Associations Between Total Sleep Duration and Cognitive Function Among Middle-Aged and Older Chinese Adults: Does Midday Napping Have an Effect on It

Aim: The effect of sleep duration on cognitive function has been reported. However, the studies about the combined effects of total sleep duration and midday napping on cognition in elders were limited and inconclusive. We aimed to investigate the associations between total sleep duration, midday napping and cognitive function among middle-aged and older Chinese adults. Methods: Based on the 3rd wave of the China Health and Retirement Longitudinal Study (CHARLS) in 2015, a total of 9218 participants aged >45 years with completed cognition measurements were included. Cognitive functions were assessed by a combined global cognition score of episodic memory and mental status. Information about sleep-related variables, demographic characteristics, and health status were collected by validated questionnaires. Multivariate linear regression models were performed to evaluate the associations between total sleep duration, midday napping, and cognitive function. Stratified analyses were used to explore the potential effect modifier. Results: Overall, the global cognition score was 10.38 +/- 4.30 among the participants (mean age: 61.5 +/- 8.7 years). For sleep duration, both short sleep and long sleep duration were significantly associated with the increased risk of cognitive impairment after controlling for demographics and other confounders. Compared with sleeping for 7h per day, the adjusted beta-coefficient and 95% CI of the risk of cognitive impairment was -0.967 (95% CIs: -1.191, -0.742) for <5h, -0.257 (-0.498, -0.016) for 6h, -0.424 (-0.650, -0.198) for 8h and -0.664 (-0.876, -0.452) for >9h. The combined effect analysis indicated that subjects with extended or without naps had a significantly higher risk of cognitive impairment in the <5h sleep time group (ref: 7h) and in extended nappers or without naps group (ref: short nappers). Subjects with extended naps or without naps might increase the risk of cognitive impairment, especially those having <5h total sleep time. Stratified analysis showed that participants aged >60 years without social activity increased the risk of cognitive decline. Conclusion: An inverted U-shaped association was observed between total sleep duration and cognitive function in Chinese elders, especially in those aged >60 years or without social activity. Short midday naps could mitigate the deleterious effects of poor sleep quality and shorter sleep duration on cognitive function. The findings could help us identify the vulnerable population and decrease the burden of cognitive impairment.