As technology developed, limitations relative to time spent in workplaces have disappeared. As lifestyles have changed, the numbers of people that work irregular hours are increasing. According to the Second European Survey on Working Conditions in the European Union that was conducted in 2000, only 24 percent of employees worked regular day shifts. Approximately 17 percent in U.S. and 16 percent in Australia worked day and night shifts. From the survey of Ministry of Employment and Labor Work Environment of Korea in 2011, approximately 16 percent were night or shift workers.

Many studies have found that shift work causes various negative side effects. Psychological issues: disruption of biological rhythm, depression and anxiety disorder. Medical issues: cardiovascular diseases, gastrointestinal disease such as dyspepsia, gastric ulcer, back pain and chronic fatigue. Occupational issues: poor performance at work, job dissatisfaction and social isolation [1,2]. Especially, many shift workers suffer from sleep disturbance. This is the reason that shift work schedules cause serious issues in circadian rhythms and sleep-wakefulness patterns [3]. Disturbed circadian rhythm affects human autonomic regulation such as heart rate, blood pressure, and immune and endocrine systems [4].

Compared to daytime or fixed schedules, shift work nurses have psychological, medical and occupational issues as well. For example, they experience issues with disturbed natural human circadian rhythm, low sleep quality, somatic disturbances, decreased heart rate variability and burnout syndrome (emotional exhaustion, depersonalization, and decreased personal accomplishment) [2]. Occupationally they experience errors in patient care such as information loss, less perception of nurse manager ability, leadership and support [5].

Morningness-eveningness indicates that an individual has a preference in diurnal performance, sleep-wake cycle for activity and alertness during the day. These differences are related to individual differences in functioning of the intrinsic circadian system [6]. Morningness-eveningness type influences individual temperament and character. For example, morningness subjects have more patience and harm avoidance behavior while eveningness subjects scored higher in novelty seeking and disinhibition [7]. In addition, morningness-eveningness type may manifest different health behavior patterns. Morningness was significantly related to lower odds of engaging in alcohol consumption, smoking and higher propensity of physical activity [6]. Morningness-eveningness type affects human physiologic homeostasis, too. In stress situations, morningness-eveningness type influences heart rate and rate pressure product (heart rate x systolic blood pressure). For example, eveningness type revealed higher heart rate and rate pressure product in the afternoon at rest and during stress situations [8]. Psychologically, eveningness type experience more depression that may be mediated via circadian effects on the behavioral approach system [9], and eveningness chronotype has cognitive and behavioral issues including attention deficit, hyperactivity and impulsiveness [3]. A study revealed that morningness type students and nurses experience better sleep quality and it was a positive predictor of resilience [10,11], but other studies insist morningness chronotypes experience poor sleep quality [12].

The purpose of this study was to investigate the effect of morningness-eveningness type of shift nurses on their sleep quality, depressive symptoms and occupational stress.

In this study, we revealed the effect of morningness-eveningness type of shift nurses on their sleep quality, depressive symptoms and occupational stress. As we age, we become more morningness types. It is because our sleep cycle is steadily advanced by circadian system as we age, and people become accustomed to waking early and working during the early hours [18]. Sleep is a parameter that is markedly impacted by shift work [19]. After night shifts, people usually sleep during the day but their daytime sleep quality is poorer compared to typical night time sleep [20]. From our study, total sleep time was statistically insignificant according to morningness-eveningness type. The World Health Organization recommends 7–8 hours of sleep for adults to recover from physical and psychiatric fatigue. However, Koreans have the least sleep time in OECD countries (Korean adults’ mean sleep time was only 6.5 hours) [21]. In this study, total sleep time of shift nurses was an average of 6.3 hours but morningness-eveningness type was not a decisive factor for total sleep time. Although total sleep time was insignificant between groups, Subjective Sleep Quality score revealed significant differences between morningness-eveningness chronotype. Post hoc analysis indicates that morningness type has better Subjective Sleep Quality compared to eveningness type. Previous studies have validated that various kinds of shift workers experience depressive symptoms [9,22]. Mean BDI score of subjects was more than 10 points, indicating mild depression but there were insignificant differences between groups.

Shift nurses have moderate occupational stress that was measured by KOSS but it did not reveal significant differences between groups. Shift nurses experience stress due to physical environments such as physical burden, risk and pollution. They have severe stress due to job demand including time pressure, excessive occupational responsibility and workload. When the stress due to job demand score is more than 70, immediate reduction of work load is recommended. Shift nurses experience moderate stress due to insufficient job control conditions including low decision-making rights and task predictability. However, occupational stress due to physical environment, job demand and insufficient job control were not associated with morningness-eveningness type. Participants revealed relatively low stress due to interpersonal conflict and job insecurity. It is because St. Vincent’s Hospital has a relatively strong support system, friendly workplace culture and stable employment system, but interpersonal conflict and job insecurity were not associated with morningness-eveningness type. Shift nurses have moderate stress due to organizational systems. Reward systems consist of financial payment, mutual respect, inner motive and skill development opportunity. These two measures (organizational system and lack of reward) were not associated with morningness-eveningness type. Stress due to occupational climate means that workers suffer from inflexible decision-making processes, workplace sexual discrimination and unwanted official dining after work hours. Subjects experienced moderate level occupational climate stress.

There are limitations of this study. First, it was conducted in one university nurse population with a self-administered questionnaire, so it may have biased responses from participants. Second, in U.S., male to female ratio of nurses is 1 to 9.5 or 8.1 percent, but all subjects of this study are females that work in the same hospital with small sampling size. Morningness type ratio was too limiting (4.3 percent) compared to other studies (range from 16.4 percent–30.5 percent) [7,23]. Therefore, generalizations should be considered with precaution. Third, this was a cross-sectional study, and therefore it was difficult to find causal relationship. Issues of health caring shift workers were not rare and eventually compromised service quality including patients’ safety. For shift nurses, eveningness chronotypes were vulnerable to shift work, especially sleep (Subjective Sleep Quality score). Poor Subjective Sleep Quality could cause various problems including day time sleepiness [24]. Naps decrease sleep pressure and reduce next day slow wave activity, so taking a nap before night shift work may be helpful to eveningness types [23]. Flexible and adaptive sleep strategies may also be helpful in decreasing sleep-wake cycle dysregulation [25]. Modafinil may be helpful for increasing awakening during night work and may be effect in modifying disturbed circadian rhythm [26,27]. More than 12-hour shift work was associated with poor job performance and lower satisfaction [28]. Therefore, limiting shift work time is necessary for shift workers’ wellbeing and job efficiency. In addition, when organizing shift work, workers’ chronotypes should be considered as well. Eveningness types usually suffer from daytime sleepiness, so regular breaks are necessary to increase work efficiency. Further studies for managing occupational stress, improving sleep quality and overall well-being with larger populations are needed for shift workers.