Interaction Effect of Midday Napping Duration and Depressive Symptoms on Subjective Memory Impairment Among Elderly People in China: Evidence from the China Health and Retirement Longitudinal Study Database

doi:10.21203/rs.3.rs-2618333/v1

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Research Article

Interaction Effect of Midday Napping Duration and Depressive Symptoms on Subjective Memory Impairment Among Elderly People in China: Evidence from the China Health and Retirement Longitudinal Study Database

https://doi.org/10.21203/rs.3.rs-2618333/v1

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published 13 Oct, 2023

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Background

Subjective memory impairment (SMI) is common in the elderly. We aimed to reveal the interaction effect of midday napping duration and depressive symptoms on the risk of SMI.

Methods

Using a dataset representative of the Chinese population from a longitudinal study of health and retirement in China, subjects with SMI were screened using the question “how do you feel about your memory now?” and the Mini-Mental State Examination. A logistic regression model was applied to explore the factors affecting SMI. Additive and multiplicative models were used to analyze the interaction effect of midday napping duration and depressive symptoms on the risk of SMI.

Results

We enrolled 8,254 subjects included and the incidence of SMI was 63.9%. Depressive symptoms, nap time, and physical activity were influencing factors of SMI. Midday napping duration and depressive symptoms had positive additive interaction effects on the risk of SMI. When extended-length naps and depressive symptoms coexisted, the risk of SMI was 1.06 times greater than that for either alone (RERI = 0.27, 95% CI = 0.07–0.43; AP = 0.14, 95% CI = 0.01–0.23; S = 1.06, 95% CI = 0.57–1.62). When short naps and depressive symptoms coexisted, the risk of SMI was 1.2 times higher than that for either alone (RERI = 0.12, 95% CI=-0.14–0.39; AP = 0.13, 95% CI=-0.07–0.22; S = 1.20, 95% CI = 0.79–1.82).

Limitations

Since this was a cross-sectional study, the cause-and-effect relationships between the associated variables cannot be inferred.

Conclusions

The interaction effect that exists between nap time and depressive symptoms in the elderly is important for the identification and early intervention of people at risk for SMI.

Subjective Memory Impairment

Elderly People

Napping Duration

Depressive Symptoms

It is estimated that the number of individuals aged ≥ 65 years will reach 449 million by 2050 in China, and the aging rate will be as high as 37.3% ^[1]. A larger aging population will result in a significantly increased number of people with Alzheimer’s disease (AD) ^[2]. AD is a neurodegenerative disease, and its onset relies on a continuous pathophysiological process that begins before AD patients show obvious impairments in cognitive performance. A large number of patients’ neurons begin to undergo apoptosis in the late stage of AD, which subsequently causes irreversible cognitive damage ^[3]. Because of the invisibility of the development of AD’s pathology, early identification and intervention appear to be particularly important.

Subjective memory impairment (SMI) is the prodromal stage of mild cognitive impairment (MCI). SMI is a self-reported memory decline in some elderly people who still score within the normal range on objective cognitive tests ^[4]. The prevalence of SMI in the elderly population ranges approximately from 20–70% worldwide ^[5–9]. The cognitive function of elderly individuals with SMI declines faster than that of those without SMI, and the risk of AD is higher in the former group ^[10–11]. A 4-year cohort study further proved that 24.4% of the elderly with SMI would progress to showing MCI ^[12]. Elderly individuals with SMI are 2 times more likely to develop AD compared to those without SMI ^[13]. Elderly people with SMI also experience abnormal neural activity in memory-related brain regions before the external memory behavior has been damaged ^[14].

Previous studies have found that midday napping duration is associated with the memory quality of elderly individuals ^[15]. The proportion of elderly people who habitually take midday naps is 22–69% globally, which is significantly greater than the proportion of young people who do the same ^[16–17]. A national cohort study found that > 51% of elderly people in China have a habit of taking midday naps, which are regarded culturally as part of a healthy lifestyle ^[18–19]. Previous studies have shown that daytime napping duration also directly affects the increase in β-amyloid deposition in the brain, which correlates with individual cognitive function ^[20–21]. A large-scale study in China has directly indicated that appropriate midday napping after lunch has a protective effect on memory and cognitive function in elderly people ^[22]. To date, many studies have explored the causal relationship between nighttime sleep and SMI in elderly people. However, although these studies found that the mechanism by which nighttime sleep impairment affects the occurrence of SMI is related to β-amyloid deposition in the brain, daytime mental state, etc ^[21,23−25]. More research to clarify the relationship between midday napping duration and SMI remains necessary.

