As the desire for higher education and self-actualization has increased among women in Korea, their participation in the economy has also increased, with the rate of employment for women rising from 54.9% in 2011 to 60.0% in 2019 [1,2]. However, this remarkable increase in women’s employment rate does not mean that women’s self-actualization and quality of life have necessarily improved. This is because a high proportion of women undertake non-regular shift work and part-time work compared to men, and inequality has been identified as a new problem related to the different employment rates and employment patterns of men and women [3]. Married women in the workforce in particular, must participate in economic activities on a par with men, while also performing additional household roles such as housekeeping and childcare; these coexisting demands can be considered hazardous to their health [1,4].

Shift work refers to a type of work in which people are divided into two or more groups, and the daily work is divided into two or more shifts [5,6]. With the growing emphasis on convenience in everyday life, shift work has been increasing in popularity in recent years [5], and there has been a growing interest in the impact of shift work on workers’ physical, mental, and social health.

Compared to regular daytime workers, shift workers are more likely to experience disruptions in their daily routines and lives due to irregular working hours, and thus have more difficulty maintaining positive everyday health behaviors [7,8]. It is well-known that shift work interferes with workers’ normal sleep-wake cycles, especially night shift workers, for whom the risk of safety accidents is high due to decreased concentration. Thus, a high proportion of shift workers in general tend to engage in unhealthy behaviors such as not getting enough sleep, smoking, and drinking alcohol [8,9]. As a result, shift work can lead to difficulties maintaining good health in daily life and increase workers’ vulnerability to diseases. Shift workers frequently experience digestive, musculoskeletal, metabolic, and cerebrovascular diseases [8-10], and women shift workers have been reported to have an elevated risk of breast cancer [9,10] and premature birth [10] or miscarriage [10].

Studies on the mental health of shift workers have found that shift workers tend to experience a variety of mental health problems such as high stress [11], depression, and anxiety [7]. Serious depression among women shift workers was also found in preceding studies [11,12]. According to the Korean National Health and Nutrition Examination Survey (KNHANES), women shift workers were 2.94 times more likely to develop depression than their male counterparts [12], and the degree of depression experienced by women shift workers was 1.73 times higher than men according to a meta-analysis of research on shift workers [11]. However, previous studies have not provided sufficient evidence regarding why women shift workers experience more serious depression than men shift workers. Some studies have suggested that women generally have a higher risk of depression than men [11], while others have suggested that hormonal reactions can cause women to be more stressed about rotated or shift work, making them more vulnerable to depression than men [12]. One explanation for the high levels of depression among women shift workers may be that childcare or housework duties increase their fatigue and stress levels in addition to the negative physical and mental effects of shift work [1,3]. In order to identify the causes and factors that influence depression among women shift workers, studies that are designed to exclude the effects of exogenous variables should be conducted.

Unmet medical care needs, which occur when people are unable to use medical services despite experiencing illness or health problems that require medical attention, is an important direct indicator of medical inequality [13,14]. These variables are meaningful indicators of the health of women who perform shift work. According to an analysis of adult women in Korea, the extent to which women received insufficient medical care was at least 1.2 times and up to 1.5 times higher for employed women than for unemployed women, and the reasons for insufficient medical care among women were costs, long wait times at doctor’s offices, difficulties obtaining health services during working hours, and childcare responsibilities [15]. For employed women, analysis of unmet medical conditions of daytime workers and shift workers with modified working schedules is sorely lacking and more in-depth research is needed.

Prior studies have found that shift work and depression are related. Although women shift workers reported more serious depression than men shift workers, most studies on depression that included women shift workers classified all participants simply as shift workers, not accounting for gender differences in their analyses [11,12], or examined health behaviors, depression, and quality of life among all women workers regardless of work type [1,15]. In many studies, women shift workers were not distinguished from other women workers according to their working patterns. In addition, studies of depression among shift workers have typically compared workers by sex, and many of the interpretations resulting from these studies have been fragmented [11,12].

