Long working hours is one of the prominent characteristics of South Korea. According to 2010Organization for Economic Cooperation and Development(OECD) statistics, Korea had 45.9 hours per week, placing itself in second place out of all OECD countries following closely behind Turkey(49.3 hours per week) [1]. Moreover, the fifth European Working Condition Survey(EWCS) claimed that Korea had an additional 8.4 hours per week than the average 37.5 hours, which is the 2010 average weekly working hours of the European Union (EU) [2]. Long work hours are associated with its negative effects on overall health, such as poorly perceived general health, increased injury rates, increased reports of illnesses(myocardial infarction, hypertension, neck or musculoskeletal discomfort), increased mortality, and increased risk of premature birth in pregnancy [3]. A study conducted in Korea claims that increase in work hours were associated with increased severity of depression(as measured by Beck Depression Inventory(BDI)) and depressive symptomatology [4,5] and more specifically, working more than 60 hours per week was closely related to increase in suicidal thoughts [6]. However, the study supervised by Heo et al (2012) [4] concerning depression and length of work hours was limited to Labor Union members and reported results from a simple analysis done with a standard of 48 hours per week and thereby could not specifically isolate long work hours and its effects. Similarly, a study done by Kim et al (2013) [5] which was conducted by asking the question, ‘Have you ever experienced extreme feelings of sadness or despair that continuously impaired daily activities for more than two weeks in the past year?’ as a means of obtaining data did not prove to be a good indicator of overall emotional health. Similarly, Kim et al (2012) [5] could not deduce that suicidal thoughts are associated with long work hours due to inconsistencies in controlled variables such as temporary employment status and diverse occupations among subjects. Therefore, it is necessary to do research where depressive symptoms are included in emotional status and select a group of participants with a steady work schedule in order to observe how extended work hours affect emotional state.

Positive emotional states have the potential to espouse healthy perceptions, beliefs, and physical well-being. Not only do emotional states have the capacity to directly induce physiological responses, especially by influencing the immune system, they can also influence patterns of health behavior and affect progress of illness by either appropriately responding to health problems or eliciting social support through interpersonal relationships [7]. In order to appropriately evaluate mechanisms of emotional states, testing motor expressions of motor neuronsby using tools such as physiological arousal, Emotion Facial Action Coding System (EMFACS) and recognition of emotion in speech, or utilizing more subjective forms of testing such as PAD Emotion Scales, Self-Assessment Manikin (SAM), Geneva Emotions Wheel, and Product Emotion Measure Instrument v7.0 (PrEmo), is common [8]. However, such tests require a long-term commitment and have to be performed by a specialist, which is not applicable in studying a large-scale population. WHO (Five) Well-Being Index was initially developed to monitor emotional states of diabetic patients, and it is an effective tool that utilizes a short questionnaire to monitor emotional functioning of patients. Emotional well-being that is measured from the Index is an important measure of overall perceived quality of life, and it is a dependable screener for emotional function and depression [9]. Therefore, it is an appropriate mechanism for a study that requires a large-scale population.

One widely used tool is WCS (Working Condition Survey), a survey that is used to select a significant group of subjects from the working population and comprehend an economically active population and their work environment, which is used as a reference when deciding policies that increase the quality of work while pursuing improvement in productivity and increase in employment. EU is known for conducting this WCS every 5 years. Similarly, Korea Occupational Safety and Health Agency(KOSHA) has been conducting Korean Working Condition Survey(KWCS) to not only emulate EU’s WCS to analyze working conditions and reform working environment, but also in order to grasp causation of work-related risk factors in labor markets and provide baseline data concerning occupational safety and health policies that are appropriate for Korea. Risk factors associated with work environment including long work hours can be analyzed by examining studies representatives of the working population of a country, such as data from working condition survey.

Therefore, we conducted a study including regular employees of manufacturing industry who were chosen as subjects as they are representative of Koreans with relatively regular and set work schedules. WHO well-being index was used to control demographical characteristics, personal health behaviors, socioeconomic status, and work-related characteristics, and various studies were performed in order to understand an association between long work hours and emotional well-being.

This study observed the emotional well-being associated with work hours by utilizing WHO (five) well-being index. A trend test between the possible depression group that scored between 0–28 and the other two groups showed that when work hours increased, possibility of developing depression also increased. Moreover, odds ratio of the possible depression group increased as work hours grew longer compared to the reference group even as general characteristics and work-related characteristics were controlled as variable. Putting sleep cycle and imbalance between work and life in perspective, long work hours are likely to cause negative impact on emotional well-being. Chances to be develop social support such as physical activity and relationships outside the workplace decreases as work hours become longer. Of the sample that EWCS reported, only 14% was considered to be working long hours of 48 hours or more, which is significantly different compared to the 47.9% from this study. The previous study warns and calls for awareness of health problems that arises from imbalance between work and life, intensity of labor and increased exposure to danger related to long work hours [2], and Korea is predicted to have more health and social problems compared to EU due to a higher percentage of employees with long work hours.

Weekly work hours of an average manufacturing industry laborer was 47.1 hours in this study, while other studies such as Survey on Labor Conditions by Type of Employment [12] and Korean Labor and Income Panel Study (KLIPS) [13] each reported slightly different results of 46.1 hours and 43.2 hours for manufacturing employees in 2010. The survey on labor conditions by type of employment only focused on regular employees who worked at companies with 5 or more employees, while KLIPS focused on same sized companies as reported by the employer. This study includes smaller sized companies (less than 5 employees), unpaid rest breaks and meal periods and depends on self-report methods so discrepancies exist compared to official government statistics on work hours.