Depression is a common mental health problem in the elderly, and the prevalence of depressive symptoms in elderly Chinese people has been on the rise in recent years ^[26–27]. A large number of studies have concluded that depressive symptoms are also associated with the occurrence of SMI ^[23,28−29]. For example, Yates and colleagues found that individuals with depressive symptoms were more likely to develop SMI after 2 years ^[30]. Research has also shown that depressive symptoms in couples are associated with the onset of SMI in later life ^[31]. While, the research about the interaction of depression and napping time on cognitive function was scarce.

Many scholars have studied the relationship between midday napping duration, depressive symptoms, and SMI, but most of the available observational data come from Western countries, and similar empirical studies of the Chinese elderly population are scarce. Differences in social and cultural backgrounds, lifestyle, and environmental factors may variably influence SMI in the elderly. Also, the questions of (1) whether there is an interaction effect between midday napping duration and depressive symptoms in the elderly and (2) to what extent the effect is associated with SMI in elderly people remain inadequately answered, as few studies to date have explored these topics in depth. As a prodromal state of MCI, SMI plays a vital role in future cognitive decline and/or dementia in the elderly. Thus, identifying risk factors for SMI and developing an intervention plan targeting SMI risk factors are essential steps for reducing the incidence rates of MCI and dementia. This study therefore used a presentative cross-sectional survey data in China to analyze the influences of midday napping duration and depressive symptoms on SMI in middle-aged and elderly people and to explore whether an interaction effect exists between them.

Design and Sample

We used the China Health and Retirement Longitudinal Study (CHARLS) data from 2018, which are publicly available at http://charls.pku.edu.cn. These survey data cover 28 provinces in China and contain information on individual factors, lifestyles, and health status of respondents. The data are of a high quality and have the nature of a large sample, thus providing real and effective data support for the analysis in this paper. Based on the purpose and needs of this study, the sample was screened, and 8,254 microsamples were obtained after excluding those with missing values of key variables. Our research was performed in accordance with the Declaration of Helsinki. The original CHARLS project was approved by the ethical review committee of Peking University (IRB00001052–11015), and all participants signed an informed consent form at the time of enrollment.

Individuals initially eligible for participation were those who (1) underwent the modified Mini-mental State Examination (MMSE) in 2018 and (2) were aged ≥ 60 years old. Hence, a total of 8,583 subjects were included. Then, the following criteria were applied for exclusion purposes: (1) data concerning midday napping duration were missing (n = 10) and (2) the individual had a self-reported diagnosis of mental illness or neurological disease (n = 319). Finally, we identified 8,254 subjects for our analysis.

Measurement Of Smi

The respondents were asked to respond to the question “how do you feel your memory is now?” by stating excellent, very good, good or fair, or poor. Then, the MMSE was used to measure the cognitive function of all elderly participants. The MMSE includes items that focus on aspects such as orientation, memory, attention and computation, and language. The selected cutoff scores for our investigation were 17 points for illiterate individuals, 20 points for participants with ≤ 6 years of education, and 24 points for participants with > 6 years of education, and cognitive impairment was suggested when the participant had an MMSE score lower than the appropriate cutoff score according to their years of education ^[32]. Subjects who complained of poor memory and who showed a normal MMSE test score were considered to have SMI ^{[33, 4]}.

Measurement Of Napping Duration

The sleep variable was investigated by examining data on post-lunch napping, which were collected by asking the following question during the face-to-face interview: “during the past month, for how long (in minutes) did you take a nap after lunch?” Respondents were categorized according to their answer into short nap (0–30 min), moderate-length nap (31–90 min), and extended-length nap (> 90 min) groups. The cutoff values for napping time were chosen according to a previous study ^[34], and our study used a moderate-length nap time (31–90 min) as the reference value for nap time ^[22].

