated the relationship between self-reported usual sleep duration determined at baseline and subsequent risk of breast cancer in the prospective, population-based cohort of the Singa-pore Chinese Health Study. We excluded from the study women with <2 years of follow-up due to possible change in sleep pattern among breast cancer cases close to the time of diagnosis. Five hundred and twenty-five incident cases of breast cancer were identified among the remaining 33 528 women after up to 11 years of follow-up. Results: Among women postmenopausal at baseline, breast cancer risk decreased with increasing sleep duration (P trend 5 0.047); those who reported 91 h of sleep showed a rel-ative risk of 0.67 (95 % confidence interval 5 0.4–1.1) compared with women who reported £6 h of sleep. This inverse association was observed primarily in lean women [i.e. body mass index be-low the median value (23.2 kg/m2)] (P 5 0.024). In this study population, irrespective of gender, urinary 6-sulfatoxymelatonin levels increased with increasing self-reported hours of sleep (P trend 5 0.035) after adjustment for age and time of day of urine collection. Melatonin levels were 42 % higher in those with 91 versus those with £6 h of sleep. Conclusion: Sleep duration may influence breast cancer risk, possibly via its effect on mela-tonin levels