Sleep duration in midlife and later life in relation to cognition.

Objectives To evaluate associations between sleep duration at midlife and later life and change in sleep duration over time and cognition in older women. Design Participants reported sleep duration in 1986 and 2000, and a subgroup of older participants began cognitive testing in 1995 to 2001; follow-up testing was conducted three times, at 2-year intervals. Setting Prospective Nurses' Health Study cohort. Participants Female nurses aged 70 and older free of stroke and depression at the initial cognitive assessment (N=15,385). Measurements Validated, telephone-based cognitive battery to measure cognitive function; four repeated assessments over 6years were averaged to estimate overall cognition at older ages, and trajectories of cognitive change were evaluated over follow up. Results Extreme sleep durations in later life were associated with worse average cognition (P<.001 for the quadratic term for a global score averaging all six cognitive tests). For example, women sleeping 5h/d or less had worse global cognition than those sleeping 7h/d, as did women sleeping 9h/d or more; differences were equivalent to nearly 2 additional years of age. Associations were similar, although slightly attenuated, for sleep duration in midlife. Women whose sleep duration changed by 2h/d or more over time had worse cognition than women with no change in sleep duration (e.g., for the global score, P<.001 for the quadratic term). Sleep duration was not associated with trajectories of cognitive function over 6years, which might be attributable to short follow-up for detecting cognitive decline. Conclusion Extreme sleep durations at midlife and later life and extreme changes in sleep duration over time appear to be associated with poor cognition in older women.