Measurement Of Depressive Symptoms

Depressive symptoms were assessed using the 10-item Center for Epidemiological Studies Depression Scale, which has been highly validated for use in general populations with adequate reliability and validity confirmed for the assessment of the community-dwelling older population in China. The respondents were asked to rate “how often you felt this way during the past week,” with total scores ranging from 0–30 points and a lower score indicating a lower level of depressive symptoms. A score of ≥ 10 points was used to identify the respondents who had significant depressive symptoms ^[24]. Cronbach’s α value was 0.71, indicating good reliability ^[24].

Measurement Of Physical Activity

The CHARLS questionnaire investigated the physical activity of participants in the preceding week. Participants were asked to indicate both the number of days that they performed ≥ 10 min of physical activity and the duration of their daily activity. We then estimated the physical activity energy expenditure or metabolic equivalent (MET) for 1 week using the following equation: MET-mins = MET level activity time (60 min) × number of activities per week. Then, based on the International Physical Activity Questionnaire, the physical activity level was divided into low-intensity physical activity (< 600 METs/week), medium-intensity physical activity (600–3000 METs/week), and high-intensity physical activity (> 3000 METs/week). We used a moderate-intensity physical activity level as the criterion for adequate physical activity according to expert consensus ^[15].

Covariates

The covariates included in the analysis were sociodemographic characteristics such as age, sex, education level (illiterate, primary education, or secondary education or above), marital status (currently married or not), and economic status (monthly income).

Statistical Analysis

Data analyses were performed using Stata version 26 (STATA Corp., College Station, TX, USA). A descriptive statistical analysis was used to demonstrate the status of the sample, and the chi-squared test was used for univariate analysis. Logistic regression models were used to analyze the factors affecting SMI. Additive and multiplicative models were used to analyze the interaction effect of midday napping duration and depressive symptoms on SMI ^[35–36]. The relative excess risk due to interaction (RERI), the attributable proportion (AP) due to interaction, and the interaction index (i.e., the synergy index) (S) estimates and 95% confidence interval (CI) values were calculated. Rothman suggests that if there is no additive interaction between the two factors, then the confidence interval of RERI and AP should contain 0, and the confidence interval of S should contain 1 ^[35]. A 2-tailed P value of < 0.05 was considered to indicate statistical significance.

Participant Characteristics

As shown in Table 1, more than half of the 8,254 elderly people included in this study were female (n = 4240). The average age of participants was 68.5 ± 6.5 years old, and the 60–74 age group contained the largest proportion of participants (80.9%). A total of 4,401 participants were illiterate (53.3%), 6,642 (80.4%) were married, and the most common monthly income was 5,000 (67.5%).

Table 1

Comparison of subjective memory impairment in older adults with different characteristics (N = 8254)
Variable		n (%)	Participants with SMI (%)	Participants without SMI (%)	χ²	P
Gender	Female	4,240 (51.37)	2933 (69.17)	1307 (30.83)	34.05	< 0.01
Gender	Male	4,014 (48.63)	2347 (58.47)	1667 (41.53)	34.05	< 0.01
Age group (years)	65–74	6,685 (80.99)	4,076 (60.97)	2069 (39.03)	350.22	< 0.01
	75–84	1,408 (17.06)	1,066 (75.71)	342 (24.29)
	≥ 85	161 (1.95)	138 (85.71)	23 (14.29)
Educational level	Illiterate	4,401 (53.32)	2,985 (67.83)	1416 (32.17)	27.15	< 0.01
	Primary school	1,808 (21.90)	893 (49.39)	915 (50.61)
	Middle school or above	2,045 (24.78)	1,402 (68.56)	643 (31.44)
Marital status	Married	6,642 (80.47)	4,083 (61.47)	2559 (38.53)	130.73	< 0.01
Marital status	Unmarried	1,612 (19.53)	1,197 (74.26)	415 (25.74)	130.73	< 0.01
Midday napping duration	0–30 min	3,785 (45.86)	2,506 (66.21)	1279 (33.79)	14.90	< 0.01
	31–90 min	3,210 (38.89)	1,903 (59.28)	1307 (40.72)
	> 90 min	1,259 (15.25)	871 (69.18)	388 (30.82)
Depressive symptoms	Yes	3,206 (38.84)	2,228 (69.49)	978 (30.51)	9.38	0.002
Depressive symptoms	No	5,048 (61.16)	3,052 (60.46)	1996 (39.54)	9.38	0.002
Economic situation (RMB)	< 5000	5,578 (67.58)	3,862 (69.24)	1716 (30.76)	60.24	< 0.01
	5000–10000	411 (4.98)	236 (57.42)	175 (42.58)
	> 10000	2,265 (27.44)	1,182 (52.19)	1083 (47.81)
Physical activity	Low intensity	1,690 (20.47)	1,192 (70.53)	498 (29.47)	35.87	< 0.01
	Medium intensity	5,575 (67.54)	3,437 (61.65)	2139 (38.35)
	High intensity	989 (11.98)	651 (65.82)	38 (34.18)

