Insufficient, excess sleep may cause cognitive decline among middle-, older-aged people

Monitoring of cognitive function may be warranted among middle- and older-aged individuals who sleep for 4 or fewer or 10 or more hours per night, according to results of a pooled cohort study published in JAMA Network Open.

“Previous studies have reported a strong association between sleep and cognitive function in older adults,” Yanjun Ma, BA, of Peking University Clinical Research Institute in China, and colleagues wrote. “Although a number of cohort studies have examined whether baseline sleep duration was associated with cognitive decline and incident dementia, the results were inconsistent. Some cohort studies observed sleep duration to be statistically significantly associated with cognitive decline and incident dementia, whereas other studies did not reach this conclusion.”

Limitations of prior studies included small sizes and unclear results regarding the magnitude of the association between sleep duration, particularly extreme sleep duration and several cognitive domains. Ma and colleagues aimed to address these limitations in the current study by evaluating the link between sleep duration and cognitive decline by analyzing data of several waves of the English Longitudinal Study of Ageing and the China Health and Retirement Longitudinal Study among a population-based setting. The studies featured two randomly enrolled cohorts that comprised 20,065 total individuals who lived either in England and were aged 50 years or older or in China and were aged 45 years or older. Participants provided self-reported nightly sleep duration via face-to-face interviews. Global cognitive z scores served as the main outcome.

Results showed faster declines in global cognitive z scores among individuals with 4 hours of sleep or less and 10 hours of sleep or more that were faster vs. those in reference group of 7 hours per night, after adjusting for numerous covariates and during 100,000 person-years of follow-up. Further, the investigators observed an inverted U-shaped association between sleep duration and global cognitive decline.

“The inverted U-shaped association indicates that cognitive function should be monitored in middle-aged and older individuals with insufficient or excessive sleep duration,” Ma and colleagues wrote. “Future mechanism studies and intervention studies examining the association between sleep duration and cognitive decline are needed.”

In a related editorial, Yue Leng, MD, and Kristine Yaffe, MD, both of the department of psychiatry at University of California, San Francisco VA Medical Center, provided a potential path forward for related future research.

“Prospective studies using objective measures of sleep duration and comprehensive neuropsychological batteries are needed to test the association in different populations and to compare different ethnicities,” they wrote. “Use of neuroimaging techniques and biomarkers will help clarify the pathophysiological pathways between sleep duration and cognitive aging. Furthermore, novel statistical approaches (e.g., mendelian randomization or causal mediation analysis) should be considered to overcome limitations of traditional observational studies and to elucidate pathways underlying sleep duration and cognitive decline.

“Ultimately, the study of sleep and cognition needs to go beyond sleep duration,” they added. “Both sleep quality and sleep quantity should be considered in developing prevention and management strategies for dementia.”