The current study evaluated the mediating role of sleeping quality in the relationship between cognitive decline and depression. Several mechanisms have been proposed to explain how sleep quality impacts both cognition and depression. 60 to 70% of people with cognitive impairment or dementia have sleep disturbances. Research has shown that poor sleep quality as measured by the PSQI is associated with multiple markers of metabolic dysfunction, including insulin resistance which is related with bad performance on executive function tasks among older adults19,20. Furthermore, several mouse models have demonstrated strong relationships between diet induced insulin resistance and memory dysfunction21,22. Besides, good sleep plays a protective role in human emotional homeostasis and regulation23. And in depressed individuals, dysregulated sleep was often-reported24. As many as 90% of patients with depression will have sleep quality complaints25. It was reported that as many as 24–58% of individuals with sleep disordered breathing (eg, obstructive sleep apnea) meet the criteria for depression26. And it was found that all the symptoms of depression, sleep problems were the most, prevalent, (13.6%), and those with sleep problems had the highest, relative odds (7.6 times) of developing a new-onset major depressive episode during the next year compared with those without sleep problems. Thus, sleep disturbance was associated with having more depressive symptoms27. Another mechanism through which poor sleep could affect both cognitive function and depression is through oxidative stress. Higher levels of oxidative stress biomarkers were found in patients with bad sleeping quality28. In addition, sleep deprivation is related with an increased rate of oxidative pentose phosphate pathway activity29. High levels of oxidative stress has also been implicated in depression among older adults30 as well as those with cognitive decline31. Our study demonstrates that bad sleeping quality partially mediates the effects of cognitive decline on depression in older adults. Our results thus contribute to the current knowledge by providing evidence that improving sleeping quality may ameliorate the negative impact of cognitive decline on depression.
Among the 7 components of sleeping assessment, we found that daytime dysfunction had a highest mediation effect with a proportion of mediation up to 14.56%, the following was sleep disturbance which had a mediation effect of 5.18% and subjective sleep quality which had a mediation effect of 4.19%. This was consistent with previous studies. A recent research found that the effects of sleep disturbance, subjective sleep quality and daytime dysfunction scores were most obvious on anxiety in the elderly aged 60 years and older in China, and the ORs (95%CI) were 4.63 (3.55–6.04), 2.75 (2.33–3.23) and 2.50 (2.19–2.86), respectively32. An earlier study also found that symptoms of short sleep duration, daytime sleepiness and sleep disturbances are independently related to anxiety while the use of sleep medication is independently associated to depression in a random sample of 2393 individuals aged 65 years or older33. Another longitudinal study found that short sleep duration, especially on weekdays, was significantly associated with subsequent depressive (OR = 0.86, 95%CI: 0.80–0.92)34. Besides, shorter sleep duration has been found to be associated with a greater rate of ventricular enlargement, which similarly reflects loss of brain volume35. And sleep disturbances were studied to be linked to cortical thinning, a marker of cortical atrophy found in many dementia subtypes36,37. As we discussed before, numerous studies provide findings indicating the remarkable relationship between sleep alterations and depression. Our study found the most three relevant components (eg. daytime dysfunction, sleep disturbance, subjective sleep quality) mediated the relationship between cognitive decline and depression, which might be the target to focus on improving sleeping quality.
According to the World Health Organization, depression is the leading cause of disability, affecting over 300 million people. Depression is also the commonest mental disorder in older adults worldwide, affecting 7% of the world's older population and accounting for 5.7% of years lived with disability among adults aged over 60 years38. For many individuals with depression, the major impairment they experience is cognitive decline39. Our study found a high prevalence of depression that was 17.4% and after adjusting numerous confounders, the association between cognitive decline and depression was still significant. This is most likely regulated by several mechanisms. Firstly, depression's duration has a significant impact on left hippocampal volume, indicating that the time since first depressive episode plays an important role in hippocampal degeneration which leading to cognitive decline40. And lower hippocampal volumes are associated with a poorer clinical outcome and more depressive episodes41. Secondly, accumulated evidence highlighting the major role of systemic inflammation, which were existed both in cognitive decline and depression42. Thirdly, as we discussed before, oxidative stress was a common mechanism in cognitive decline and depression30,31. Thus, the relationship between cognitive decline and depression is complex and bidirectional. The ultimate goal of treatment in depression is full functional recovery, and assessing patients for cognitive impairment and selecting treatments that address cognitive dysfunction.
There are several limitations in this study. Firstly, our sampling did not cover all the cities in west China. Secondly, our study design was a cross-sectional study. Thirdly, we conducted a centralized investigation and not a household survey. Furthermore, most of the participants who came to the site of investigation on their own were relatively healthy. This might existed some bias in the analysis. So a critical next step would be to replicate this study with longitudinal data to establish the relationship. In addition, it would be crucial to examine if clinically established sleep interventions are able to prevent or reverse depression in cognitive decline adults.