Subjects
The present study used data from the 4th KWCS, which was conducted between June and September 2014 on employed workers, 15 years or older. Among a total of 50,007 people, 25,138 wage workers were selected as the subjects of the study, after excluding military personnel and those who failed to or refused to respond to the questions required for analyses.
General characteristics
General characteristics included sex, age, education level, and income.
Occupational characteristics
With respect to occupational characteristics, job types were divided into blue collar (skilled agricultural and fishery workers, craft and related trades workers, plant and machine operators and assemblers, and elementary occupations), service work (service workers and sales workers), and white collar (manager, professional, technicians and associate professionals, and office workers). Occupational characteristics also included type of employment, number of employees, working time, shift work, and emotional labor. Emotional labor was defined if the response to the question “In my work, I have to hide my feelings” was either”always” or “most of the time”. Being exposed to physical risk factors (vibration, noise, high and low temperatures), biochemical risk factors (dust, organic solvents, chemical handling, second hand smoke, and infectious materials handling and contact), and ergonomic risk factors (posture that cause fatigue or pain, lifting or moving people, heavy load handling, continuous standing posture, and repetitive movement) was defined as exposure for ≥1/4 of working time.
Psychosocial risk factors were divided into 5 categories of job demand, job control, social support, job insecurity, and lack of reward. Job demand used 7 questions, which consisted of; “very fast speed,” “strict deadline,” and “experience of stopping task at hand for unexpected work” for work speed; “strict product quality standards,” “evaluating one’s own quality of work,” and “resolve unexpected problems on one’s own”for occupational characteristics; and “enough time to complete the task at hand” for work situation. Job control used 7 questions, which consisted of “possible to take care of personal business during work hours,” along with “order of work,” “work method,” and “work speed/work rate” for possibility of work selection and “I have a say in the choice of my working partners,” “I can take a break when I wish,” and “I can influence decisions that are important for my work” for work situation. Social support used 8 questions, which consisted of “support of colleagues,” “support of boss,” and “I have a very good friend at work” for job situations and “gives feedback to my work,” “respects me as an individual,” “resolves conflicts well,” “plans and organizes work very well,” and “encourages me to participate in important decision making” for disposition and attitude of direct superior. Job insecurity used only 2 questions, which were “I will lose this job within the next 6 months” and “Even if I quit or get fired from this job, it will be easy for me to find another job with similar pay.” Lack of reward only used 1 question, which was “I am receiving fair compensation for my work.” Each question was scored and the subjects were divided into high or low group according to the median score. With respect to groups, score of <3 points was considered low job demand group and score of ≥4 was considered high job demand group; <3 points was considered low insufficient job control group and score of ≥4 was considered high insufficient job control group; <1 point was considered low inadequate social support group and score of ≥2 was considered high inadequate social support group; and 0 point was considered low job insecurity group and score of ≥1 was considered high job insecurity group. Since lack of reward contained only 1 question, inadequate compensation was given a score of 1 point. The internal consistency of each of the psychosocial factors is as follows. The Cronbach alpha value was 0.630 for job demand (7 questions), 0.670 for job autonomy (7 questions), and 0.793 for social support (8 questions).
Workplace violence and perpetrators of violence
Workplace violence was assessed using the following 6 questions; “In the past 1 month, have you experienced the following?” A) verbal abuse, B) unwanted sexual attention, or C) threatening or humiliating behavior?” and “In the past 1 year, have you experienced the following? A) physical violence, B) bullying/harassment, or C) sexual harassment?” Six types of workplace violence were measured each with Yes/No response, No opinion/Refusal categories. If one or more items were answered “Yes”, the respondent was identified as a member of workplace violence experience group.. The workplace violence experience group was divided based on whether their response to the question on perpetrator of violence was a client (client, student, patient, etc.) and/or colleague (boss, colleague, subordinate, etc.).
Sleep disturbance
Anyone who answered “Yes” to sleep disturbance or insomnia for the question “Have you had any of the following health problems in the past 12 month” was considered as having sleep disturbance. KWCS includes a survey item regarding sleep disorder symptomswhich considers a job-related problem as a cause. However, in the current study, the survey item regarding sleep disorder symptoms irrelevant to job was used as the dependent variable because we hypothesized that sleep disorder may be caused by workplace violencenot recognized as directly related to problems at work.
Analysis method
In order to examine the associations of general characteristics, occupational characteristics, workplace violence, and perpetrators of violence with sleep disturbance, a chi-squared test was performed. Based on the factors which showed statistical significance in the chi-squared test results, a multiple logistic regression analysis was used to examine the effects of general characteristics, occupational characteristics, workplace violence, and perpetrators of violence on symptoms of sleep disturbance. All analyses were performed using SPSSver. 22.0 (Chicago, IL, USA).