The “24-h society” has changed the timing of work towards longer and non-standard business hours. Because companies attempt to secure competitiveness by raising productivity to 24 h a day or extending business hours and as customer demands for out-of-hour services are increasing, flexibility in work hours has been requested. As a result, the arrangement of working hours has become a critical element in work organization, and the number of nontraditional working systems consisting of night work and shift work has been increasing [1, 2]. Although shift work was mainly found in the manufacturing industry in the past, the scope of shift work has extended to the retail and service industries due to globalization and development of information technology [3]. Approximately 15 ~ 18 % of employers are estimated to be involved in shift work in industrialized countries worldwide. Surveys show that 17.7 % of the United States' workforce is estimated to have shift work [4]. In Europe, based on the result of Fourth European Working Conditions Survey published in 2007, shift workers made up about 17 % of the workforce [5]. The Ministry of Employment and Labor investigated the work-hour conditions of companies with 10 or more regular employees (3414 samples) in Korea in June 2011. The average rate of implementing the shift system for all industries was 15.2 % [6].

Shift work is vital in hotel businesses as these businesses run 24 h daily regardless of holidays to accommodate customers. Recently, more hotels are opening supplementary facilities such as in-room dining, bars, pools, and gyms for 24 h to accommodate higher customer demands for such services along with front desk service. Consequently, the ratio of shift work in hotel workers is high, and various types of shift work exist including continuous day shift without night work, rotating night shift, every-other-day (24-h) shift, and fixed night shift. According to the results of the previously mentioned 2011 survey on working hours in Korea, the shift system was introduced at a rate of 34 % in the accommodation and food service industry, more than twice the introduction rate in all industries (15.2 %) and similar to the rate found in a European Union survey [5, 6]. In addition, the growth of hotel businesses in Korea due to increased number of tourists from foreign countries is noteworthy. In 2012, the number of foreign inbound tourists to Korea was over 11 million. This number increased by 107 % compared to 5.32 million in 2000. This number increased by 6.2 % each year for the past 12 years [7]. In this case, it is only natural that the demands for hotels and hotel workers are greatly increasing. The country's tourist accommodation business status estimates released by Tourism Knowledge and Information System of the Korea Culture and Tourism Institute showed that the number of tourist hotels increased from 621 in 2009 to 734 in 2013 and the number of the guest rooms increased from 67,171 to 79,393. Each number surprisingly increased by 18.2 % [8]. According to the Economically Active Population Survey of Statistics Korea (KOSTAT), the number of workers in the accommodation and food service industry, including hotels, increased by about 15 % in three years from 1.85 million in 2011 to 2.12 million in September, 2014 [9]. Therefore, high rates of shift work in the accommodation and food service industry are a global trend, and the number of hotel shift workers is expected to increase even more.

Shift work causes discordance between the endogenous circadian timing system and environmental synchronizers and interrupts the normal circadian rhythm [10, 11]. This gets workers to experience “jet lag,” which is a combination of feelings of fatigue, sleepiness, insomnia, disorientation, digestive troubles, irritability, decreased mental agility, and reduced performance efficiency. Besides these short-term effects, shift work also has long-term effects such as severe diseases that cause individuals and the society to spend significant amounts of financial and social cost [2]. Shift work is known to be related to physical disorders such as stroke [12], cardiovascular disease [13, 14], metabolic syndrome [15, 16], breast cancer [17, 18], and gastrointestinal disease [14, 19], and mental disorders such as sleep disorder [20, 21] and depression [22, 23]. Shift workers can also be isolated from their families and society; this is because most familial and social activities are based on the day-oriented rhythm of the general population, and therefore, shift workers find it difficult to participate in such activities [24, 25].

Depression is an important global public-health issue, because its lifetime prevalence ranging from 2 % to 15 % is relatively high, and it is associated with substantial disability [26]. Rated as the fourth leading cause of disease burden in 2000, depression accounted for 4.4 % of total disability adjusted life years (DALYs). Projections indicate that after heart disease, depression is expected to become the second leading cause of disease burden by the year 2020 [27]. Depression is problematic not only in the general population but also in the working population. The result of research analysis on the rate of mental disorders in workers demonstrated that depression had the highest prevalence rate after simple phobia [28]. In addition, depression in workers causes a significant amount of business and social costs as it leads to the decrease of productivity. Recently, several studies reported the cost-effectiveness of treatment and intervention for depression. Early discovery of depression and the importance of intervention were emphasized [29, 30].

