It is commonly believed that 8 hours of sleep per night is optimal for good health. However, recent studies suggest the risk of death is lower in those sleeping 7 hours. We prospectively examined the association between sleep duration and mortality in women to better understand the effect of sleep duration on health. Prospective observational study. Community-based. Women in the Nurses Health Study who answered a mailed questionnaire asking about sleep duration in 1986. None. Vital status was ascertained through questionnaires, contact with next of kin, and the National Death Index. During the 14 years of this study (1986–2000), 5409 deaths occurred in the 82,969 women who responded to the initial questionnaire. Mortality risk was lowest among nurses reporting 7 hours of sleep per night. After adjusting for age, smoking, alcohol, exercise, depression, snoring, obesity, and history of cancer and cardiovascular disease, sleeping less than 6 hours or more than 7 hours remained associated with an increased risk of death. The relative mortality risk for sleeping 5 hours or less was 1.15 (95% confidence interval [CI], 1.02–1.29) for 6 hours, 1.01 (95% CI, 0.94–1.08), for 7 hours, 1.00 (reference group), for 8 hours, 1.12 (95% CI, 1.05–1.20), and for 9 or more hours 1.42 (95% CI, 1.27–1.58). These results confirm previous findings that mortality risk in women is lowest among those sleeping 6 to 7 hours. Further research is needed to understand the mechanisms by which short and long sleep times can affect health.