Participants
The China Longitudinal Study on Health and Retirement (CHARLS) is a nationally representative survey in the middle-aged and older population (≥45 year) , covering 150 county-level units distributed in 28 provinces in mainland China except Tibet. In this study , after excluding individuals younger than 45 years old and losing the necessary information, a total of 4957 participants (2529 males and 2428 females) from CHARLS wave 3 during 2015 to 2016 were included.
Arthritis and other health conditions
Participants were defined as arthritis and other health conditions based on self- report derived from the doctor's diagnosis. New Respondents in the third waves were asked “Have you been diagnosed with [conditions listed below, read one by one] by a doctor? The details are as follows:1.Hypertension;2. Dyslipidemia (elevation of low density lipoprotein, triglycerides (TGs),and total cholesterol, or a low high density lipoprotein level);3.Diabetes or high blood sugar;4.Cancer or malignant tumor (excluding minor skin cancers);5.Chronic lung diseases, such as chronic bronchitis , emphysema ( excluding tumors, or cancer);6.Liver disease (except fatty liver, tumors, and cancer);7.Heart attack, coronary heart disease, angina, congestive heart failure, or other heart problems;8.Stroke;9.Kidney disease (except for tumor or cancer);10.Stomach or other digestive disease (except for tumor or cancer);11.Emotional, nervous, or psychiatric problems;12.Memory-related disease;13.Arthritis or rheumatism;14.Asthma.If the answer is "yes", it is considered as patients with the disease diagnosed by the doctor. For remaining respondents since last waves reported that he/she had [conditions listed],asked “Our records from your last interview show that you have had/not had [conditions listed below], is this right?” If the answer of the last visit was correct, then asked the next chronic disease. If it was wrong, asked if they had this chronic disease since last interview. If the interviewees reported that they did not have [load chronic disease] at last visit and confirmed that the answer was correct, then they were asked whether they had this chronic disease since last wave. If the answer in the last interview was wrong, it indicated that the interviewees had suffered from this chronic disease in the last visit, and then asked the accesses (through routine or CHARLS physical examination, or any other?)Finally, based on the results of the above questionnaires, we included a total of 1719 patients (including 740 males and 979 females)with arthritis, including self-report of new and previous respondents with arthritis, and self-reported arthritis since last visit.
As hypertension, diabetes and coronary heart disease are the main risk factors of arthritis, we included these three diseases as covariates.
Exposure
The nighttime sleep duration item was set under the part of lifestyle and health behaviors in a standardized questionnaire. And the hours of actual sleep were considered instead of time in bed. According to the answers, they were divided into five groups (Group A:< 5h, Group B: 5-7h, Group C: 7-8h, Group D:8-9h and Group E: ≥ 9h), and the Group A (< 5h) was set as the reference group.
The sleep restless was evaluated through the self-report question “My sleep was restless” with four options: Rarely or none of the time (< 1 day);Some or a little of the time (1-2 days);Occasionally or a moderate amount of the time (3-4 days); Most or all of the time (5-7 days). According to the answers, they were divided into four groups: I Rarely or none, II Some or a little, III Occasionally and IV Most or all, and the group I (Rarely or none) was set as the reference group.
Participants were asked “Have you started menopause?” and “When did you begin the menopause?” If the respondents answered “yes” or the specific time of menopause, they were considered to be postmenopausal, based on the answer, 2068 postmenopausal women were included.
Other Covariables
The biological age was obtained by asking “What’s your actual date of birth?”. Then, people aged 45-60 years old were defined as middle-aged, and over 60 as old, and the middle-aged was set as the reference group. “The main city zone” was defined as urban, and other residential types, including “Combination zone between urban and rural areas, the town center, Zhenxiang area, special area, township Central, village” were defined as rural, and the urban was set as the reference group. Marital status was divided into three groups: Married with spouse present and Cohabitated; Married but not living with spouse temporarily for reasons such as work; Separated/Divorced/Widowed/Never married. “Married with spouse present and Cohabitated” was set as reference. A subsample of the participants was randomly selected to answer questions regarding physical activity. The questionnaire classified the levels of physical activity intensity as vigorous, moderate and walking, and then asked the duration they spent of each. High-intensity physical exercise is a risk factor for arthritis, and the prevalence increases for people with more than four continuous hours[33]. So only vigorous activity was taken into consideration, which required hard/high-intensity physical effort, such as heavy lifting, digging, ploughing, aerobics and fast bicycling, and cycling with a heavy load. BMI was calculated by dividing weight in kilograms by height in meters squared. BMI status were divided into following groups, normal weight range of 18.5–23.9 kg/m2,thin <18.5 kg/m2, overweight 24.0–27.9 kg/m2, obesity>28.0 kg/m2.The ever and current smoking was defined as smoking, never smoking was defined as non-smoking, the ever and current drinking was defined as drinking, never drinking was defined as non-drinking.
Blood samples were collected from each respondent by trained staff from the Chinese Center for Disease Control and Prevention (CDC), and respondents were asked to fast overnight. More detailed information regarding blood sample storage, transportation, and measurement has been published [31]. The protocol of the blood- based biomarker sample collection study was approved by the ethical review committee (IRB) of Peking University (IRB 00001052-11014). Written informed consent was obtained from all study participants.
Statistics
The frequency and percentage of samples were used to show the characteristics of the participants. T test was used for continuous variables and chi square test for categorical variables. To evaluate the association between sleep duration, restless and arthritis, a multivariate logistic model was fitted to estimate the adjusted odds ratio (OR) and its 95% CI. Model 1 only included sleep duration group (Group A,B,C,D,E) and age. Model 2 adjusted for model 1 and the variable sleep restless. Model 3 included additional adjustments including: residence, marital status, vigorous exercises, drinking status, smoking status, BMI status, hypertension, diabetes, heart problems, blood indexes ( including: Total cholesterol, Triglycerides, HDL cholesterol, LDL cholesterol, Glucose). Model 4 adjusted for model 3 and the variable sleep restless. Participants were stratified by sex ,postmenopausal status and sleep restless status. Considering the balance of sample size, the sleep restless stratification was based on participants with sleep restless (Group II: Some or a little, III Occasionally and IV Most or all) and without sleep restless (Group I: Rarely or none).
We did all statistical analyses using version 14 (Stata Corp LP, College Station, Texas, USA) ,plotted graphs using R version 3.5.3 and Graphpad prism 8. A p-value < 0.05 was defined as statistically significant.