Depression and relevant influencing factors among elderly adults with different sleep quality in pension institutions in Northeastern China

Objective This study aimed to assess depression among elderly adults with different sleep quality in pension institutions and to clarify its factors. Methods We conducted a cross-sectional study using a staged sampling method in six pension institutions located in Northeastern China from May to September 2017. Our study population comprised all elderly adults willing to participate in this study, who were not suffering from dementia and had lived in these pension institutions for six months or longer. All the participants were interviewed using a questionnaire that explored cognitive function, demographic characteristics, and factors influencing depression. The questionnaire also made use of the Pittsburgh Sleep Quality Index (PSQI) and Geriatric Depression Scale (GDS). After screening for cognitive function, we chose 507 elderly adults without cognitive defects as our participants. We used logistic regression to analyze the factors influencing depression among elderly adults with poor or good sleep quality.


Introduction
Population aging is becoming an increasingly serious issue globally, posing serious challenges to the maintenance and/or improvement of human health and socioeconomic development, particularly in China 1 . China has the largest aged population-it is the only country in the world with over 100 million elderly adults (aged 60 years and above) 2 . With aging population and changing family structures, traditional home-based care in China has gradually weakened, and an increasing number of elderly people are inclined to receive long-term care in pension institutions 1 . Elderly adults who live in pension institutions rather than in their own homes are unable to receive care from their adult children. Accordingly, they are more likely to suffer from mental disorders than other elderly adults.
Depression is one of the most important indicators of mental health. Among elderly adults, depression has become the second largest mental disorder after Alzheimer's disease 3 . It is also the only mental disorder that ranks among the top in terms of disease burden in both high and low income countries (high-income countries: rank 2; low and middle-income countries: rank 4) 4 . It has been reported that about 80% of elderly adults who commit suicide do so as a result of depression 4 . Therefore, it is imperative to analyze depression among elderly adults living in pension institutions.
Sleep is a physiological need of the human body. Numerous studies have shown that the quality of sleep among the elderly is closely related to mental disorders, such as depression and anxiety 5,6 . Elderly adults' sleep quality certainly affects their lives, work, and mental status to varying degrees, and as such affects their quality of life.
Therefore, it is necessary to conduct targeted research on the characteristics of the elderly population with different sleep quality. To date, there have been some studies on the relationship between sleep and depression among elderly adults.
However, most of them focus on the correlation between sleep and depression among elderly adults who are either hospitalized or reside in a community; there are few studies on elderly adults living in pension institutions 5 .
In addition, previous studies in China have relied on different methods of measuring depression 7,8 , making comparison of depression levels among elderly adults difficult. Moreover, research conclusions from developed countries cannot be directly applied to elderly adults in China because of differences in race, culture, income level, and lifestyle. Most previous studies also did not screen for cognitive impairment among elderly adults, leading to insufficient reduction of information bias and decreasing the generalization of previous conclusions 9 .
This study attempted to analyze depression and factors influencing depression among elderly adults with different sleep quality living in pension institutions in Northeastern China. We used the Geriatric Depression Scale (GDS), which is widely used in China and abroad, to investigate depression among elderly adults living in pension institutions. Furthermore, we comprehensively analyzed and compared the depression levels among elderly adults according to their sleep quality and examined their correlation with demographic information and health status. Through this study, we provide rational prevention and intervention measures for elderly adults with different sleep quality that were suffering from depression.

Ethics approval
The study protocol and informed consent form received ethics approval from the Committee on Human Experimentation at the China Medical University (Code: AF-SOP-07-1.1-01). Written informed consent concerning the conduct of the survey was obtained from each participant.

Study area and subjects
This cross-sectional study was performed in Northeastern China. A list of pension institutions in Northeastern China was taken from the website of the Liaoning Provincial Civil Affairs Bureau. A staged sampling method was carried out from May to September 2017. Six public pension institutions were selected as research objects: two in Shenyang, two in Anshan, one in Tieling, and one in Benxi. Our study population comprised all elderly willing to participate in this study, who were not suffering from dementia and had lived in these pension institutions for six months or longer. All the participants were interviewed using a questionnaire exploring cognitive function, demographic characteristics, and influencing factors. The questionnaire also made use of the Pittsburgh Sleep Quality Index (PSQI) and Geriatric Depression Scale (GDS). A total of 553 valid questionnaires were obtained, for an effective response rate of 84.04%. After screening for the cognitive function using the Mini-Mental State Examination (MMSE) 10 , 507 elderly adults without cognitive defects were chosen as participants.

Survey scales
The GDS 11 is a 30-item self-report assessment used to identify depression among the elderly. In the GDS, 20 questions are answered "yes" (scored 1) or "no" (scored 0), while the remain 10 questions are evaluated the opposite way (where "yes" is scored 0 and "no" is scored 1). The total score of the GDS ranges from 0 to 30, with a score of ≥ 11 indicating depression. The PSQI 12 is a self-report questionnaire that assesses sleep quality over a onemonth time interval. The measure scale consists of seven components, including subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbance, use of sleeping medication, and daytime dysfunction. The score for each component ranges from 0 to 3, where 0 indicates "no difficulty" and 3 indicates "very difficult." The total score of the PSQI ranges from 0 to 21, with a score of > 7 denoting poor sleep quality or having a sleep disorder.
The MMSE has been widely used to screen for cognitive defects among the elderly 10 . The total score of the MMSE ranges from 0 to 30. Combined with the educational level of the elderly, the diagnostic criteria for cognitive impairment among the elderly are as follows: illiteracy is ≤ 17 points, primary school is ≤ 20 points, secondary school (including technical secondary school) is ≤ 22 points, and university is ≤ 23 points.
We also used a self-designed questionnaire for baseline information including age, sex, marital status, pension, economic status, chronic disease, self-care, body aches, chewing ability, ability to go out alone, exercise, regular diet, smoking, filial piety, and loneliness.

