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.
Keywords: cognitive impairment; logical memory; older adults; sleep duration.
© 2020 John Wiley & Sons Ltd.