Study objectives: 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.
Design: Prospective observational study.
Participants: Women in the Nurses Health Study who answered a mailed questionnaire asking about sleep duration in 1986.
Measurements and results: 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).
Conclusions: 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.