Japanese Working Population Research Articles

Relationship of airflow limitation severity with work productivity reduction and sick leave in a Japanese working population.

BackgroundThe aim of this study was to reveal the association between airflow limitation (AL) severity and reduction with work productivity as well as use of sick leave among Japanese workers.MethodsThis cross-sectional study included 1,378 workers who underwent a lung function test during a health checkup at the Japanese Red Cross Kumamoto Health Care Center. AL was defined as forced expiratory volume in 1 second/forced vital capacity of <0.7. Workers completed a questionnaire on productivity loss at work and sick leave. The quality and quantity of productivity loss at work were measured on a ten-point scale indicating how much work was actually performed on the previous workday. Participants were asked how many days in the past 12 months they were unable to work because of health problems. Logistic regression analysis was used to assess the associations between AL severity and the quality and quantity of productivity loss at work as well as use of sick leave.ResultsCompared with workers without AL, workers with moderate-to-severe AL showed a significant productivity loss (quality: odds ratio [OR] =2.04, 95% confidence interval [CI]: 1.12–3.71, P=0.02 and quantity: OR =2.19, 95% CI: 1.20–4.00, P=0.011) and use of sick leave (OR =2.69, 95% CI: 1.33–5.44, P=0.006) after adjusting for sex, age, body mass index, smoking status, hypertension, hyperglycemia, dyslipidemia, sleep duration, work hours per day, and workplace smoking environment.ConclusionAL severity was significantly associated with work productivity loss and use of sick leave. Our findings suggested that early intervention in the subjects with AL at the workforce might be beneficial for promoting work ability.

Strength training and risk of type 2 diabetes in a Japanese working population: A cohort study

Aims/IntroductionMuscle strength training has been suggested to improve glucose metabolism; however, epidemiological evidence regarding strength training's effects on diabetes risk is scarce. We prospectively examined the association between strength training and the risk of type 2 diabetes in Japanese men and women.Materials and MethodsThe sample included health checkups on 26,630 Japanese male and female workers aged 30–64 years without diabetes at baseline. Weekly time spent on strength training was elicited using a self-reported questionnaire. Type 2 diabetes was diagnosed based on hemoglobin A1c, fasting glucose, random plasma glucose and self-report in an annual health checkup. Hazard ratio (HR) and its 95% confidence interval (CI) for incident diabetes was estimated using a Cox proportional hazards model.ResultsDuring a mean follow up of 5.2 years with 139,748 person-years, 1,770 individuals developed diabetes. Age- and sex-adjusted HR for diabetes was 0.58 (95% CI 0.42–0.79) in those who engaged in strength training compared with those who engaged in no strength training. After further adjusting for potential confounders, the corresponding HR was 0.66 (95% CI 0.48–0.90). Additional adjustment for body mass index did not materially change the result; the HR was 0.70 (95% CI 0.51–0.96). The association was more pronounced in individuals aged 50 years or older than those aged <50 years, although the difference in the association by age was not significant.ConclusionsThese results suggest that engagement in strength training could help to reduce the risk of type 2 diabetes in a Japanese working population.

Mortality benefit of participation in BOOCS program: a follow-up study for 15 years in a Japanese working population.

Objective:This study aims to demonstrate the protective effect on mortality among participants of a health education program, Brain-Oriented Obesity Control System (BOOCS).Methods:A quasi-experimentally designed, 15-year (1993 to 2007) follow-up study was conducted with a total of 13,835 male and 7791 female Japanese workers. They were divided into three groups: participants in the program (1565 males and 742 females), nonparticipant comparative obese controls (1230 males and 605 females), and nonparticipant reference subjects (11,012 males and 6426 females). Hazard ratios were calculated with survival curves drawn to evaluate the mortality effects by the program participation.Results:The male participants showed significantly lower mortality risk for all causes of death at hazard ratio = 0.54 (95% confidence interval: 0.31 to 0.94) with significantly different survival curves (P = 0.014 by log-rank test) than obese controls.Conclusions:The results support a protective effect on mortality by participating in BOOCS program.