Among women workers, those who perform shift work tend to face many health vulnerabilities. Studies that examine women workers should further classify them as daytime or shift-based workers, according to the type of work performed, in order to more accurately identify differences in health status and depression. Furthermore, in order to better understand women shift workers’ health, identifying their unmet medical needs, which tend to be indicators of general characteristics, health behaviors, and medical inequality, is also of value. Depression among women shift workers has already been shown to be at a serious level [11,12]. Thus, it is important and urgent to identify the factors affecting depression among women shift workers as a next step.

This study was therefore conducted to examine shift working women’s health behaviors, health care use, and depression according to their type of work, and analyze factors related to depression. This would improve the existing understanding of women shift workers’ mental health, and enable suggestions for future health promotion programs targeted to women shift workers, and provide basic data to support policies that address their needs.

In this study, depression was higher among women shift workers than among their daytime working counterparts. This is consistent with the results of a study of employed women showing a higher rate of depression among women who performed shift work than among women who were daytime workers [19] and a study of male and female workers that also showed a higher rate of depression among shift workers than among daytime workers [8,20]. Considering that the rate of depression is higher among women workers than men [20,21], these results indicate that women who perform shift work experience the most depression among various categories of workers. Because shift work is different from the 24-hour biological rhythm [22], it interferes with workers’ biological rhythm, leading to a variety of physical health problems as well as various mental health problems such as anxiety and depression [23,24]. However, shift work is inevitably performed due to industrial development and flexible working hours [8]. Therefore, efficient and practical interventions for depression prevention and management are needed for women shift workers, and to do so, it is necessary to accumulate evidence through repeated studies that seek to identify the factors related to their experience of depression. In addition, the prevalence of shift work has continued to increase and is expected to further increase in the future [25,26]. This requires community-level and national attention and flexible measures such as adjusting shift work cycles, reducing shift work hours, and ensuring sufficient rest during shift work [8].

This study identified common influential factors on depression for women workers: age, personal income, perceived health status, smoking status, and unmet medical needs. For daytime working women drinking status and average hours slept per weeknight were additional factors; whereas unmet dental needs were an extra factor for shift working women.

Age affected depression differently, however, between daytime and shift workers, with daytime workers experiencing higher levels of depression in the 19–29 years age range than in the 60 years and older range, while shift workers experienced higher levels of depression in the 30–39 years range than in the 60 years and older range. This is somewhat consistent with the results of a study on paid workers that showed higher rates of depression among younger workers, especially those between 19 and 39 years old [27]. The high level of depression among workers aged 19 to 39 years compared to other age ranges reflects the socioeconomic environment of South Korea, where there has been an increase in youth unemployment, nontraditional employment, and job insecurity [27]. In addition, people in their 30s often experience a new family environment due to getting married and having children, which can lead to conflicts between work and home roles, and possibly deepen depression [21]. This suggests that depression intervention and prevention programs for women workers need to focus more on those in their 20s and 30s. In addition, differences were observed between daytime and shift working women in terms of which age groups experienced the most depression, so further research is needed.

The analysis of personal income related to depression showed that lower personal income for both daytime and shift workers corresponded to higher levels of depression. This is consistent with previous studies which have found that lower income groups experience higher levels of depression [27-29]. In addition, this study found differences in depression levels according to personal income were greater among shift workers than among daytime workers. These findings support the need for depression prevention and management programs for women workers to target low-income workers, with a particular focus on shift workers with low income.

The analysis of smoking status found that women who smoked showed higher rates of depression than non-smokers among both daytime workers and shift workers, while women who drank alcohol showed a higher rate of depression than non-drinking women only among daytime workers. Prior studies have shown that women who smoked and drank alcohol had higher levels of depression than non-smoking women and non-drinking women [28,30]. However, some studies have found no link between smoking or drinking and depression, reflecting the difficulty of determining the factors that influence the decision to smoke or drink as they relate to depression [28]. In this study, drinking had an influence on depression only among daytime work women, unlike previous studies. Therefore, greater evidence should be accumulated through additional research on the influence of smoking and drinking on depression among women workers. In addition, age at first exposure to smoking or drinking, the degree of exposure, and the effects of smoking or drinking, as well as current smoking and drinking status, can affect depression [30]. Therefore, more in-depth understanding of issues related to smoking and drinking is required, such as how long women have been regular smokers/drinkers, their regular amount of smoking/drinking, and the frequency with which they smoke/drink.