Status Of Depressive Symptoms, Midday Napping Duration, And Incidence Of Smi

Among the respondents, 5,280 (63.9%) had SMI. A total of 3,785 (45.86%) took short naps (0–30 min), while 3,210 took moderate-length naps and 1,259 (15.25%) took extended-length naps (> 90 min). A total of 3,206 (38.8%) participants had depressive symptoms. Univariate analysis detected significant differences in gender, age, education level, marital status, economic status, depressive symptoms, midday napping duration, and physical activity between the non-SMI and SMI groups (P < 0.05). Detailed results are shown in Table 1.

Affecting Factors Of Smi

Multi-factor logistic regression analysis showed that gender (odds ratio [OR] = 1.49), age (OR = 2.05, 3.71), education (OR = 0.62, 1.99), marital status (OR = 0.73), economic status (OR = 0.70, 0.52), depressive symptoms (OR = 1.29), midday napping duration (OR = 1.27, 1.52), and physical activity (OR = 0.82, 0.99) were significant factors affecting SMI in the elderly. More details can be found in Table 2.

Table 2

Logistic regression analysis of SMI in elderly
Variable	OR (95%CI)	S	Z
Age (years) (ref = 65–74 years)
75–84	2.03 (1.76,2.33)	0.14	9.91
≥ 85	3.70 (2.32,5.88)	0.87	5.54
Gender (ref = male)
Female	1.50 (1.36,1.66)	0.07	7.93
Marital status (ref = unmarried)
Married	0.73 (0.64,0.84)	0.04	−4.59
Educational level (ref = illiterate)
Primary school	0.62 (0.55,0.70)	0.03	−7.73
Middle school or above	2.03 (1.77,2.32)	0.13	10.40
Economic situation (ref = < 5000RMB/monthly)
5000–10000	0.70 (0.56,0.86)	0.07	−3.30
> 10000	0.52 (0.46,0.59)	0.03	−10.4
Physical activity (ref = low intensity) (ref = Low intensity)
Medium intensity	0.83 (0.73,0.94)	0.05	−2.93
High intensity	0.99 (0.83,1.19)	0.09	−0.04
Depressive symptoms (ref = no)
Yes	1.29 (1.17,1.44)	0.07	4.87
Midday napping duration (ref = 31-90min) duration30-90min)
Short nap (0–30 min)	1.27 (1.14,1.40)	0.06	4.60
Extended-length nap (> 90 min)	1.52 (1.31,1.76)	0.11	5.69

Multiplicative Interaction Effect Of Depressive Symptoms And Midday Napping Duration On Smi

A multiplicative model ^[35] was used to analyze the interaction effect of midday napping duration and depressive symptoms on SMI. After controlling the confounding factors, there was no multiplicative interaction effect between depressive symptoms and short nap or extended-length nap on the occurrence of SMI in the elderly (P > 0.05). Detailed results are shown in Table 3.