Dietary patterns and sleep symptoms in Japanese workers: the Furukawa Nutrition and Health Study

ObjectiveExperimental studies have shown that some nutrients are involved in initiating and maintaining sleep, but epidemiological evidence on overall dietary patterns and insomnia is scarce. We investigated the relationship between dietary patterns and sleep symptoms in a Japanese working population. MethodsThe participants were 2025 workers, aged 18–70 years, who participated in a health survey during a periodic checkup in 2012 and 2013. Dietary intake was assessed with a self-administered diet history questionnaire. Dietary patterns were extracted by principal component analysis on the basis of the energy-adjusted intake of 52 food and beverage items. Sleep duration, difficulty initiating and maintaining sleep, and poor quality of sleep were self-reported. Logistic regression was used to estimate the odds ratios of each sleep symptom according to quartile categories of each dietary pattern with adjustment for potential confounding variables. ResultsWe identified three major dietary patterns. A healthy pattern, characterized by a high intake of vegetables, mushrooms, potatoes, seaweeds, soy products, and eggs, was associated with a decreased prevalence of difficulty initiating sleep once or more a week (P for trend = 0.03); the multivariate adjusted odds ratio in the highest quartile of this score compared with the lowest was 0.75 (95% CI: 0.57–0.99). This association persisted after the exclusion of individuals with severe depressive symptoms. However, there was no significant association with difficulty initiating sleep at least three times a week. ConclusionsOur findings suggest that a healthy dietary pattern may be associated with difficulty initiating sleep at least once a week.

Association of overtime work and hypertension in a Japanese working population: A cross-sectional study

Long working hours have been associated with an increased risk of cardiovascular disease, but its relationship with hypertension remains unclear. The objective of this study is to examine the relationship between overtime and presence of hypertension using data from a large-scale multi-company study in Japan. Participants were 52 365 workers of four companies that provided both health-checkup data and self-reported data on overtime worked. Hypertension was defined as systolic blood pressure ≥140 mmHg, diastolic blood pressure ≥90 mmHg, and/or the use of antihypertensive drug. Logistic regression analysis was performed to determine the odds ratio for hypertension for each category of overtime work (<45, 45–79, 80–99 or ≥100 h/month) with adjustments for age, sex, company, smoking status and body mass index. The prevalence of hypertension tended to decrease with increasing overtime work: 17.5, 12.0, 11.1 and 9.1% for the shortest (<45 h/month) through the longest overtime category (≥100 h/month). The age-, sex- and company-adjusted odds ratios (95% confidence interval) were 1.00 (reference), 0.81 (0.75–0.86), 0.73 (0.62–0.86), 0.58 (0.44–0.76), respectively (p for linear trend <0.001). In a sub-cohort, the inverse association remained statistically significant after an additional adjustment for other potential confounders. Results of the present large-scale study among Japanese workers suggest an inverse association between overtime work and presence of hypertension.

Association of serum leptin and ghrelin with depressive symptoms in a Japanese working population: a cross-sectional study.

BackgroundLeptin and ghrelin have been implicated in the pathogenesis of major depression. However, evidence is lacking among apparently healthy people. This study examined the relationship of these appetite hormones to depressive symptoms in a Japanese working population.MethodsA cross-sectional study was conducted in 2009 among 497 Japanese employees (287 men and 210 women) aged 20–68 years. Fasting serum leptin and ghrelin levels were measured using a Luminex suspension bead-based multiplexed array. Depressive symptoms were assessed using the Center for Epidemiologic Studies Depression (CES-D) scale. Logistic regression analysis was performed to estimate odds ratio (OR) and 95% confidence interval (CI) for depressive symptoms with adjustment for potential confounders.ResultsThe prevalence of depressive symptoms (CES-D ≥16) was 26.5% and 33.3% among men and women, respectively. Women in the middle and highest tertiles of leptin levels showed lower odds for depressive symptoms compared with those in the lowest level, although the trend association was not statistically significant (Ptrend = 0.14). Higher ghrelin levels were associated with increased odds for depressive symptoms in women (Ptrend = 0.02). The multivariable adjusted OR (95% CI) of having depressive symptoms for the lowest through highest tertiles of ghrelin levels were 1.00 (reference), 1.71 (0.76 - 3.86), and 2.69 (1.16 - 6.28), respectively. Neither leptin nor ghrelin was associated with depressive symptoms in men.ConclusionsResults suggest that lower leptin and higher ghrelin levels may be related to higher prevalence of depressive status among Japanese women.