Shift work and depression have been examined in workers of various job categories. Although there was no significant difference in depression between shift workers and non-shift workers in a study of university hospital nurses [31], depression level in shift workers was significantly higher in studies conducted with security guards [32], automobile manufacturing plant workers [33], steel manufacturers [34], and police officers [35], showing an overall association between shift work and depression. However, previous studies conducted comparative analysis merely on shift workers and non-shift workers, thereby overlooking the possibility of a difference in the association with depression depending on the type of shift work within shift workers. Furthermore, these studies did not discuss the continuous day shift without night work, which is currently rising as an issue. In addition, studies have rarely been conducted on hotel workers, who are one of the most representative shift worker groups. Studies about the impact of shift work on health conditions, especially mental health conditions, of hotel workers are rarely found. As mentioned previously, the number of shift workers in hotel businesses is expected to increase consistently and it is crucial to study the impact of shift work on hotel workers' mental health. Studies about depression with its high disease burdens are considered to be especially important.

Therefore, this research was to understand depression in hotel workers, to examine the association between shift work and depression, and to confirm whether different shift types affected how strongly these two were related to each other.

This research was to understand hotel workers' depression, to confirm the relationship between shift work and depression, and to see whether the degree of relationship was different based on shift types.

First, the research subjects' prevalence rate of depression was 16.4 %. This was lower than the rates of hotels and restaurants workers (28.8 %) [40], bankers (20.6 %) [41], and clinical nurses (36.3 %) [42] based on previous studies conducted in Korea using 21 points as the CES-D cut off point. The subjects of this research showed lower prevalence rates of depression because the subjects worked at the first class hotels and 90.1 % of these subjects were permanent employees. In addition, these subjects have relatively high incomes; 62.7 % of the subjects had an annual income of 30 million won or higher. Therefore, this result was due to the fact that job stability and appropriate compensation contribute to maintaining social and economic status and the fact that this eventually helps lower the depression level [43]. The prevalence rate of depression was higher in females (22.1 %) than in males (12.9 %) and the rate for females were significantly higher. When this result was compared with previous studies conducted in Korea using 21 points as the CES-D cut off point, the gender differences were greater than research conducted on Korean employees [40] with a rate of 14.7 % for males and 18.6 % for females and research conducted on bankers [41] with a rate of 19.8 % for males and 21.5 % for females. This research result is limited for direct comparison since it was not clinical major depression and was only point prevalence. However, population-based epidemiologic studies from 10 different countries, including Korea and the United States that compared lifetime rates of major depression [44] showed that females were two times more likely to have depression than males in all countries. This gender distribution was very similar to that of this study. In addition, depression level significantly increased with younger age, shorter work duration, and lower annual income. It is likely that younger age and shorter work duration make it difficult to adapt to work or result in establishing less mature relationships with employers or coworkers, thereby increasing fatigue levels and depression [45]. If one's position is low due to short work duration, one's control over the work is low while workload demands are high and income is relatively low. This can reduce one's satisfaction with work and increase one's depression level [43, 46]. From the perspective of hotel workers, their positions tend to be low when their age is younger and work duration is shorter. This increases their interaction with customers, which also increases their exposure to emotional labor and experience of violence, leading to increased risk of depression [47]. This is likely to be especially high in workers in charge of rooms and food & beverage whose main job is to interact with customers and satisfy their demands. Although there was no statistically significant difference in depression level according to department, the prevalence of depression was highest at 22.1 % and 17.8 % in rooms and food & beverage/facilities, respectively, while it was 3.1 % in housekeeping, which rarely involves interaction with customers. This shows that the risk of depression may differ according to work-related characteristics.

A comparison between the relationship of shift work and depression and the relationship based on shift types showed that when relevant general and work related characteristics were adjusted, the risk of being in the depression group was 2.22 times higher in the rotating day shift group, 2.63 times higher in the rotating night shift group, 3.46 times higher in the fixed night shift group compared to the fixed day shift. The result showed that the prevalence rate of depression is higher in the traditional rotating night shift. This corresponds to the results of studies previously conducted in Korea: night shift security guards were more likely to be depressed [32]; automobile manufacturing plant workers with shift work had a significantly higher Beck Depression Inventory score, which was a depression indicator [48]; and police officers with shift work were more likely to have depression [35]. This result also matches results of previous studies that showed that shift work including nighttime work negatively influences one's depression level [22, 23].

