The relationship between sleep duration and activities of daily living (ADL) disability in the Chinese oldest-old: A cross-sectional study

Introduction

Population ageing has become an irreversible trend as the fertility rate of the world’s population declines and life expectancy increases (Chen, 2016). China is now in a phase of accelerated ageing, with an average annual increase of 6.2 million elderly people (≥ 65 years-old); therefore, it is particularly important to identify the health patterns, general health status and service needs of the elderly population (Shi, 2021). There is growing concern about the impact of sleep on the health and well-being of elderly people (Frohnhofen, 2020; Rodriguez, Dzierzewski & Alessi, 2015), and activities of daily living (ADL) disability is also an important health issue for the elderly population (Chatterji et al., 2015; Gill et al., 2015; Zeng et al., 2017). ADL is a set of basic human functions that people must perform repeatedly every day to live independently, such as basic movement and skills to perform clothing, eating and transportation (Katz & Akpom, 1976). Frailty can result in ADL disability (hardly completing daily activities), which places a huge burden on their family, society and health care systems (Vermeulen et al., 2011).

The National Sleep Foundation recommends a sleep duration of 7-8 h for older adults (Hirshkowitz et al., 2015). Too little or too much sleep duration can increase the risk of morbidity and mortality from cardiovascular disease (CVD), osteoporosis and stroke in older adults (Da Silva et al., 2016; Moradi et al., 2017; Ren et al., 2020; Titova, Michaëlsson & Larsson, 2020; Wang et al., 2019; Zhou et al., 2020) and is associated with cognitive impairment and dementia (Ferrie et al., 2011; Lo et al., 2016; Potvin et al., 2012; Schmutte et al., 2007). Moreover, both cognitive dysfunction and cardiovascular risk factors are associated with ADL disability among elderly people (Andersen et al., 2015; Covassin & Singh, 2016; Cukierman-Yaffe et al., 2019; Karssemeijer et al., 2017; Sousa et al., 2009; Štefan et al., 2018; Zeng et al., 2017; Zhang et al., 2019).

Previous studies have mostly focused on the association between sleep duration and cognitive function, morbidity and mortality risk of other chronic diseases in elderly people (Chen et al., 2016; Kawada, 2019; Ma et al., 2020; Suh et al., 2018), but few studies have examined the relationship between sleep duration and ADL disability. In a study based on US night shift workers, Yong, Li & Calvert (2017) found that sleep duration was associated with ADL disability; Ishimaru et al. found that long sleep duration was associated with ADL disability in Japanese patients with dementia (Ishimaru et al., 2021). However, the relationship between sleep duration and ADL disability in the oldest-old population (≥ 90 years-old) has not been studied.

The relationship between ADL disability and sleep duration in the oldest-old population is therefore unclear. With the improvement of lifestyle, medical conditions and population health awareness, living a long and healthy life is becoming more common in China and across the world, and the oldest-old people, especially the natural oldest-old population in the Guangxi region, are examples of absolute health and longevity. Therefore, these individuals have very important research value. Research among this populations will provide clues to healthy lifestyles for the general population and to the development of rehabilitation treatment programs for oldest-old people in clinical practice. Considering the decline in physical recovery in the oldest-old people, we suspected that their optimal sleep duration may be different from that of other populations, because they need more rest time to recover their energy. Therefore, the present study aims to investigate the relationship between sleep duration and ADL disability in the oldest-old population based on the Guangxi Longevity Population Database.

Methods

ADL disability

ADL disability was assessed by using the Katz index in six essential parts: bathing, transferring, dressing, eating, toileting, and continence (Katz et al., 1963). The Chinese version of the Katz index scale, which has been extensively tested in previous studies, has been widely proven to have its reliability as well as validity (Gu, 2008; Liu et al., 2018; Yi et al., 2001). Participants were asked if they could independently perform five activities (bathing, transferring, dressing, eating, and toileting) or had trouble with incontinence (Katz et al., 1963). We defined those who could independently complete the above five activities and be continent as ADL normal, and ADL disability was identified as requiring assistance to complete one or more of the above five activities or being incontinent.