High dietary acid load is associated with increased prevalence of hypertension: The Furukawa Nutrition and Health Study

ObjectiveAcid–base status has been suggested to influence blood pressure, but there is a paucity of epidemiologic evidence linking dietary acid load to hypertension. We examined cross-sectionally the association between dietary acid load and hypertension in a Japanese working population. MethodsData were derived from health surveys from 2028 employees, ages 18 to 70 y, in two workplaces in Japan. A validated brief diet history questionnaire was used to assess diet. Two measures were used to characterize dietary acid load: potential renal acid load and estimated net endogenous acid production, which were derived from nutrient intakes. Multilevel logistic regression was used to examine the association between dietary acid load and hypertension with adjustment of potential confounding variables. ResultsHigh dietary acid load was suggestively associated with increased prevalence of hypertension. The multivariable adjusted odds ratios (95% confidence interval) of hypertension for the lowest through highest tertiles of net endogenous acid production were 1.00 (reference), 1.07 (0.80–1.42), and 1.33 (0.998–1.78), respectively (P for trend = 0.053). This positive association was statistically significant among normal-weight (body mass index <23 kg/m2; P for trend = 0.03) and non-shift workers (P for trend = 0.04). Similar positive associations were observed between potential renal acid load and hypertension. ConclusionsThe present findings suggest that high dietary acid load may be associated with increased prevalence of hypertension among those who were normal weight and non-shift workers.

Coffee and green tea consumption is associated with insulin resistance in Japanese adults

ObjectiveHigher coffee and green tea consumption has been suggested to decrease risk of type 2 diabetes, but their roles in insulin resistance (IR) and insulin secretion remain unclear. This study examined the association between habitual consumption of these beverages and markers of glucose metabolism in a Japanese working population. Materials/MethodsParticipants were 1440 Japanese employees (1151 men and 289 women) aged 18–69years. Consumption of coffee and green tea was ascertained via a validated brief diet history questionnaire. Multilevel linear regression was used to estimate means (95% confidence intervals) of fasting insulin, fasting plasma glucose, homeostatic model assessment of IR (HOMA-IR), homeostatic model assessment of β-cell function (HOMA-β) and glycated hemoglobin (HbA1c) with adjustment for potential confounding variables. ResultsCoffee consumption was significantly, inversely associated with HOMA-IR (P for trend=0.03), and the association appeared to be confined to overweight subjects (BMI≥25kg/m2) (P for trend=0.01, P for interaction=0.08). Unexpectedly, green tea consumption was positively associated with HOMA-IR (P for trend=0.02), though there was no dose–response relationship among daily consumers of green tea. Neither coffee nor green tea consumption was associated with HOMA-β and HbA1c. ConclusionsOur findings indicate that coffee consumption may be associated with decreased IR, but not with insulin secretion. The positive association between green tea consumption and IR warrants further investigation.

Knowledge of HBV and HCV and Individuals’ Attitudes Toward HBV- and HCV-Infected Colleagues: A National Cross-Sectional Study among a Working Population in Japan

Prejudice and discrimination in the workplace regarding the risk of transmission of Hepatitis B virus (HBV) and Hepatitis C virus (HCV) are increased by excess concerns due to a lack of relevant knowledge. Education to increase knowledge about HBV and HCV and their prevention could be the first step to reduce prejudice and discrimination. This study aimed to determine the association between the level of knowledge and negative attitudes toward HBV- and HCV-infected colleagues among the Japanese working population. An online anonymous nationwide survey involving about 3,000 individuals was conducted in Japan. The questionnaire consisted of knowledge of HBV and HCV, and attitudes toward HBV- and HCV-infected colleagues in the workplace. Knowledge was divided into three categories: “ensuring daily activities not to be infected”; “risk of infection”; and “characteristics of HBV/HCV hepatitis”, based on the result of factor analysis. Multiple logistic regression analysis was applied. A total of 3,129 persons responded to the survey: 36.0% reported they worried about the possibility of transmission of HBV and HCV from infected colleagues; 32.1% avoided contact with infected colleagues; and 23.7% had prejudiced opinions about HBV and HCV infection. The participants were classified into tertiles. A higher level of knowledge of HBV and HCV was significantly associated with these three negative attitudes (P for trend < 0.005). This study suggests that increasing knowledge may decrease individuals’ negative attitudes towards HBV- and HCV-infected colleagues. Thus, we should promote increased knowledge of HBV and HCV in stages to reduce negative attitudes toward HBV- and HCV-infected colleagues.

Mortality risks in relation to occupational category and position among the Japanese working population: the Jichi Medical School (JMS) cohort study

ObjectivesA well-known demographic finding in modern society is the inverse association between socioeconomic status and mortality. The purpose of the study was to examine socioeconomic indicators, such as occupational category...