Among women with unmet medical needs, the rate of depression was high for both daytime workers and shift workers, while for women with unmet dental needs, the rate of depression was high only for shift workers. Unmet health care needs lead to a higher likelihood of negative health-related consequences [31], and if one has unmet health care needs, there is also a strong possibility of having unmet dental care needs [32]. The relationship between mental health and unmet health care needs is strong [31], and the high rate of depression among female daytime and shift workers with unmet medical needs in this study can be understood in this context. In addition, if the demand for health services is high, even if people are provided consistent health services, they might not perceive those services as having been sufficient [32]. It has been found that one reason for the high rate of unmet dental care needs is that those with high income feel that dental care is a low priority compared to other problems, while those with low income cannot always afford dental care [33]. Given the high proportion of female shift workers with a low income in this study, unmet dental needs were likely related to depression only for shift workers because of their inability to afford sufficient dental care or lack of desire to obtain dental care due to economic strain. However, this study did not identify the reasons for unmet dental needs, so it is necessary to identify both the presence of unmet dental needs and the reasons for unmet dental needs in future studies.

The number of average hours slept on weeknights was related to depression only among female daytime workers, and depression was high among women who slept fewer than 5 hours on average per night. This can be understood in the same context as prior studies finding that workers who slept for fewer than 7 hours per night on average had a 2.16 times higher incidence of depression [20], and that fewer hours of sleep resulted in a higher degree of depression [27]. Sleep and depression are closely related [25], and sleep disorders can lead to depression [19]. Therefore, it is important to provide interventions for the prevention and management of depression among women workers that consider time spent sleeping and factors related to sleep disorders. In addition, the relationship between time spent sleeping and depression was significant only among daytime workers, which could be understood in relation to the results of a study [25] finding that shift nurses had adapted to irregular sleep patterns, while daytime nurses were accustomed to regular sleep. However, for shift workers, the risk of sleep disorders is higher than that of daytime workers, so it is necessary to conduct further studies on the relationship between sleep and depression according to type of work. In addition, it is necessary to identify sleep-related characteristics such as sleep quality and time spent sleeping, to determine their relationship with depression.

In this study, women daytime and shift workers differed in age, education, personal income, marital status, and time spent working. The lowest proportion of daytime workers were ages 19–29 years, while the highest proportion of shift workers were ages 19–29 years, indicating that the typical age of women shift workers was lower than that of daytime workers. This can be understood in the context of the results of prior studies [8,26] that showed that shift workers tended to be younger than daytime workers. A prior study on nurses found that most new (younger) nurses are assigned to perform shift work, while experienced nurses are assigned to daytime work [26]. In general, workers prefer daytime work over shift work and is likely that the phenomenon of assigning shift work to new employees is also true of other professions besides nursing, potentially explaining the lower average age of women shift workers than daytime workers.

There was a difference in education level, as the largest share of women daytime workers were college graduates, while the largest share of shift workers were high school graduates. This is consistent with prior studies [8,20] that reported that daytime workers tended to have a higher education level than shift workers. Considering previous studies that found that a high percentage of non-regular workers performed shift work [34] and that shift workers had a low average education level [8], this study’s results can also be understood in this context. While this difference may be due to the characteristics of daytime and shift work, our analysis did not take that possibility into account and may be an area for future research to clarify.

Personal income was lower and lower education levels were found for women shift workers than for daytime workers. A prior study reported that daytime workers tended to have a higher economic status than shift workers, but there was no statistical significance [20], which differed from this study. Education levels are related to income levels, and low education levels tend to correspond to low income levels [18,28]. In addition, this study found that the average weekly time spent working tended to be lower for shift workers than for daytime workers, which is also related to low income levels.