Results

The prevalence of ADL disability by age, race and sex

The age- and gender-specific rates of ADL disability are illustrated in Fig. 2. Overall, the prevalence of ADL disability was higher in centenarians (87%) than nonagenarians (61%), and the prevalence was also higher in females than in males (62% vs. 58%, 88% vs. 83%).

Figure 3 shows the prevalence of ADL disability by gender for different ethnicities. The prevalence was 71% in the Han population (68% in males and 72% in females), 60% in the Zhuang population (54% in males and 62% in females) and 53% in other ethnic populations (53% in males and 53% in females).

The relationship between sleep duration and the risk of ADL disability

The dose–response relationship between sleep duration and the risk of ADL disability among the oldest-old individuals (≥90 years) is illustrated in Fig. 4. The restricted cubic spline function showed a nonlinear dose–response relationship between sleep duration and the risk of ADL disability. Furthermore, the oldest-old individuals with 8–10 h of sleep duration had a decreased risk of ADL disability.

Table 2 indicates that sleep duration (≥12 h) is associated with an increased risk of ADL disability among the oldest-old individuals after adjusting for confounding factors (OR = 1.46, 95% CI [1.02, 2.10], p < 0.05). There was no significant association between sleep duration (<10 h) and the risk of ADL disability after adjustment.

Table 2:

Association between sleep duration and ADL disability among the oldest-old (≥90 years).

Variable	All	ADL disability	ORs (95% CI)	p
Sleep duration
<10 h	1,213	760	1.11 (0.90–1.37)	0.345
10–12 h	434	264	Ref
≥ 12 h	151	97	1.46 (1.02–2.10)	0.041

DOI: 10.7717/peerj.14856/table-2

Notes:

Adjustment for age, gender, ethnicity, smoking, drinking, meat, field crop, BMI, sleep quality, heart disease, diabetes, hypertension, respiratory disease, cancer, dementia and stroke; Bold values are statistically significant.

Discussion

In the current cross-sectional study, we observed that the overall prevalence of ADL disability among the oldest-old individuals was 62%, which was much higher than the reported rate of 32% (Jiang & Liu, 2020), and the female and Han populations were vulnerable to ADL disability. In addition, we found that sleep duration was associated with ADL disability in the oldest-old population, long sleep duration (≥ 12 h) could increase the risk of ADL disability by 47%, and the optimal sleep duration could be approximately 8-10 h. To our knowledge, this is the first study to estimate the association between sleep duration and ADL disability in the oldest-old Chinese population, and we further evaluated their optimal sleep duration.

The prevalence of ADL disability varies by region and gender and is related to factors such as the level of medical care and ethnicity. A national study recruited 52,667 Chinese participants over 80 years-old and found a decreasing trend in the prevalence of ADL disability from 18% (1998) to 12% (2008) (Hou et al., 2018). A cross-sectional study conducted in 2020 showed that the prevalence of ADL disability among the Chinese elderly population was 32%, and females (36%) had a higher prevalence than males (27%) (Jiang & Liu, 2020). Considering the relatively low level of health resource allocation in Guangxi as well as the obvious disparity between urban and rural areas (Luo, Nong & Tang, 2017), the overall prevalence of ADL disability in the Guangxi oldest-old population (63%) was significantly higher than the reported rate (32%) (Jiang & Liu, 2020). Consistent with our findings, Li, Tang & Wang (2016) revealed that the prevalence of ADL disability was higher in females than in males. In addition, inconsistent with our findings, several studies found that elderly people of Han nationality had a higher risk of ADL disability (Ran et al., 2019; Xu et al., 2020), which may result from multiple factors, such as the environment and heredity. The oldest-old population of Zhuang nationality has a lower risk of ADL disability than the oldest-old Han individuals due to their long life in mountainous areas and better rope ladder dexterity (Lv et al., 2003). Moreover, the frequency of carrying the Apo E ɛ 2 allele was higher in the Zhuang population (15%) than in the Han population (9%) (Yin et al., 2008), while the Apo E ɛ 2 allele was associated with a decreased risk of ADL disability (Kulminski et al., 2008). Therefore, a lower prevalence of ADL disability was found in the Zhuang population.