Table 3

Multiplicative interaction effect of midday napping duration and depressive symptoms on the incidence of SMI in the elderly
Variable	OR	S	Z	P	95% CI
Short nap (0–30 min)	1.26	0.08	3.60	< 0.01	1.11	1.43
Extended-length nap (> 90 min)	1.57	0.14	4.94	< 0.01	1.31	1.87
Depressive symptoms	1.30	0.10	3.31	< 0.01	1.11	1.53
Short nap * depressive symptoms	1.01	0.10	0.09	0.92	0.81	1.24
Extended-length nap * depressive symptoms	0.91	0.14	−0.54	0.58	0.67	1.24
Note: The model was adjusted for variables of gender, age, education, marital status, and economic status. Subjects without depressive symptoms and moderate-length naps was considered as the reference group.

Additive Interaction Effect Of Depressive Symptoms And Midday Napping Duration On Smi

An additive model was used to analyze the interaction effect of midday napping duration and depressive symptoms on SMI. The elderly people without depressive symptoms and moderate-length naps were considered as the reference group. The analysis detected the positive additive interaction effects between depressive symptoms and short naps or extended-length naps, respectively. However, the interaction effect between short naps and depressive symptoms was greater than that found between extended-length naps and depressive symptoms (RERI = 0.12, 95% CI = -0.14–0.39; AP = 0.13, 95% CI = -0.07–0.22,S = 1.20, 95% CI = 0.79–1.82 vs. RERI = 0.27, 95% CI = 0.07–0.43; AP = 0.14, 95% CI = 0.01–0.23,S = 1.06, 95% CI = 0.57–1.62). When extended-length naps and depressive symptoms coexisted in an elderly individual, the risk of SMI was 1.06 times higher than that associated with the presence of either extended-time naps or depressive symptoms alone (RERI = 0.27, 95% CI = 0.07–0.43; AP = 0.14 95% CI = 0.01–0.23,S = 1.06, 95% CI = 0.79–1.82). Also, when short naps and depressive symptoms coexisted, the risk of SMI was 1.2 times higher than that of either short naps or depressive symptoms alone (RERI = 0.12, 95% CI = − 0.14 to 0.39; AP = 0.13, 95% CI = − 0.07 to 0.22, S = 1.20, 95% CI = 0.57–1.62). The AP values were 0.12 and 0.14, respectively, which indicated that the interaction effect of depressive symptoms and short naps was responsible for the state of 12% of subjects with SMI in the total population, while the interaction effect of depressive symptoms and extended-length naps could be blamed for the state of 14% of subjects with SMI. Tables 4 and 5 provide detailed results.

Table 4

Additive interaction of different napping durations (short naps or extended-length naps), depressive symptoms, and SMI in elderly participants
Different napping duration		Depressive symptoms	OR	S	Z	P	95% CI
Short nap (0–30 min)	Yes	No	1.20	0.06	3.29	< 0.01	1.07	1.35
	No	Yes	1.35	0.07	5.27	< 0.01	1.21	1.52
	Yes	Yes	0.84	0.07	−1.99	0.04	0.71	0.99
Extended-length nap (> 90 min)	Yes	No	1.41	0.10	4.82	< 0.01	1.23	1.63
	No	Yes	1.40	0.08	5.62	< 0.01	1.24	1.57
	Yes	Yes	0.77	0.07	−2.5	< 0.01	0.63	0.94

Table 5

Estimated value of the additive interaction effect
	Short nap	Extended-length nap
RERI	0.12 (− 0.14–0.39)	0.27 (0.07–0.43)
AP	0.13 (− 0.07–0.22)	0.14 (0.01–0.23)
S	1.20 (0.79–1.82)	1.06 (0.57–1.62)

AP, attributable proportion due to interaction; CI, confidence interval; RERI, relative excess risk due to interaction; S, synergy index.

The findings of the present study indicated that the incidence of SMI among the enrolled elderly participants was 63.9%, which is significantly higher than the rate (22%) reported by a previous study that enrolled community-dwelling elderly people in Korea ^[37] and Tobiansky R's findings (25%). However, it was lower than the prevalence of SMI reported in another study conducting in registrants aged above 65 years old in two primary care services in south London (66.7%) ^[9].This variation may result from the difference in screening tools of SMI. Kim and associates adopted the Geriatric Mental State Scale to assess memory function, while Tobiansky and associated used a semi-structured interview called short-CARE. A single subjective question asking the elderly whether they felt memory decline and the MMSE were used to screen for SMI in the present study. However, the incidence of SMI in the present study was lower than that (70%) reported by another study ^[33], in which the respondents were from rural areas with lower education and economic status levels. The findings of this earlier study suggested that SMI is very common among elderly individuals. Overall, the various incidence rates of SMI in the literature may be attributed to the heterogeneity of respondents from different regions with specific social and cultural backgrounds and the difference in screening tools used. Community health care personnel should pay more attention to elderly individuals with SMI, adopt more sensitive screening tools for early screening, enhance the regularity of assessments to identify memory changes, and provide memory or cognitive training as early as possible to protect the cognitive function of the elderly.

