Sleep duration and endometrial cancer risk.

Purpose: Recent data indicate that night shift work is associated with increased endometrial cancer risk, perhaps through a pathway involving lower melatonin production. Melatonin is an antiestrogenic hormone, with production in a circadian pattern that is dependent on presence of dark at night. Sleep duration is positively associated with melatonin production and may be an indicator of melatonin levels in epidemiologic studies.

Methods: We evaluated associations between self-reported sleep duration and endometrial cancer risk using publicly available prospective data on 48,725 participants in the Women's Health Initiative Observational Study, among whom 452 adjudicated incident cases of endometrial cancer were diagnosed over approximately 7.5 years of follow-up. Sleep duration was self-reported at baseline. Cox proportional hazards regression was used to estimate hazard ratios (HR) and 95% confidence intervals (CI) for endometrial cancer risk with adjustment for potential confounders.

Results: Most women reported sleeping ≤ 6 (33.3%) or 7 (38.5%) h each night; fewer reported sleeping 8 (23.4%) or ≥ 9 (4.8%) h each night. In adjusted analyses, there was an indication of reduced risk associated with longer sleep duration, though no statistically significant association was observed. Women who slept ≥ 9 h had a nonsignificant reduced risk of endometrial cancer compared with women who slept ≤ 6 h (HR = 0.87; 95% CI = 0.51-1.46).

Conclusions: We found weak evidence of an association between sleep duration and endometrial cancer risk. Self-reported sleep duration may not adequately represent melatonin levels, thus further studies utilizing urinary melatonin levels are necessary to establish the mechanism by which night shift work increases endometrial cancer risk.