STUDY OBJECTIVES: To investigate the association between sleep duration and breast cancer incidence, we examined the association in a large UK prospective study and conducted a meta-analysis of prospective studies. METHODS: In the Million Women Study, usual sleep duration over a 24-hour period was collected in 2001 for 713,150 participants without prior cancer, heart problems, stroke or diabetes (mean age=60 years). Follow-up for breast cancer was by record linkage to national cancer registry data for 14.3 years on average from the 3-year resurvey. Cox regression models yielded multivariable-adjusted breast cancer relative risks (RR) and 95% confidence intervals (CIs) for sleep duration categories. Published prospective studies of sleep duration and breast cancer risk were included in a meta-analysis, which estimated the inverse-variance weighted average of study-specific log RRs for short and for long versus average duration sleep. RESULTS: After excluding the first five years to minimise reverse causation bias in the Million Women Study, 24,476 women developed breast cancer. Compared with 7-8 hours of sleep, the RRs for <6, 6, 9, and >9 hours of sleep were 1.01 (95%CI, 0.95-1.07), 0.99 (0.96-1.03), 1.01 (0.96-1.06), and 1.03 (0.95-1.12), respectively. In a meta-analysis of 14 prospective studies plus the Million Women Study, including 65,410 breast cancer cases, neither short (RR<7 hours=0.99 [0.98-1.01]) nor long (RR>8 hours=1.01 [0.98-1.04]) versus average duration sleep was associated with breast cancer risk. CONCLUSIONS: The totality of the prospective evidence does not support an association between sleep duration and breast cancer risk.