SLEEP DURATION AND BREAST CANCER: A PROSPECTIVE COHORT STUDY

Abstract

ISEE-298 Introduction: Breast cancer incidence has increased during recent decades for reasons that are only partly understood. Some evidence suggests that sleep duration has decreased, and many people suffer from poor sleep. Aim: We hypothesized that there is an inverse association between sleep duration and breast cancer risk, possibly due to greater melatonin production in longer sleepers. Methods: This population-based and nationwide study includes information on over 12,000 women born before 1958. Sleep duration, other sleep variables and breast cancer risk factors were assessed by self-administered questionnaires in 1975 and in 1981. Breast cancer incidence data for years 1976 to 1996 were obtained from the Finnish Cancer Registry. Relative risks and 95% confidence intervals were obtained by using Cox proportional hazards models, adjusting for potential confounders. Results: Overall, there was no clear change in breast cancer risk by sleep duration among the 12,222 women with sleep duration data for 1975. When the analysis was restricted to the 7,396 women who reported the same sleep duration in 1975 and in 1981, we observed a significantly lower risk of breast cancer in those who reported a long duration of sleep (nine or more hours per night, compared to the average duration (of seven or eight hours). This finding was based on seven observed cases. No effects were observed for sleep insufficiency and sleep quality. Conclusion: This study provides some support for a decreased breast cancer risk in long sleepers.