Association of Knowledge of HIV and Other Factors with Individuals’ Attitudes toward HIV Infection: A National Cross-Sectional Survey among the Japanese Non-Medical Working Population

BackgroundThe stigma of and discrimination because of HIV has been described as the most important obstacle to prevention and treatment efforts. The purpose of this study was to investigate negative attitudes and prejudice toward HIV among the Japanese non-medical working population and to explore contributing factors.MethodsAn online anonymous nationwide survey involving approximately 3,000 individuals was conducted in Japan. Questions ranged from background information and HIV knowledge to individuals’ attitudes towards HIV infection in the workplace. Descriptive statistics and logistic regression were applied for analysis.ResultsThirty-three percent of participants feared transmission of HIV from infected colleagues, 34% tended to avoid contact with them and 40% had prejudiced opinions about HIV infection. Despite a relatively high level of knowledge of HIV/AIDS overall (11.9±3.3 from 15 points), only 50% of individuals were aware of some issues. Greater knowledge was associated with less negative attitudes towards HIV infection (OR 0.39, 95% CI 0.31–0.48 for prejudiced opinion, high compared with low level of knowledge), whereas greater health consciousness was inversely related to attitude (OR 1.97, 95% CI 1.50–2.58 for prejudiced opinion, high compared with low health consciousness).ConclusionKnowledge neutralizes peoples’ negative attitudes towards HIV infection, whereas greater health consciousness may worsen them. Educational programs should balance knowledge with health consciousness to improve the efficacy of HIV interventions.

Green tea and coffee consumption is inversely associated with depressive symptoms in a Japanese working population – CORRIGENDUM

Green tea and coffee consumption is inversely associated with depressive symptoms in a Japanese working population – CORRIGENDUM

Green tea and coffee consumption is inversely associated with depressive symptoms in a Japanese working population.

To examine the association between the consumption of green tea, coffee and caffeine and depressive symptoms. Cross-sectional study. Consumption of green tea and coffee was ascertained with a validated dietary questionnaire and the amount of caffeine intake was estimated from these beverages. Depressive symptoms were measured using the Center for Epidemiologic Studies Depression Scale. Multiple logistic regression analysis was performed to compute odds ratios and 95% confidence intervals for depressive symptoms with adjustments for potential confounders. Two workplaces in north-eastern Kyushu, Japan, in 2009. A total of 537 men and women aged 20-68 years. Higher green tea consumption was associated with a lower prevalence of depressive symptoms. Compared with participants consuming ≤1 cup/d, those consuming ≥4 cups green tea/d had a 51% significantly lower prevalence odds of having depressive symptoms after adjustment for potential confounders, with significant trend association (P for trend = 0·01). Further adjustment for serum folate slightly attenuated the association. Coffee consumption was also inversely associated with depressive symptoms (≥2 cups/d v. <1 cup/d: OR = 0·61; 95% CI 0·38, 0·98). Multiple-adjusted odds for depressive symptoms comparing the highest with the lowest quartile of caffeine consumption was OR = 0·57 (95% CI 0·30, 1·05; P for trend = 0·02). Results suggest that higher consumption of green tea, coffee and caffeine may confer protection against depression.

Dairy consumption is associated with decreased insulin resistance among the Japanese

Beneficial effects of dairy consumption in preventing obesity and metabolic syndrome have been suggested, but the association between dairy intake and insulin resistance (IR) is not clear. To test the hypothesis that higher dairy consumption is associated with lower insulin resistance, the authors investigated the association between dairy intake (taking fat content into consideration) and markers of IR in a Japanese working population. A cross-sectional study was conducted in 2009 among 496 subjects aged 20–68 years who participated in a health survey during a periodic checkup. Dairy consumption was assessed using a validated brief dietary history questionnaire. Multiple regression analysis was used to assess the association between fasting serum insulin, plasma glucose, and the homeostatic model assessment of IR (HOMA-IR) and dairy consumption with adjustment for potential confounding variables. The subjects in the highest quartile for total dairy intake showed the lowest mean IR markers compared to those in other categories, although the difference was not statistically significant. Intake of full-fat dairy products was inversely associated with HOMA-IR (P for trend = 0.02). The multivariable adjusted mean HOMA-IR values (with 95% confidence intervals) for the four groups, from those who consumed the fewest to those who consumed the most servings of full-fat dairy products, were 1.04 (0.96-1.12), 1.04 (0.96-1.13), 1.00 (0.91-1.08), and 0.86 (0.76-0.96), respectively. Low-fat dairy intake was not significantly associated with any IR markers. The results suggest that the consumption of full-fat dairy products may be associated with lower IR among Japanese adults.

