The longitudinal mediating role of sleep in the bidirectional relationship between depression and functional disability among older adults: A systematic review and conceptual framework

BackgroundThe increasing prevalence of depression and functional disability in older adults highlights the need for targeted interventions, with sleep as a potentially modifiable factor, yet the longitudinal effects and mediating role of sleep remain poorly understood. MethodsThis review and conceptual framework aimed to examine the pairwise bidirectional associations between sleep, depression, and functional disability and identify the longitudinal mediating role of sleep in the bidirectional relationship between depression and functional disability in older adults. The academic databases PsycArticles, PubMed, MEDLINE, Science Citation Index, Social Sciences Citation Index, ProQuest Dissertations and Theses Global, Cochrane, and Scopus were searched for research published in English between January 2000 and June 2024. Systematic review and cohort study designs were eligible. All included studies were assessed for quality using the Critical Appraisal Skill Programme checklist (CASP 2024). Results397,289 citations were identified, and 82 studies meeting the inclusion criteria were included. Cohort studies and reviews provide evidence that there is a dynamic reciprocal correlation between sleep, depression, and functional disability in the older population. We propose that sleep may increase the risk of depression and functional disability in the follow-up years, with sleep acting as a potential mediating factor between depression and functional disability. There was a selection bias in the study samples, as most studies focused on specific populations or regions. Moreover, some of the cohort studies included lacked sufficient follow-up time to observe long-term effects. ConclusionsThis review and conceptual framework highlight that sleep health can provide crucial insights for mitigating the adverse effects experienced by older adults due to depression and functional disability. For healthcare professionals and policymakers, it provides evidence about prioritizing sleep health as an accessible step to foster a healthy lifestyle. PROSPERO registration numberCRD42024556536. What is already known on this topicWith the increasing aging population, improving the physical and mental health of older adults has become a key social issue. Substantial epidemiological studies have confirmed the existence of bidirectional relationships between depression, sleep disorders, and functional disability in older adults, with all three variables influencing each other. However, the complex interaction mechanisms among these three variables remain unclear, and further research is needed to explore whether sleep plays a longitudinal mediating role between depression and functional disability. What this study addsThis study significantly enhances our understanding by providing robust evidence of the dynamic, bidirectional relationships among sleep, depression, and functional disability in older adults. Unlike previous research that primarily examined pairwise relationships, our study delves deeper by proposing a comprehensive conceptual framework. This framework underscores the potential mediating role of sleep, suggesting that sleep disturbances are not merely consequences of depression and functional disability but also active contributors to their interaction and progression. By elucidating these underlying mechanisms and potential pathways, our study sheds light on the complex interplay among these three variables, ultimately enhancing the quality of life for older adults. How this study might affect research, practice or policyThis study paves the way for deeper investigation into the causal mechanisms connecting sleep, depression, and functional disability. It highlights the critical importance of prioritizing resources for sleep-related research and interventions, recognizing their significant potential to enhance the well-being of an aging population. This holistic approach aims to foster a more comprehensive understanding and effective strategies for promoting healthy aging.