There is a nonlinear correlation between sleep duration and ADL disability, which could be mediated by chronic disease (hypertension, CVD) or immune inflammation. This study found a U-shaped relationship between sleep duration and the risk of ADL disability, which is consistent with previous research findings (Gangwisch et al., 2008; Yin et al., 2017), and long sleep duration was associated with an increased risk of ADL disability after adjusting for potential confounders. As a potential confounding factor, meat consumption was associated with the sleep duration and quality of the elderly by influencing neurotransmitters (serotonin and melatonin) (Lana et al., 2019). Smoking, alcohol consumption and obesity are also potential factors affecting sleep and ADL by influencing the nervous system and hormone levels in the body (Bayon et al., 2014; Htoo et al., 2004; Thakkar, Sharma & Sahota, 2015). Although the mechanisms underlying the association between sleep duration and ADL disability are unclear, one study has shown that short and long sleep durations increase the risk of hypertension among Chinese adults (Luo et al., 2021), which may lead to cardiovascular diseases (CVDs) that lower the quality of life and daily activity ability in the elderly individuals. Another Japanese study found that short and long sleep durations increase the risk of dementia (Ohara et al., 2018), which has an impact on communication skills, behaviour and discrimination.

In addition, sleep has a bidirectional association with the body’s immune system (Irwin, 2019). Acute infectious illnesses lead to fatigue and sleepiness by activating central nervous system (CNS) responses (Dantzer & Kelley, 2007), and it has often been found that cytokines (IL-1, TNF, IL-6) reach their highest circulating levels in the early morning and during sleep (Cermakian et al., 2013; Lange, Dimitrov & Born, 2010). In addition, several studies have reported a relationship of inflammatory markers (CRP, IL-6) with short sleep duration (Chiang, 2014; Ferrie et al., 2013). Multiple studies have revealed that long sleep duration was associated with elevated inflammatory markers (Patel et al., 2009; Prather, Vogelzangs & Penninx, 2015), which have an influence on the impairment of cognition (Leigh & Morris, 2020) and motion (Visser et al., 2002) in the elderly individuals. Dysregulation of inflammation, therefore, could be a potential mechanism of association between sleep duration and ADL disability. Related studies have shown that sleep is associated with changes in the epigenome, such as DNA methylation and histone modification (Gaine, Chatterjee & Abel, 2018), which provides another way to explore the mechanism linking sleep duration to ADL disability.

Studies on optimal sleep duration are currently limited to the general population (30–70 years-old), but there are no relevant studies about oldest-old people. One study involving 116,632 participants (35–70 years-old) from 21 countries indicated a sleep duration of 6–8 h was associated with the lowest risk of demise, and the optimal value of sleep duration was approximately 7 h (Wang et al., 2019). However, our results showed that two more hours of sleep could be beneficial for daily activities in the oldest-old population. Similarly, one Swedish study revealed that sleeping longer than 7 h could decrease the risk of all-cause mortality among people over approximately 87 years-old (Akerstedt et al., 2017), which indirectly validated our results. Explanations for the benefits of longer sleep duration in oldest-old individuals are uncertain, and further research is needed. One possible reason for this result is that a sleep duration of two more hours is advantageous for the oldest-old individuals to restore functions and sustain energy.

Our results reveal that the optimal sleep duration of the Chinese oldest-old population is higher than that of other populations, which could provide clues to the development of a healthy lifestyle for the oldest-old population. In clinical practice, medical staff should be aware of the potential risk for ADL disability in oldest-old people caused by excessive sleep duration during treatment and try to keep their sleep duration from 8 h to 10 h. There are some limitations in this study. (1) This study is a cross-sectional study without a causal relationship. It is possible that the oldest-old people with ADL disabilities need longer hours of sleep; therefore, further large cohort studies are needed for validation. (2) There may be some information bias in collecting sleep duration through participants’ self-assessment, and all questionnaires used self-report scales. (3) Our study was working in a specific part of China, and the results lacked national representation. (4) Our study participants were composed of multiple ethnic groups. Therefore, future nationally representative cohort studies focusing on each ethnic group are warranted.

Conclusion

In summary, females and the Han population are more likely to have ADL disability in the oldest-old population. A sleep duration longer than 12 h may be associated with the increased risk of ADL disability among the oldest-old Chinese. The optimal sleep duration among this population could be 8–10 h.

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