Occupational factors and problem drinking among a Japanese working population.

Problem drinking is a serious public health problem in the workplace. However, few Japanese epidemiological studies have investigated the occupational characteristics of problem drinking. The purpose of this study is to clarify the occupational risk factors for problem drinking among a Japanese working population. We used data from a random-sampling survey about mental health and suicide, conducted among Hamamatsu City residents aged 15 to 79 yr old during May and June in 2008. The relation between occupational factors and problem drinking was analyzed with multiple logistic regression models stratified by gender. CAGE questionnaire was used to assess problem drinking. With regard to employment types, problem drinkers were more prevalent among self-employed women. With regard to occupational types, clerical and service professions had more problem drinkers of either sex, while administrative/managerial and sales professions had more women with such problem. With regard to company size, male problem drinkers were more prevalent in smaller companies than in larger ones. These results indicate that the prevalence of problem drinkers in the workplace depends on where one works. It is necessary to consider these characteristics to provide effective measures to address problem drinking in the workplace.

Dietary patterns and metabolic syndrome in a Japanese working population

BackgroundMetabolic syndrome has become a major public health concern, but the role of diet in the etiology of this syndrome is not well understood. This study investigated the association between major dietary patterns and prevalence of metabolic syndrome in a Japanese working population.MethodsThis cross-sectional study was conducted among 460 municipal employees (284 men and 176 women), aged 21–67 years, who participated in a health survey at the time of periodic checkup. Dietary patterns were derived by using the principal component analysis of the consumption of 52 food and beverage items, which were assessed by a validated brief diet history questionnaire. Metabolic syndrome was defined according to the modified NCEP-ATP III criteria. Logistic regression was used to examine the association between dietary patterns and metabolic syndrome with adjustment of potential confounding variables.ResultsThree dietary patterns were identified. Westernized breakfast pattern characterized by high intakes of bread, confectionaries, and milk and yogurt but low intakes of rice and alcoholic beverages was inversely associated with prevalence of metabolic syndrome and high blood pressure (P for trend = 0.02 and 0.049, respectively). Animal food pattern characterized by high intakes of fish and shellfish, meat, processed meat, mayonnaise, and egg was not associated with prevalence of metabolic syndrome, but was positively associated with high blood glucose (P for trend = 0.03). Healthy Japanese dietary pattern characterized by vegetables and fruits, soy products, mushrooms, and green tea was not appreciably associated with prevalence of metabolic syndrome or its components.ConclusionsThe results suggest that westernized breakfast pattern may confer some protection against metabolic syndrome in Japanese. The causality of these associations needs to be confirmed.

Serum ferritin is associated with markers of insulin resistance in Japanese men but not in women

ObjectiveSeveral epidemiological studies have reported that high concentrations of circulating ferritin, a marker of iron stores, are related to insulin resistance (IR); however, questions remain regarding inconsistent data between Asian men and women and the inadequate consideration of potential confounding effects on the relationship between ferritin and IR. Our aim was to examine the relationship between serum ferritin concentrations and IR markers in the Japanese population. Materials/MethodsWe analyzed data (n=493) from a cross-sectional survey conducted in 2009 among a Japanese working population aged 20–68years. Fasting serum ferritin and insulin levels and fasting plasma glucose levels were determined, and the homeostatic model assessment of IR (HOMA-IR) was calculated. Multiple regression analysis was performed with adjustments for demographic and lifestyle factors, body mass index and serum C-reactive protein. ResultsFasting insulin and HOMA-IR significantly increased with increasing levels of serum ferritin after adjustment for covariates in men (P for trend=.005 and .001, respectively). Compared with men in the lowest tertile of serum ferritin, those in the highest tertile had a 24% higher HOMA-IR score. Additional data suggested a positive association between iron intake and HOMA-IR (P for trend=.07) in men. Neither serum ferritin nor iron intake was related to IR markers in women, even in postmenopausal women. ConclusionsSerum ferritin concentrations were positively associated with fasting insulin and HOMA-IR in men but not in women, suggesting an important role of iron storage in the pathogenesis of IR in Japanese men.