Quality control
All the surveyors were trained uniformly. While conducting the investigation, they had to ensure that the respondents could grasp and understand the meaning of each question, avoid inducement of any bias as far as possible, and ensure the reliability of the information collected.

Statistical analysis
Items for which over 95% of participants had the same answers were excluded from the data analysis; these items included race ("Han" accounted for 96.8%) and medical insurance ("Have" accounted for 98.9%).
In addition, the proportion of missing items was < 5% among elderly adults with poor or good sleep quality. No missing items were processed. We performed a statistical analysis of the data using SPSS Statistics 20.0. We conducted a univariate analysis using the chi-square test, and a multivariate analysis using binary logistic regression analysis. A P < 0.05 was considered statistically significant.
The overall prevalence of depression was 21.7%; the ratios among elderly adults with good and poor sleep quality were 12.3% and 31.1%, respectively. The results of the chi-square test showed a significant difference in the depression rate between the two groups of elderly adults (P < 0.05).

Univariate analysis of factors influencing depression among the elderly living in pension institutions
The results of the univariate analysis of factors influencing depression among elderly adults living in pension institutions are reported in Table 1. Marital status, pension, economic status, chronic illness, self-care, body aches, chewing ability, ability to go out alone, regular exercise, and regular diet were significantly related to the prevalence of depression among all elderly adults (P < 0.05). For elderly adults with good sleep quality, marital status, pension, economic status, chronic illness, self-care, body aches, chewing ability, ability to go out alone, regular exercise, eating patterns, smoking, and filial piety were significantly related to depression (P < 0.05). As for elderly adults with poor sleep quality, marital status, pension, chronic illness, self-care, chewing ability, ability to go out alone, regular exercise, diet, and loneliness were related to the detection rate of depression (P < 0.05).

Multivariate analysis of the interaction between factors influencing depression and sleep quality among elderly adults living in pension institutions
Considering the significant difference in prevalence of depression among elderly adults with good and poor sleep quality, we wanted to examine the interaction between sleep quality and the factors influencing depression among elderly adults living in the pension institutions. As seen in Table 3, after adjustment for age, regular exercise and child filial piety significantly interacted with sleep quality to influence depression (P < 0.05). Therefore, we went forward with analyzing the specific factors influencing depression among elderly adults according to their sleep quality (good or poor).

Multivariate analysis of risk factors of depression according to sleep quality among elderly adults living in pension institutions
The results of the multivariate analysis of risk factors for depression among elderly adults with different sleep quality living in pension institutions are reported in Table 4. After adjusting for age as a fixed factor in the model, the logistic regression analysis revealed that the factors influencing depression among elderly adults with good sleep quality were ranked (in descending order of effect size) as follows: marital status, chronic disease, regular exercise, body aches, child filial piety, and chewing ability (P < 0.05). By contrast, the factors influencing depression among elderly adults with poor sleep quality were ranked as follows: loneliness, regular diet, chewing ability, and chronic disease (P < 0.05; Table 4).

Discussion
This study conducted a cognitive function screening for elderly adults living in pension institutions, and adopted the highly valid and reliable GDS as the main assessment tool for evaluating depression and its influencing factors among elderly adults without cognitive dysfunction. The findings of this study can be characterized as a good representation of the population (and thus bearing high information reliability). The conclusions drawn also have strong generalizability. The results of this study showed that the depression rate among elderly adults without cognitive dysfunction living in pension institutions in Northeastern China was 21.7%, which is much lower than that in Beijing 13 (n = 107, 32.71%) and Guangdong Province 14 (n = 379, 38.5%). The depression rate among elderly adults with poor sleep quality was 31.1%, which was significantly higher than that among elderly adults with good sleep quality (12.3%). The results were similar to those pertaining to elderly adults living in pension institutions in Beijing 13 .
Elderly adults showed significant differences in depression, marital status, economic status, chronic disease, self-care, chewing ability, ability to go out alone, regular exercise, regular diet, filial piety, and loneliness according to sleep quality.
Similarly, Dai and Li et al. 5 Marital status is an important risk factor for depression among elderly adults with good sleep quality. Spousal support and care can reduce the incidence of depression: indeed, the incidence of depression in widowed elderly adults was significantly higher than that among married elderly adults 15 . Divorced elderly adults or those in poor marriages are also more likely to suffer from mental health problems and increased depression 16 . The elderly population is a high-risk group in terms of the incidence of various chronic diseases, which harm the physical and mental health of elderly adults and impact their normal lives and social

Ethics approval and consent to participate
We strictly followed the ethical principles.

Consent for publish
Not applicable

Availability of data and materials
All data generated or analysed during this study are included in this published article and its supplementary information files.