Study design and population
This study was based on a cross-sectional data from the occupational population, which conducted among steelworkers at 11 steel production departments owned by the HBIS Group’s Tangsteel Company in Tangshan City, Hebei Province in north China. All workers at this company underwent a legally required health examination each year. A total of 7661 participants who underwent the annual required legally occupational health examinations were recruited from February to June 2017. There were 4084 workers who volunteered and completed carotid ultrasound examinations. After excluding 97 workers without complete items in the insomnia scale; 205 workers without sufficient shift work data, 200 workers without complete information on main covariates on the questionnaire, a total of 3582 participants were included for the final analysis. All participants gave informed consent before taking part in this study. The research was approved by the Ethics Committee of North China University of Science and Technology (No.16040).
Assessment of insomnia
The entire 8-item Athens Insomnia Scale (AIS) based on the International Classification of Diseases, Tenth Revision (ICD-10) diagnostic criteria of insomnia was employed as the insomnia assessment tool in this study . The first 5 items evaluate difficulty with sleep induction, awakening during the night, early-morning awakening, total sleep time, and overall quality of sleep; the last 3 items focus on sense of well-being, overall functioning, and sleepiness during daytime. Each item was scored on a 4-point Likert scale from 0 (no problem at all) to 3 (a very serious problem), with the total score ranging from 0 to 24. A score of ≥ 6, which being the widely accepted cut-off value for insomnia, classified the workers into the insomnia group; other workers were classified as the non-insomnia group.
Measurement of plaque in the carotid artery
Assessment of plaque from both left and right carotid artery systems was performed using a high-resolution B-mode topographic ultrasound system (PHILIPS, HD7, China) by two trained sonographers who were blinded to the research purpose and the study design. Participants were examined in the supine position with their head rotated in the opposite direction to the probe and with a lateral probe orientation. For this study, atherosclerotic plaques were defined as focal structures encroaching into the arterial lumen of at least 0.5 mm or 50% of the surrounding IMT value, or demonstrates a thickness > 1.5 mm as measured from the intima-lumen interface to the media-adventitia interface . When a local protrusion was defined as a plaque, its maximum thickness (mm) was measured using ultrasound calipers . The carotid plaque score indicates the severity of atherosclerosis, which is the sum of the cumulative maximum thickness of plaques obtained in the longitudinal sections of the common carotid artery, bifurcation, and internal carotid artery of the left and right carotid systems .
Assessment of covariates
Information on demographic characteristics, work lifestyle behavior, clinical characteristics were collected via face-to-face questionnaire survey, physical and biochemical examination. The duration of sleep was the weighted averages of sleep on working days and rest days, and divided into two groups according to 6 hours. Habitual snoring was defined as a self-report of snoring > 4 times per week. Workers were asked if they had taken any sleeping pills in the past month. Smoking status was divided into “never”, “ever” and “current”. Those who regularly consumed ≥1 cigarette/day over the past 12 months were defined as current smokers. Drinking status was divided into “never”, “ever” and “current”. Those who usually consumed some alcohol at least once a week over the past 12 months were defined as current drinkers. Considering that current lifestyle habits have a more significant effect on insomnia, and the proportion of past smokers, past drinkers and former shift workers were smaller, we divided these three variables into two categories according to "current" state. Dietary patterns were assessed based on the DASH (dietary approaches to stop hypertension, DASH) diet score . The calculation of metabolic equivalents was based on the International Physical Activity Questionnaire (IPAQ) . The workers with metabolic equivalent task (MET) [min/week] values < 600, 600-3000 and > 3000 were classified as having a low, moderate, and high level of physical activity respectively. Body mass index (BMI) was defined as body weight (kg) divided by the square of the body height (m2). The main work schedule of the present study population has been introduced in detail in our previous research . Shift status was classified as never/ever and current in this study. Hypertension was defined as current systolic blood pressure ≥ 140 mmHg, or diastolic blood pressure ≥ 90 mmHg, or if the patient was receiving antihypertensive therapy. Diabetes was defined as fasting blood glucose ≥ 7.0 mmol/L or if the patient was receiving hypoglycemic therapy. Total cholesterol (TC) ≥ 6.22 mmol/L or low-density lipoprotein (LDL-C) ≥ 4.11 mmol/L or high-density lipoprotein (HDL-C) ≤ 1.04 mmol/L or triglycerides (TG) ≥ 2.32 mmol/L, or patients undergoing lipid-lowering therapy were considered to demonstrate dyslipidemia.
Continuous variables are presented as means and standard deviations, and between-group comparisons were performed using Student’s t-test or analysis of variance (ANOVA) if the data were normally distributed. Otherwise, the median (upper quartile-lower quartile) and Wilcoxon rank sum test were used to describe and compare these continuous variables between groups. Categorical variables are presented as numbers and percentages, and the chi-square test was used to compare differences among groups. Associations between insomnia and carotid plaque were reported as odds ratios (OR) and corresponding 95% confidence intervals from multivariable adjusted logistic regression models. The risk factors and potential confounders were included in the analysis. We fit an age-adjusted model (model 1) and a multivariable (model 2) model additionally including sex and a final multivariable model (model 3) additionally adjusted for other confounders and known risk factors: marital status, educational level, BMI (categorical), smoking, drinking, DASH score, physical activity, shift work, sleep duration (categorical), sleep drug, snore, hypertension, diabetes, dyslipidemia. Subsequently, in subgroup analysis, we introduced multiplicative interaction terms using the insomnia and the stratifying factors including sex, BMI (< 25 kg/m2 or ≥ 25 kg/m2), smoking status (no/current), drinking status (no/current), snore (no/yes), shift work (no/current), sleep duration (< 6 hours or ≥ 6 hours), diabetes (no/yes), hypertension (no/yes), and dyslipidemia (no/yes) to assess potential effects modification. Log likelihood ratio test was used to compare models with and without cross-product interaction terms. A two-tailed p < 0.05 was considered statistically significant. All statistical analyses were performed using SAS V.9.4.