Social isolation is prevalent and associated with dementia, yet the directionality and mechanisms are less understood. This study examined the association between social isolation and cognitive functioning and explored the mediating role of sleep disturbance on the social isolation-cognition relationship. Data from 5,753 dementia-free Americans aged ≥50 of 2006 (T1), 2010 (T2), and 2014 (T3) waves of the Health and Retirement Study. Social isolation was measured by the Steptoe Social Isolation Index. Cognitive functioning was measured by the Telephone Interview of Cognitive Status. Sleep disturbance was measured with the modified Jenkins Sleep Scale. We used cross-lagged panel models to determine the associations between social isolation, sleep disturbance, and cognitive functioning. Social isolation is significantly associated with subsequent cognitive functioning (T1 to T2: β = -0.055, standard error [SE] = 0.014, p < 0.001; T2 to T3: β = -0.044, SE = 0.016, p < 0.001). Lower cognitive functioning is significantly associated with greater subsequent social isolation (T1 to T2: β = -0.101, SE = 0.020, p < 0.001; T2 to T3: β = -0.058, SE = 0.011, p < 0.001). Sleep disturbance at T2 partially mediated the effect of social isolation (T1) on cognitive functioning (T3), accounting for 6.2% of the total effect (β = -0.003, SE = 0.001, p < 0.01). Social isolation may deteriorate cognitive functioning and vice versa. The association between social isolation and cognition is partially explained by sleep disturbance.

Previous longitudinal studies have found that cognitive function affected oral health, and vice versa. However, research is lacking on the reciprocal relationships between cognitive function and edentulism simultaneously, especially in developing countries. The present study aimed to examine the reciprocal relationship between cognitive function and edentulism among middle-aged and older adults in China. Data were derived from the 2011 and 2015 waves of the China Health and Retirement Longitudinal Study. The sample included 14,038 respondents aged 45 or older. A two-wave cross-lagged analysis was adopted to test the hypothesized model. Among respondents aged 45–59, baseline cognitive function was associated with subsequent edentulism [b = −0.017, standard deviation (SD) = 0.006, P < 0.01]. In contrast, baseline edentulism was not significantly associated with poorer cognitive function at the follow-up wave (b = −0.744, SD = 0.383, P > 0.05). However, among respondents aged 60 or older, baseline cognitive function was associated with subsequent edentulism (b = −0.017, SD = 0.005, P < 0.01), and baseline edentulism was also associated with follow-up lower levels of cognitive function (b = −0.419, SD = 0.143, P < 0.01). These findings demonstrated the reciprocal relationships of cognitive function and edentulism. However, such relationships varied across age groups. This study demonstrates the importance of developing programs and services to promote both cognitive and oral health, especially for those in older age.