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 6 years 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 5 h/d or less had worse global cognition than those sleeping 7 h/d, as did women sleeping 9 h/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 2 h/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 6 years, 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.