In recent decades, the proportion of people aged 60 years or older has increased rapidly and is projected to be greater than one-fifth of the global population in 2050 (Prince et al., 2013). Consequently, the burden of dementia and cognitive impairment is also increasing. China has the largest population of patients with dementia in the world (Jia et al., 2020) and is expected to host almost half of the global dementia population by 2050 (Chhetri et al., 2020). Motoric cognitive risk (MCR) syndrome is a predementia syndrome characterized by subjective cognitive complaints and objective slow gait in older individuals without dementia or any mobility disability (Verghese et al., 2013). It has recently been suggested that providers use MCR syndrome diagnosis as an indicator to screen for adverse health outcomes (Chhetri et al., 2017) including falls (Callisaya et al., 2016), disability (Doi et al., 2017), dementia (Doi et al., 2017), cardiovascular disease (Meiner et al., 2020), and mortality (Beauchet et al., 2019). A clearer understanding of potentially modifiable determinants of MCR, enabling effective preventative intervention design, would help alleviate the increasing individual and societal burdens of dementia.

Sleep is a complex and crucial physiological process for the brain; it can restore the brain’s functional capacity, maintain homeostasis, and is necessary for survival. Older adults tend to experience less nighttime sleep and have more sleep disturbances compared to younger adults (Ohayon et al., 2004). As previous experimental research demonstrated, a decrease in sleep quality and quantity might elevate the concentration of β-amyloid, a hallmark of Alzheimer’s disease (AD; Xie et al., 2013), and therefore lead to a higher risk of dementia and different types of cognitive impairment. Numerous studies have investigated the various associations between sleep duration and cognition, but the results are conflicting. Some cohort studies observed sleep duration to be statistically significantly associated with cognitive decline and incident dementia (Virta et al., 2013; Devore et al., 2014; Bokenberger et al., 2017; Ohara et al., 2018), whereas others did not reach this conclusion (Tworoger et al., 2006; Benito-León et al., 2009; Yaffe et al., 2011; Keage et al., 2012; Potvin et al., 2012; Blackwell et al., 2014). Some reported an inverted U-shaped association between sleep duration and the risk of dementia (Chen et al., 2016; Bubu et al., 2017) or cognitive decline (Xu et al., 2011), which means that, compared to intermediate sleep duration, short or long sleep durations are associated with cognitive decline or dementia. However, still, others found that longer sleep duration was associated with poorer cognitive function (Benito-León et al., 2009; Faubel et al., 2009). This discrepancy may be due to various evaluations of sleep duration and cognitive function, different characteristics among the studied cohorts, and differences in study length. Along with sleep duration, the relationship between sleep quality and cognitive performance has also been studied, and mixed findings have been reported. Some studies showed that poorer sleep quality measured by the Pittsburgh Sleep Quality Index (PSQI) was associated with weaker working memory (Noort et al., 2016) and executive function (Nebes et al., 2009), while others failed to find an association between subjective sleep quality and cognitive performance (Stepanski et al., 1989; Miyata et al., 2013). However, to the best of our knowledge, there is currently little evidence supporting the U-shaped association between sleep duration and MCR in developing countries, especially in China. Even fewer have investigated the relationship between sleep quality and MCR among Chinese older adults.

Sleep-related factors are potentially modifiable. Early identification and control of sleep-related predictors of MCR might have important clinical implications and generate early avenues for the prevention of progression to dementia, specifically in the large Chinese elderly population. Therefore, this study used data from the China Health and Retirement Longitudinal Study (CHARLS), a nationally representative longitudinal study, to examine the inverted U-shaped association between sleep duration and probability of MCR, and to investigate the associations between sleep quality and MCR among community-based Chinese older adults.

In the nationally representative cohort of community-dwelling Chinese older adults, we found that, compared to median night sleep duration, shorter and longer night sleep durations were significantly associated with increased odds of MCR diagnosis, indicating an inverted U-shaped association between nighttime sleep duration and cognitive performance. Moreover, an increasing frequency in self-reported poor sleep quality was also significantly associated with higher odds of MCR after adjusting for potential confounders, including demographic characteristics, lifestyle factors, and self-reported comorbidities.

Several epidemiological studies on Asian populations have also reported a U-shaped association between sleep duration and cognitive function, which is in concordance with our results. Ohara et al. (2018) investigated the association between self-reported daily sleep duration and risk of dementia by grouping all participants into five categories (<5.0, 5.0–6.9, 7.0–7.9, 8.0–9.9, ≥10.0 h/night) in a Japanese elderly cohort. They observed a U-shaped association between sleep duration and dementia during the 10-year follow-up period. Xu et al. (2011) found that short (3–4 h/day) or long sleep durations (≥10 h) were an important factor independently associated with memory impairment among 28 67 + -year-old community-dwelling adults in Guangzhou. A prospective study among 7,444 community-dwelling women (aged 65–80) showed that both short-duration (≤6 h/night) and long-duration (≥8 h/night) sleepers had a greater cognitive decline and an increased risk of cognitive impairments, including dementia, compared to those with self-reported sleep of 7 h/night during an average of more than 7 years of follow-up. Another study among older Chinese women identified a significant U-shaped association between a higher dementia risk with either short (≤6 h/night) or long (≥8 h/night) sleep durations (vs. 7 h/night; Chen et al., 2016). In contrast, Benito-León et al. (2009) observed a significant positive association between long self-reported sleep duration (≥9 h) and incident dementia. For short-duration sleepers (≤5 h), after controlling for relevant confounders, the relationship was attenuated and no longer significant (Benito-León et al., 2009; Kawada, 2015).