The results of the current study revealed that elderly people with SMI tend to have certain demographic factors, including older age, female sex, an unmarried status, and a lower economic status (monthly income), which are consistent with the findings of previous studies ^{[7, 38, 39]}. This study also found that elderly individuals with a high level of education (secondary or above) were more likely to report SMI, while previous studies have suggested that high education is a protective factor of memory in the elderly ^[40–41]. Whereas there was also evidence that the risk of Alzheimer's disease associated with subjective memory complaints is higher among those with higher education than those with lower education ^[42]. Elderly individuals with greater education levels tend to access to obtain more health knowledge ^[43], who are more aware of their health problems and sensitive to memory impairments, and thus are more likely to provide external feedback on their memory impairment problems. Especially in highly educated individuals, who still perform well on formal cognitive tests, subjective memory impairment may be an important first sign of impending Alzheimer's disease ^[42]. Therefore, we need to pay attention to whether older people with high level of education have complaints or behaviors regarding complaints of memory problems, and further develop more accurate screen tools for early identification of SMI and avoid the under detection of SMI.

This study also found that midday napping duration is one of the important factors influencing the occurrence of SMI. Short naps (0–30 min) and extended-length naps (> 90 min) are greater risk factors for SMI compared to moderate-length naps (31–90 min). Previous studies have also found that moderate-length naps benefit memory and cognitive function in the elderly ^[44]. Blackwell and associates further indicated that the longer the midday napping duration is, the higher the risks of memory decline and cognitive impairment ^[17]. Extended-length naps affect the occurrence of SMI in the elderly by influencing the sleep/wake circadian rhythm and disrupting nighttime sleep, leading to impairments in memory and cognitive function ^[45]. Midday napping is a low-cost strategy that can easily be extended to a large population as a means of potentially protecting memory function. Community health care personnel should actively give professional advice to elderly people who do not nap or who take short or extended-length naps to help them establish the better habit of taking moderate-length naps.

We further found that depressive symptoms are another important factor influencing the occurrence of SMI in the elderly, which is supported by previous studies ^{[36, 46, 47, 44]}. For example, a cross-sectional study of community adults indicated that the occurrence of SMI was strongly associated with depressive symptoms ^[48]. Another study further revealed that higher levels of depression were independently associated with more severe memory impairment ^[49]. Several mechanisms can explain the associations between depressive symptoms and SMI. First, elderly people with depressive symptoms tend to evaluate their memory performance negatively and to amplify their memory impairment ^[50]. Second, patients with depression have higher levels of inflammatory cytokines, and elevated inflammatory markers are associated with poorer memory and cognitive function ^[51–52]. Most mechanistic studies have mostly focused on groups with MCI; however, SMI is a precursor state of MCI, and more studies focusing on the mechanism by which depressive symptoms relate to SMI are warranted.

It is worth emphasizing that short naps, extended-length naps, and depressive symptoms had positive additive interaction effects, respectively, on the risk of SMI in the elderly. Notably, the interaction effect between short naps and depressive symptoms was greater than that between extended-length naps and depressive symptoms. Our results were consistent with previous studies ^{[34, 45]}, which showed that extended-length naps and short naps are more detrimental than moderate-length naps to memory and cognitive function in elderly people. Wang and associates found that subjects who do not nap had higher risks for memory and cognitive impairments than those who take moderate-length naps ^[44]. More than 80% (3119, 82%) of subjects never take midday naps in the current study. It has been reported that insufficient napping duration is more likely to promote neuroinflammation and nerve destruction, particularly in the hippocampus, a key neuroanatomical region of memory, leading to neurodegenerative degeneration ^[53]. However, the mechanisms of the interaction effect between different napping duration and depressive symptoms on the occurrence of SMI in the elderly have not yet been determined. In the future, more empirical studies are needed to explore the dose–effect relationship between midday napping and SMI. Community health professionals could develop effective interventions to relieve depressive symptoms and help patients form regular midday napping habits; for example, in a previous study, a homemade gravity blanket was provided to depressive disorder co-morbid insomnia patients to influence adjustments to their nap time in order to reduce depressive symptoms ^[54].