Serum 25-hydroxyvitamin D and markers of insulin resistance in a Japanese working population

The association between vitamin D status and insulin resistance (IR) has been less studied among Asians, and it remains elusive whether calcium could modify such an association. We examined the association of serum 25-hydroxyvitamin D (25(OH)D) concentrations with IR markers and the potential effect modification by calcium intake among a Japanese population. The authors analyzed data (n=494) from a cross-sectional survey conducted in 2009 among a Japanese working population aged 20-68 years. Fasting serum 25(OH)D and insulin, as well as fasting plasma glucose were determined, and homeostatic model assessment of IR (HOMA-IR) was calculated. Calcium intake was assessed using a validated diet history questionnaire. Multiple linear regression was done with adjustment of potential confounding variables. Fasting insulin and HOMA-IR were significantly inversely associated with 25(OH)D concentration across quartiles of 25(OH)D after fully adjusting for covariates (P(trend)=0.04 and 0.02, respectively). Across clinically relevant categories of 25(OH)D, compared with participants in the vitamin D sufficiency group, those in the vitamin D insufficiency group had a 5% higher HOMA-IR score, and those in the hypovitaminosis D group had an 18% higher HOMA-IR score (P(trend)=0.01). In an analysis by calcium intake, the HOMA-IR score was highest among participants with both a low calcium intake and lowest 25(OH)D concentrations, with significant inverse trend being observed in the group with lower calcium intake (P(trend)=0.02). Our findings suggest that low vitamin D status is associated with IR among Japanese adults.

Dietary patterns and C-peptide concentrations in a Japanese working population

ObjectiveIt remains unsettled whether dietary patterns play a role in insulin resistance. We assessed the association of major dietary patterns with C-peptide concentrations in a Japanese working population. MethodsA cross-sectional study was conducted in 456 municipal employees (270 men and 186 women) 21 to 67 y old who participated in a health survey at the time of their periodic checkup. The dietary patterns were derived by using the principal component analysis of the consumption of 52 food and beverage items, which was assessed by a validated brief dietary history questionnaire. Multiple regression analysis was used to estimate the means of C-peptide concentrations across tertiles of each dietary pattern score with the adjustment of potential confounders, including age, body mass index, physical activity, smoking, alcohol drinking, and energy intake. ResultsWe identified three dietary patterns: healthy, animal food, and Westernized breakfast patterns. The Westernized breakfast pattern was characterized by high intakes of bread, confectionaries, and milk and yogurt but low intakes of rice and alcohol and was inversely associated with C-peptide concentrations in women but not in men. The multivariable-adjusted means of C-peptide concentrations were 1.03 ng/mL (95% confidence interval 0.95–1.12), 0.95 ng/mL (95% confidence interval 0.88–1.03), and 0.89 ng/mL (95% confidence interval 0.82–0.97) for the lowest through the highest tertiles of the Westernized breakfast pattern score (P for trend = 0.015) in women. Other dietary patterns were not appreciably associated with C-peptide concentrations. In a subgroup, similar associations were observed between dietary patterns and the homeostasis model assessment of insulin resistance. ConclusionThe Westernized breakfast pattern may be associated with a lower insulin resistance in Japanese women.

Effect of the Interaction Between Employment Level and Psychosocial Work Environment on Insomnia in Male Japanese Public Service Workers

Little is known about the combined risk of socioeconomic status and psychosocial environment for insomnia. The purpose of this study was to investigate the combined risk of employment level and psychosocial work environment for insomnia in a large Japanese male working population. We investigated 5,951 male employees aged 34-59 years from two local governments in Japan. Data were obtained from a questionnaire distributed in advance of their annual health check-up. Employment level was categorized as higher-level non-manual workers, lower-level non-manual workers, and manual workers. Psychosocial work environment was assessed using the Demand-Control Model and the Effort-Reward Imbalance Model (ERI). Insomnia was assessed using the Athens Insomnia Scale. The results showed that 1,382 (23.2 %) participants suffered from insomnia. Lower employment level was significantly associated with a higher risk of insomnia. Job strain (ratio of job demand to job control), E/R (ratio of job effort to job reward), and OC (over commitment) were also significantly associated with insomnia. The relative excess risk due to the interaction between employment level and psychosocial environment (job strain, E/R, and OC) was 0.09 (95 % CI -0.57 to 0.76), 2.61 (0.74 to 4.48), and 3.14 (0.82 to 5.45), respectively; synergy index = 1.07 (0.66 to 1.74), 1.99 (1.37 to 2.90), and 2.25 (1.46 to 3.46), respectively. We found supra-additive interactions between employment level and psychosocial environment, assessed by the ERI (E/R and OC). Our findings suggested that lower level workers are more vulnerable to an adverse psychosocial environment than those at a higher level.