The odds ratio of the rotating day shift was 2.22 and this had a significantly higher prevalence rate of depression compared to the fixed day shift. The odds ratio of the rotating night shift was 2.63 and this was not significantly different. This is meaningful because it confirms that there is a risk that shift work without night work may be related to depression. This is expected to be due to disruption of the circadian rhythm, which can cause mental health issues including depression, lack of familial and social support, and sleep disorders [2, 3]. Most subjects in the rotating day shift group had overlapping time with continuous two-shifts or three-shifts. Nine working hours, including one-hour mealtimes, were distributed between 06:00 and 22:00; the hours were different based on departments and hotels. Therefore, workers with morning work had to go to bed early to get to work in the morning with a sufficient amount of sleep. However, most personal activities took place at nighttime and it was difficult for these workers to reduce their participation in these activities. When they forced themselves to go to bed early, this put them close to circadian acrophase and made it difficult for them to initiate sleep [49]. Such working on a rotating day shift created significant sleep disturbances [21]. Compared to constant day workers, shift workers (with or without night shift) had a difficult time falling asleep [20]. Afternoon work or evening work less frequently interrupted circadian rhythm and caused sleep disturbance issues, but parents with evening work could not participate in child-related activities [50]. Similarly, evening workers could have reduced familial and social supports because they could not participate in social activities or family activities.

Lastly, the odds ratio of the fixed night shift was 3.46, which was the highest of all shift types; this result was caused by the decreased sleep duration and the maladjustment of the circadian rhythm. According to previous studies, fixed night workers seem to sleep somewhat less than rotating shift workers when averaged across the entire shift cycle [49, 51]. According to a review of the literature on the adjustment to fixed night work on the circadian rhythm in melatonin secretion, which is generally considered to be the best known indicator of the endogenous circadian body clock, a very small minority (<3 %) of fixed night workers show “complete” adjustment of their endogenous melatonin rhythm to night work, and less than one in four fixed night workers shown adjustment sufficiently “substantial” to derive any benefit from it [52]. There were no previous studies on the relationship between fixed night shifts and depression due to rarity of fixed night workers; there were only studies on the relationship between work injury [53] and work satisfaction level [54, 55]. Therefore, additional studies on the impact of fixed night work on mental health must be conducted to confirm this relationship. Any causal relationship must be revealed through conducting large-scale studies as the data for shift workers including fixed night workers are easily accessible due to the nighttime special health examination available since 2014.

In addition to shift work, numerous harmful occupational factors must be considered, because hotel workers serve in a variety of departments; in particular, workers in rooms and food and beverage/facilities who frequently interact with customers must be alert and attentive to their tastes and demands and are also required to maintain uncomfortable or stationary postures for long hours [47]. In the present study, the ratios of workers in rooms and food and beverage/facilities according to the type of shift work are fixed day shift (35.3 %), rotating day shift (64.6 %), rotating night shift (67.9 %), and fixed night shift (92.3 %), showing that shift workers are largely distributed in the department of rooms and food and beverage/facilities. Thus, for the association between shift work and depression in hotel workers, work-related characteristics such as emotional labor or experience of violence, which can occur with interaction with customers, must be considered. Efforts to better elucidate the association between shift work and depression in hotel workers by including such related risk factors in the model are required in future studies.

The limitations of this research are the following. First, this research is a cross-sectional study that does not provide any information about temporal relationship; it cannot investigate the causal relationship of shift work and depression. Second, this research was conducted on workers in two first-class hotels in Seoul and the depression level of the workers could have been underestimated because most of the research subjects were permanent employees. In addition, it is difficult to generalize to all hotel workers in Korea with this sample. Third, the measurement could be limited since each subject evaluated his or her own depression level when completing the survey. Finally, the “healthy worker effect” may interfere with the results of the study because the study was conducted on current employees and excluded retired workers who had health issues caused by shift work.

Despite such limitations, this research revealed that shift work and depression in hotel workers were related to each other and the differences were evident based on shift types. The depression risk was the highest with fixed night shift workers; this research showed that the depression risk of rotating day shift workers who did not have night work could also be increased. This research is noteworthy as it can be used to provide basic data for designing shift work types and developing preventative measures for mental disorders in hotel workers, especially since the number of the shift workers is expected to increase in this industry. More studies must be conducted on hotel workers' shift work types, various factors related to shift work, for example, regularity, speed, and direction of shift rotation, and mental health including depression.

This study showed that shift work and depression in hotel workers were related to each other and the differences were evident based on shift types. The depression risk was the highest with fixed night shift workers and the depression risk of rotating day shift workers who did not have night work could also be increased. These findings are noteworthy as it can be used to provide basic data for designing shift work types and developing preventative measures for mental disorders in hotel workers, especially since the number of the shift workers is expected to increase in this industry. This study also indicated the need for further research into various factors related to shift work, for example, regularity, speed, and direction of shift rotation, and mental health including depression.