The mediating role of cognition in the relationship between sleep duration and instrumental activities of daily living disability among middle-aged and older Chinese.

Objective: To explore the effect of sleep duration at baseline on the incident IADL disability among middle-aged and older Chinese, and test whether cognition mediates this causality.

Methods: Data were collected from wave 1 (2011-2012) to wave 3 (2015-2016) of the China Health and Retirement Longitudinal Study (CHARLS). Sleep duration was self-reported at baseline. Cognitive function, including episodic memory and mental intactness were measured via a questionnaire. IADL was assessed at baseline and follow-up. Baron and Kenny's causal steps and Karlson/Holm/Breen (KHB) method were conducted to examine the mediating effect.

Results: A total of 10,328 participants free of IADL disability at baseline were included in this study. Over 4 years of follow-up, 17.1% of participants developed IADL disability. Compared to 7-8 h sleep duration, both short sleep (OR=1.460; 95% CI: 1.261-1.690 for sleeping ≤5 h; OR= 1.189; 95% CI: 1.011-1.400 for sleeping 5-7 h) and long sleep (OR=1.703; 95% CI: 1.269-2.286 for sleeping >9 h) were linked with incident IADL disability. KHB method identified significant mediating effect of cognition on the relationship between extreme sleep durations (≤5 h or >9 h) and IADL disability and the proportional mediation through cognition was 21.32% and 21.06% for sleeping ≤5 h and >9 h, respectively.

Conclusion: Both short (sleeping ≤5 h) and long sleep duration (sleeping >9 h) predicted incident IADL disability. Cognition partially mediated the effect of extreme sleep durations on IADL disability.