Study design and participants
The Kailuan study was a prospective cohort study involving 101,510 participants (men: 81,110; women: 20,400; ages: 18–98 years) in the Kailuan community from June 2006 to October 2007 (visit 1). The participants were employees (including the retired) of Kailuan (Group) Co. Ltd that represent all levels of the society, and they are employed as coalminers, administrators, secretaries, accountants, as well as the supportive and service staff, such as policemen, doctors, nurses, vendors, teachers. Of the 101,510 people who participated in the survey, 34,260 adults were included in the analysis after excluding the following participants: 62,200 subjects who had a history of dyslipidemia, 1436 subjects with missing information about baseline sleep duration or serum lipid profiles, and 3614 subjects without data for 2008-2009 (visit 1), 2010-2011 (visit 2), 2012-2013 (visit 3), and 2014-2015 (visit 4) follow-up visits (Figure 1). The health interview survey was performed using self-administered structured questionnaires to obtain information on sociodemographic characteristics, health status, and health behaviors. Before the study, all doctors and nurses received rigorous unified training.
Assessment of sleep duration
Sleep duration data were collected via self-reported answers to the question “How many hours of sleep have you had on an average at night in the preceding 3 months?” We divided sleep durations into five groups according to the responses: ≤5 h, 6 h, 7 h, 8 h, and ≥9 h. Additionally, participants were asked to answer “yes” or “no” to the question “Do you generally snore when you sleep?”[25, 26].
Assessment of potential covariates
The clinical examination consisted of a medical history, a physical examination, anthropometric measurements, and self-administered questionnaires on lifestyle characteristics[25, 27], such as sleep duration,
regular leisure-time physical activity, smoking habits, and daily
alcohol consumption. Physical activity was classified as ≥4 times per week and ≥20 minutes at a time (active physical activity), <80 minutes per week, or none. Smoking status and drinking status were classified as never, former, or current according to the self-reported information. The questionnaire information was double-entered. The body mass index (BMI) was calculated as the weight (kg) divided by the height squared (meters2). Blood pressure was measured three times using a standardized sphygmomanometer while the participant was in a seated position. The average of the three measurements was used in this study. Blood samples were drawn by trained phlebotomists from participants after an overnight fast for 8–12 h. Blood indicators, including TG, TC, LDL-C, HDL-C, fasting blood glucose (FBG), and high-sensitivity C-reactive protein (h-CRP) levels, were measured. TC and TG were measured enzymatically. HDL-C and LDL-C levels were measured using the direct test method. All blood samples were analyzed using a Hitachi 747 autoanalyzer (Hitachi; Tokyo, Japan). All data was directly uploaded to the oracle database. Diabetes was defined as having a history of diabetes, the use of glucose-lowering agents, or an FBG ≥7 mmol/l. Hypertension was defined as a systolic blood pressure ≥140 mmHg, a diastolic blood pressure ≥90 mmHg, a history of hypertension and/or the use of antihypertensive agents.
Diagnosis of each lipid profile abnormality
Each lipid profile abnormality was defined according to the Chinese guidelines on the prevention and treatment of dyslipidemia in adults (2016). Specifically, abnormal TC was defined as TC ≥5.2 mmol/L, abnormal TG were defined as TG ≥1.70 mmol/L, abnormal LDL-C was defined as LDL-C ≥3.4 mmol/L, and abnormal HDL-C was defined as HDL-C <1.0 mmol/L.
Continuous variables were described by their means ± standard deviations, and groups were compared using a one-way analysis of variance (ANOVA). Categorical variables were described as percentages and compared using the Chi-square test. The Cox proportional hazards model was used to estimate the HR for the incidence of each lipid profile abnormality in relation to sleep duration and baseline covariates. The follow-up time was calculated from the 2006 interview to the date when lipid profile abnormalities were detected, date of death, or date of the last attended interview in this analysis, whichever came first. For all models, the adequacy of the Cox proportional hazards model was assessed. Model 1 was adjusted for age and sex. Model 2 was further adjusted for the level of education, smoking, alcohol consumption, physical activity, and snoring. Model 3 was further adjusted for hypertension, diabetes mellitus, BMI, and h-CRP. Because transient changes in lipid profile (e.g., reversion from dyslipidemia to normal blood lipid) can impact the current results, the above analysis was repeated after excluding individuals with these conditions (Table 3, sensitivity analysis). To investigate whether age/sex acted as an effect modifier in the relationship between sleep duration and dyslipidemia, a regression model with an interaction term between age/sex and sleep duration was used. The statistical analysis was performed using SAS 9.4. All statistical tests were two-sided, and the significance level was set at 0.05.