This study also found that physical activity is an important factor influencing the occurrence of SMI among the elderly. In this study, elderly individuals who engage in medium- to high-level intensity physical activity tended to have better memories, which is supported by previous studies indicating that the amount of total physical activity is the main factor affecting the cognitive function of the elderly ^[55–57]. Another study further indicated that moderate or above physical activity for more than 90 min during the week slowed the rate of cognitive decline in older adults and contributed to the improvement of cognitive function ^[58]. The mechanism by which physical activity benefits memory and cognitive function can be explained according to physiological and psychological aspects. First, physical activity improves cerebral blood flow and cerebral perfusion and thus has a positive effect on brain memory function ^[59]. Second, people who engage in outdoor group physical activities tend to have more social links and social roles, which increase their sources of social support and reduce social isolation.

There are several improvements that can be made to this study. First, this investigation used CHARLS cross-sectional data, making it difficult to infer a cause-and-effect relationship between SMI, depressive symptoms, and midday napping duration. Future longitudinal studies could help to establish the temporal relationship between SMI and the main influencing factors. Second, the indicator of midday napping duration used in this study originated from the subjective judgments of the respondents, which may have been misreported and could lead to some information bias in the study. Future research could employ tools to record objective nap times. Finally, future studies should do cohorts to explore causal relationships between midday napping duration, depression and SMI.

Midday napping duration (Short naps and extended-length naps) and depressive symptoms had positive additive interaction effects on the occurrence of SMI in the elderly. The interaction effect between short naps and depressive symptoms was greater than that between extended-length naps and depressive symptoms. Community healthcare worker can reduce risk of SMI by helping the elderly to maintain healthier napping habits, and teaching the elderly and their families to detect and treat depressive symptoms effectively

Ethics approval and consent to participate

Our research was performed in accordance with the Declaration of Helsinki. The original CHARLS project was approved by the ethical review committee of Peking University (IRB00001052–11015), and all participants signed an informed consent form at the time of enrollment.

Consent for publication

Not applicable.

Availability of data and materials

The datasets generated and/or analysed during the current study are available in the [the China Health and Retirement Longitudinal Study (CHARLS)] repository, [http://charls.pku.edu.cn].

Competing interests

The authors declared no conflict of interest.

Funding

This study was supported by National Natural Science Foundation of China (72274078).

Authors' contributions

The work of substantial contributions to the conception or design of the work was done by LQ, XP. The work of drafting the work or revising it critically for important intellectual content and final approval of the version to be published was done by LQ, XP, TL, WY, and interpretation of data for the work was done by ZN ,LC, ZZ. All authors read and approved the final manuscript.

Acknowledgements

The data used in this study were from the survey organized by the China Health and Retirement Longitudinal Study (CHARLS).

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Interaction Effect of Midday Napping Duration and Depressive Symptoms on Subjective Memory Impairment Among Elderly People in China: Evidence from the China Health and Retirement Longitudinal Study Database

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Journal Publication

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Abstract

Background

Methods

Results

Limitations

Conclusions

Introduction

Methods

Design and Sample

Measurement Of Smi

Measurement Of Napping Duration

Measurement Of Depressive Symptoms

Measurement Of Physical Activity

Covariates

Statistical Analysis

Results

Participant Characteristics

Status Of Depressive Symptoms, Midday Napping Duration, And Incidence Of Smi

Affecting Factors Of Smi

Multiplicative Interaction Effect Of Depressive Symptoms And Midday Napping Duration On Smi

Additive Interaction Effect Of Depressive Symptoms And Midday Napping Duration On Smi

Discussion

Conclusion

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1