Associations of time in bed and sleep timing with dementia and cognitive decline among Chinese rural‐dwelling older adults: A cohort study

Abstract Background The longitudinal associations of time in bed (TIB) (a composite indicator of sleep duration, latency, and fragmentation) and sleep timing (a marker of circadian rhythms) with dementia and cognitive decline in older adults are unclear. Method This population‐based cohort study used data from 1982 participants in the Shandong Yanggu Study of Aging and Dementia, who were aged ≥60 years, free of dementia, and living in the rural communities in western Shandong Province, China. At the baseline (2014) and follow‐up (2018) examinations, sleep parameters were assessed using the Pittsburgh Sleep Quality Index and the Epworth Sleepiness Scale. Global cognitive function was measured using the Mini‐Mental State Examination (MMSE). Dementia was diagnosed following the DSM‐IV criteria, and the NIA‐AA criteria for Alzheimer disease (AD). Data were analyzed using restricted cubic splines, Cox proportional‐hazards models, and general linear models. Result During the mean follow‐up of 3.7 (standard deviation = 0.3) years, dementia was diagnosed in 97 participants (68 with AD). Restricted cubic spline curves showed J‐shaped associations of sleep duration, TIB, and rise time with dementia risk, and a reverse J‐shaped association with mid‐sleep time. When continuous sleep parameters were categorized into tertiles, the multivariable‐adjusted hazard ratio (HR) of incident dementia was 1.69 (95%CI 1.01‐2.83) for baseline sleep duration >8 hours (vs. 7‐8 hours), 2.17 (1.22‐3.87) for bedtime before 9PM (vs. 10PM or later), and 2.00 (1.23‐3.24) for mid‐sleep time before 1AM (vs. 1‐1.5AM). Early bedtime and mid‐sleep time were significantly associated with incident AD (multivariable‐adjusted HR range: 2.25‐2.51; P<0.05). Among individuals who were free of dementia at follow‐up, baseline long TIB, early bedtime and mid‐sleep time, early and late rise time, and prolonged TIB and advanced mid‐sleep time from baseline to follow‐up were associated with greater decline in MMSE score (P<0.05). These associations with cognitive decline were statistically evident mainly among men or participants who were aged <75 years. Conclusion Long TIB and early sleep timing are associated with an increased risk of dementia and AD, and their associations with greater cognitive decline are evident only among young‐old people and men.