Emotional state is known to be associated with gender and age. While a notable discrepancy was observed between reference group and low-mood group, results of binary and multinomial logistic regression analysis showed that general characteristics and work-related characteristics were not statistically significant. It should be noted because females tend to express emotions more often, experience greater intensity of emotions and heightened sense of physiological awareness, it is perceived that gender difference in emotions exist [14,15]. It is possible that this study does not reflect such a gender difference because females may respond more positively to job security [16]. Moreover, even though it is well known that an emotional state is affected by age, as seen by observations of higher levels of both positive and negative emotional states during adolescence and senescence [17], a slight disparity was observed very likely due to underestimation of health worker effect [18].

A clear relationship between health behavior and emotional well-being was not observed in this study. Previous studies showed an association between employment status and change in health behaviors such as consumption of alcohol, weight and physical activity (excluding smoking), which is a good example of how stress due to change in employment status may cause changes in health behavior [19]. Similarly, this study took health behaviors into account such as smoking consumption of alcohol, weight and physical activity and studied their association, and both low-mood group and possible depression group showed a significant association. It is predicted that they are interrelated, where physical activity increases positive emotions and suppresses negative emotions, and negative emotions suppresses promotion of healthy behavior [5].

There are several limitations to this study. First, causal relationships are hard to be defined from a cross sectional study based on questionnaires. Not only do limitations exist as a survey to represent the Korean population, it is also a cross sectional study and difficult to say that emotional well-being has influenced work hours. Therefore it is probable that work hours and change in emotional well-being are in a causal relationship, and it is an issue that needs a confirmation through a Cohort study. This study has included various confounding variables in order to overcome such challenges; effects of shift work on health (based on previous studies) were included in the analysis and work-related characteristics (income, company size) were included as much as possible to interpret the relationship between long work hours and negative emotional state.

Secondly, there is a possibility that observational errors exist because KWCS measures work hours through a detailed questionnaire. Work hours tended to have a disparity compared to other well-known sources, and it is perceived to be caused by misunderstanding of terminology related to hours and difference in sampling. However, because the study employed the same questionnaire, it is thought that a questionnaire based work hour survey does not affect the direction or strength of association with emotional well-being.

Third, the WHO well-being index was used to diagnose a specific emotional state, but a more diverse approach is necessary in order to identify different emotional states. Even though the WHO well-being Index data is sufficient in measuring emotional functioning or diagnosing depression, it is not an applicable in correctly distinguishing an emotionate state. It is necessary that a research be conducted where an emotional state is subdivided into positive and negative emotions, or emotional states and traits. The WHO Well-Being Index is a survey based primarily on positive emotions. However, the basal ganglia and amygdala are each known to be closely involved with positive and negative emotions [20,21], so a classification between two emotional activities and a more extensive research on negative emotions are crucial. It is also possible to use a different approach where emotional state is subdivided into state emotion, a type emotion that one acquires from specific incidents, or trait emotion, a type of emotion that is constantly innate in every individual. State of emotions can be differentiated by using the Intensity and Time Affect Scale (ITAS) [22] to categorize emotional states by survey or by observing changes in emotions during elapsed time through journal entries [23]. However, aforementioned surveys are complicated, have many provisions, and require a significant amount of time and effort, which is more applicable for a smaller sized sample. It is more suitable to utilize the WHO (five) well-being index in monitoring emotional functioning, emotional well-being, possibility of developing depression, and the continual progress in a larger sample study.

The resulting work hours from this study shows an association between long work hours and emotional well-being. This result is similar to previous studies conducted in Korea where working 48 hours or more showed increase in level of depression (measured by Beck Depression Inventory (BDI)) [4] and working 60 hours or more showed increase in depressive symptoms [5] and suicidal tendencies [6]. Based on these results, conducting additional researches on variations of emotional well-being, health problems associated with this, health behaviors and progress in illnesses that arise from altering work hours is required. Furthermore, it would be necessary to subcategorize a long time frame in order to find the accurate initiation point where physical and emotional changes related to working long hours occur. This study has taken shift work into account in relation to work hours, but additional analysis of variables that might affect the result, such as work days, late night shifts, working all day long, working during the holidays, and flexibility of work schedule, is imperative.

This study was adequate in establishing an association between long work hours and emotional well-being by studying employees of manufacturing industry. Results show that with controlled variables, a clear association between long work hours and possibility of developing depression and emotional well-being was observed, as seen by trends and graphs. Not only are emotional states meaningful as a result by themselves, they also have a capacity to affect overall health and be used as a reference to indicate the current health of an individual [11]. Additional approaches concerning balance between work and life, sleep cycle, interpersonal relationships, negative effects on promotion of health, perception of health, behavior, occurrence of illness, and occurrence of natural disasters are necessary in order to create an even more diverse approach to emotional states and analyze how long work hours affect one’s health.

The authors declare that they have no competing interests.

KHL, JEK, YGK, DMG and SGP designed the research. KHL, MJY and SGL collected the data and performed the statistical analysis. KHL, MJY, JEK, and DMG interpreted the data and wrote the manuscript. All authors read and approved the final manuscript.