Association between self-reported night sleep duration and cognitive function among older adults with intact global cognition

Objectives The purpose of this study was to clarify the association between self-reported night sleep duration and cognitive functions such as word memory, story memory, attention, executive function, and processing speed of older adults with normal global cognitive function. Methods A total of 241 functionally independent older adults (mean age, 75.5 +/- 6.4 years) participated in this study. No participants had a history of dementia diagnosis, and each had a Mini-Mental State Examination (MMSE) score >= 24. Participants were evaluated for self-reported sleep duration and cognitive function using the National Center for Geriatrics and Gerontology-Functional Assessment Tool (NCGG-FAT). Results The longest sleep duration group (>= 9 h) had lower MMSE scores (p = 0.010), Z-score of word list memory II (delayed recall; p = 0.001), and Z-score of story memory II (delayed recognition; p = 0.002) than the medium sleep duration group (7-8 h). Longest and long sleep duration (8-9 h) was significantly associated with impairment of story memory II (longest sleep duration: adjusted odds ratio [OR] = 3.58, 95% confidence interval [CI] = 1.13-11.37, long sleep duration: adjusted OR = 4.30, 95% CI = 1.34-13.82) with reference to medium sleep duration, but no impairment of MMSE according to multiple logistic regression analysis. Furthermore, short sleep duration (<7 h) was not associated with cognitive impairment. Conclusions This study suggests that long sleep duration is associated with cognitive impairment, especially of story memory (delayed recognition) in older adults with normal global cognitive function.