Previous studies have also investigated the relationship between changes in sleep duration and cognitive decline, but the results were contradictory.

Hahn et al. (2014) used data from a population-based sample of 214 Swedish adults aged 75 and over and found that reduced sleep was associated with a 75% increase in all-cause dementia risk at a 3-year follow-up. However, Westwood et al. (2017) reported that transitioning to sleeping >9 h over a mean period of 13 years was associated with an increased risk of all-cause dementia. Moreover, Bokenberger et al. (2017) demonstrated that short and extended time in bed (TIB) among older adults predicted increased dementia incidence in the 17 years following the initial sleep analysis. The variation in follow-up period, outcome measurements, cohort size, and studied population might partially explain the discrepancies between results.

Although several studies have examined the relationship between sleep duration and cognitive function, few have investigated the relationship between sleep duration and odds of MCR. Only one cross-sectional study among 940 Chinese older adults aged ≥65 years in the Ningbo Community Study on Aging (NCSA) reported that longer night sleep duration (>8.5 h) was associated with higher odds of MCR compared to a shorter night sleep duration (<8 h). The relatively few participants with extremely short sleep durations prevented this study from exploring a possible U-shaped association (Zeng et al., 2021).

Besides sleep duration, we also found that subjective poor sleep quality was significantly associated with higher odds of MCR after adjusting for confounders. Few previous studies have assessed whether subjective sleep disturbance predicts cognitive decline or cognitive impairment among older adults, and the results were inconsistent. Tworoger et al. (2006) found that women aged 70–81 years who regularly had difficulty falling or staying asleep scored 0.11 units lower on the global cognition score (95% CI: −0.22, 0.01) compared to those who rarely had difficulty sleeping. Jelicic et al. (2002) reported a negative relationship between sleep complaints and cognitive performance at a follow-up among 838 middle-aged and older adults (≥50 years). Cricco et al. (2001) observed that self-reported symptoms of insomnia independently predicted the incidence of cognitive decline 2 years later in older men, but not older women. Inconsistent conclusions across the current literature about the relationship between sleep quality and cognitive function among older adults could be explained by the variability of the subjective sleep quality indicators used. More objective methods and tools to measure sleep disturbance (such as wrist actigraphy) among larger samples of older adults might be needed to clarify these associations in the future.

The mechanisms underlying the association between sleep disturbance and odds of MCR remain unclear, although several potential biological pathways between sleep disturbance and cognitive function have been identified. Firstly, reduced or prolonged sleep duration has been found to be related to changes in the brain’s ventricular volume (Lo et al., 2014) and total cerebral brain volume (Westwood et al., 2017) as well as cortical thickness (Spira et al., 2016), all of which influence cognitive performance. Secondly, neuroinflammation caused by interleukin 6 and C-reactive protein activation might occur after sleep deprivation (Atienza et al., 2018), which might mediate age-related cognitive impairment (Legdeur et al., 2018). Thirdly, poor sleep quality may derail metabolic and endocrine function, which could induce cognitive impairments. For instance, chronic sleep deprivation is linked to an increase in amyloid-β plaque formation, which is a hallmark of AD (Kang et al., 2009). Besides AD, poor sleep quality may exacerbate or accelerate the neurodegenerative process of different types of dementia.

Our study has several strengths. First, it is the first study to use a nationally representative cohort of community-dwelling Chinese adults to evaluate the association between sleep disturbance and MCR, which adds generalizability to our findings. Moreover, our results revealed that self-reported sleep duration and subjective poor sleep quality may be useful clinical tools to help predict persons at risk of progressing to dementia. Second, respondents were asked about sleep duration without being given example durations, and the classification of different nighttime sleep periods was set based on previous literature, both of which contributed to the identification of a reliable U-shaped association between sleep duration and odds of MCR. Third, a wide range of potential confounding parameters including demographic background, lifestyle factors, and history of comorbidities was adjusted for within the multivariable analyses, thus providing robust statistical power. However, several limitations should also be acknowledged. Firstly, the assessments of nighttime sleep duration were collected based on self-reporting in this study, which may have been influenced by recall bias. Although self-reported sleep has been shown to reflect objective assessments of sleep accurately (Biddle et al., 2015), more objective measurements are required in future studies. Secondly, this investigation was an observational study, so no causal relationships could be demonstrated. Lastly, the sleep quality of participants was not fully investigated. More detailed measurements, like the PSQI, a self-rated questionnaire that assesses sleep quality and disturbances over a 1-month time interval (Buysse et al., 1989), would be needed to comprehensively evaluate the sleep quality of all participants.

In summary, this study demonstrated that shorter or longer sleep durations than average and subjective poor sleep quality were associated with increased odds of MCR. These findings have substantial public health implications and emphasize the need to improve the quality and quantity of sleep to prevent the prevalence of pre-dementia symptoms in developing countries where the number of older adults with dementia is expected to increase. In the future, longitudinal studies with comprehensive assessments of sleep disturbance among a larger population are required to establish causality and further explore potential mechanisms between sleep-related factors and MCR.

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author/s.

The studies involving human participants were reviewed and approved by Peking University’s Institutional Review Board. The patients/participants provided their written informed consent to participate in this study.

WX wrote and participated in all aspects of this research, including the field investigation. AB and XH analyzed the data and interpreted the results. YG and LL designed this research. All authors reviewed the manuscript and agree to be accountable for the content of the work. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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