There was a higher proportion of unmarried women among shift workers than among daytime workers. This is consistent with previous studies [20,23] that found that there were more single women among shift workers than among daytime workers. This is likely because married women often have to balance work and family, which can lead to many problems if they perform shift work. Thus, married women likely tend to avoid shift work more than unmarried women. In addition, shift work tends to be assigned to new employees, and experienced employees are assigned daytime work [19]. Therefore, considering that women tend to get married after being employed rather than getting married and then seeking employment, new employees are more likely to be unmarried and are thus also more likely to be assigned shift work.

Drinking and smoking were more prevalent among shift workers than daytime workers. This is inconsistent with a preceding study that found no difference in the prevalence of drinking and smoking between daytime and shift workers [8]. This finding can likely be attributed to the younger age of women shift workers compared to daytime workers in this study. Prior studies on workers’ drinking and smoking habits have analyzed differences according to occupation type, such as manufacturing jobs, office jobs, and service jobs, and found differences in the drinking and smoking habits of workers across different occupations [35,36]. However, differences in drinking and smoking habits between daytime workers and shift workers are measured differently depending on the study, so further studies are needed. In addition, Korean culture emphasizes drinking as an important activity for social relationships and as an opportunity to exchange information with co-workers and bosses, so it is necessary to analyze both the prevalence of drinking and the reasons for drinking. It has also been found that the risk of smoking is high among night shift workers [29], meaning that it is also necessary to consider the type of shift when analyzing the smoking rate among shift workers.

On average, women daytime workers often slept for the recommended number of hours per weeknight, while shift workers often slept on average for fewer than 5 hours per weeknight. Prior studies, however, have found that there was no difference in average time spent sleeping between daytime workers and shift workers [8] or that the total time spent sleeping by shift workers was higher than daytime workers [25]. Thus, it is necessary to conduct further studies on the average time spent sleeping by women daytime workers and shift workers. In addition, shift work disrupts biorhythm, causing irregular sleep patterns and reducing sleep quality [24,25], and a prior study reported that shift workers experienced irregular sleep times, took more sleeping pills, and experienced a lower quality of sleep than daytime workers [25]. Therefore, identifying the qualitative characteristics of sleep as well as quantitative characteristics (e.g., time spent sleeping) may help improve sleep issues according to daytime and shift work in women.

In this study, the use of health services between women daytime workers and shift workers differed for medical check-ups and cancer screening, and the percentage of shift workers who received a medical check-up and cancer screening within the previous 2 years was lower than that of daytime workers. This is partly consistent with a previous study [29] that reported that the rate of medical check-ups was lower among women who worked at night compared to women who worked during the day, but that there was no difference in cancer screening rates. Considering that the proportion of non-regular workers is high for shift work and the health insurance subscription rate is low for non-regular workers [29], this result can likely be attributed to the high possibility that women who perform shift work are less likely to receive health insurance benefits than women who perform daytime work. In addition, this study’s finding that women shift workers were younger, have less education, and have a lower income than daytime workers likely also relates to differences in health insurance benefits and thus the rate at which shift workers attend medical check-ups and cancer screenings. Since shift work interference with biological rhythm can lead to a variety of health problems [22,24], the need for early detection and treatment of health problems through medical check-ups and cancer screenings is greater for shift working women than for daytime workers. Closely examining the reasons why women shift workers are less likely to have medical check-ups and cancer screenings may be helpful for policies to narrow this gap.

This study has the following limitations. All types of work other than daytime work were classified as shift work, and thus, the various contexts of shift work could not be considered. In addition, job-related characteristics, which can affect depression among working women, were not available. Future studies should examine different types of shift work to identify differences according to type of work, with appropriate consideration of a wider range of variables that may also affect depression among working women.

In summary, depression was found to be higher among women who performed shift work than among women who performed daytime work. The factors related to depression among women daytime workers were age, personal income, perceived health status, smoking status, drinking status, average hours of sleep per weeknight, and unmet medical needs, while those related to depression among women shift workers were age, personal income, perceived health status, smoking status, unmet medical needs, and unmet dental needs. Therefore, it is necessary to provide practical and efficient evidence-based interventions for depression prevention and management among women who perform shift work, taking into account depression-related factors. In addition, national support and various policies that specifically support women performing shift work is needed to improve their health and well-being.