Background: Short sleep duration was associated with an increased risk of colorectal adenomas in a recent report. To date, no data are available on the relationship between sleep duration and colorectal cancer risk. Method: We prospectively followed 76,368 women aged 40 to 73 years in the Nurses’ Health Study and 28,676 men aged 41 to 79 years in the Health Professionals Follow-up Study who reported their habitual sleep duration in the mid-1980s. We used Cox proportional hazards regression models to estimate multivariable relative risks (MV RRs) and 95% confidence intervals (CIs) adjusting for known risk factors (dietary and non-dietary) for colorectal cancer. Results: A total of 1,925 incident colorectal cancer cases (1,264 in women and 661 in men) were documented during up to 22 years of follow-up. The median of sleep duration was 7 hours in both cohorts and was used as the reference group. In both age-adjusted and the multivariate models, long sleep duration (i.e., at least 9 hours of sleep) was significantly positively associated with risk of colorectal cancer in men (MV RR=1.49; 95%CI: 1.10, 2.03) but not in women (MV RR=1.10; 95%CI: 0.85, 1.43). However, for the same comparison (i.e., at least 9 hours vs. 7 hours of sleep), among overweight (body mass index at least 25 kg/m2) individuals, we found significant positive associations for both men (MV RR=2.66; 95%CI: 1.81, 3.90) and women (MV RR=1.42; 95%CI: 1.01, 1.99). Short sleep duration (i.e., 5 hours or less of sleep) was not associated with colorectal cancer risk in either men (MV RR=0.65; 95%CI: 0.33, 1.28) or women (MV RR=1.14; 95%CI: 0.88, 1.48). Results were essentially unchanged when we excluded cases occurred in the first 4 years of follow-up. In addition, the association was not significantly modified by physical activity, alcohol consumption, endoscopy screening, family history of colorectal cancer, menopausal status, or shift work (women only). Conclusion: These prospective data from two large cohorts suggest that overweight men and women with a sleep duration of 9 or longer hours had a significantly increased risk of developing colorectal cancer when compared to those with 7 hours of sleep, which cannot be explained by known risk factors or reverse causality. The potential mechanisms behind this association deserve further investigation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-323. doi:1538-7445.AM2012-LB-323