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Abstract
Objectives Shift work is associated with adverse physical and psychological health outcomes. However, the independent health effects of night work and rotating shift on workers' sleep and mental health risks and the potential gender differences have not been fully evaluated.
Methods We used data from a nationwide survey of representative employees of Taiwan in 2013, consisting of 16 440 employees. Participants reported their work shift patterns 1 week prior to the survey, which were classified into the four following shift types: fixed day, rotating day, fixed night and rotating night shifts. Also obtained were self-reported sleep duration, presence of insomnia, burnout and mental disorder assessed by the Brief Symptom Rating Scale.
Results Among all shift types, workers with fixed night shifts were found to have the shortest duration of sleep, highest level of burnout score, and highest prevalence of insomnia and minor mental disorders. Gender-stratified regression analyses with adjustment of age, education and psychosocial work conditions showed that both in male and female workers, fixed night shifts were associated with greater risks for short sleep duration (<7 hours per day) and insomnia. In female workers, fixed night shifts were also associated with increased risks for burnout and mental disorders, but after adjusting for insomnia, the associations between fixed night shifts and poor mental health were no longer significant.
Conclusions The findings of this study suggested that a fixed night shift was associated with greater risks for sleep and mental health problems, and the associations might be mediated by sleep disturbance.
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What this paper adds
▸ Shift work is known to increase the risks for sleep disturbance and physical and psychological health problems.
▸ The independent effects of night shift and rotating shift on workers' sleep problems and mental health risks have rarely been examined.
▸ The study findings showed that fixed night shifts were associated with greater risks for sleep disturbances than other shift types.
▸ Fixed night shifts were associated with higher risks for burnout and mental health problems in women but the associations disappeared after controlling for insomnia, suggesting that insomnia may play a mediating role.
Introduction
Shift work is defined as a way of organising daily working hours in which different people work in succession to cover more than the usual 8-hour day up to 24 hours.1 A recent study of representative employees of Taiwan indicated that the prevalence of non-standard shifts including night shifts and rotating shifts had increased substantially during the period from 2001 to 2010, from 17% to 24% in men and 12% to 20.4% in women.2 Since shift work has been an unavoidable feature of the 24-hour society, its associated health impacts should be of important occupational health concerns.
In addition to increased risks for disturbances in cardiovascular and neuroendocrine functions as well as various types of cancer, shift work has been associated with higher risks for sleep problems, fatigue and psychological symptoms.1 ,3–7 Mechanisms linking shift work to sleep and mental health problems may involve both biological and social aspects. On the one hand, night shifts are known to disturb the circadian rhythm in melatonin secretion, stress hormones and autoimmune functions, leading to impaired sleep quality and psychological well-being. On the other hand, night shifts may also impair work–life balance and social interactions, leading to greater mental distress.8
Some studies found that rotating night shifts were associated with greater health risks than permanent night shifts, suggesting that the former shift type might involve greater disruptions in physiological rhythms.9 ,10 Early shift changeovers and backward-rotating shifts have also been found to associate with greater health risks as compared with late shift changeovers and forward-rotating shifts.11 ,12 Furthermore, gender differences in vulnerability have been reported.1 ,8 ,13 Some studies indicated that female workers had lower tolerance to shift work, probably due to greater difficulties in coordinating irregular working schedules with domestic duties. Other studies further suggested that night work had a greater negative impact on men's mental health, while women's mental health was more adversely affected by irregular shift patterns.13 It has been suggested that disturbed social life caused by shift work may affect women to a greater extent than men due to greater social expectation of women in a family role.14 Nevertheless, the independent health effects of two domains of shift work, that is, night shift and rotating shift, and the potential gender differences in the shift work–mental distress associations have rarely been studied.
While it can be expected that shift work as well as other unfavourable working situations are likely to cluster in less advantaged groups, few studies have taken into account potential confounders such as socioeconomic status and psychosocial work characteristics. A longitudinal study found elevated risks of depressive mood among shift workers; however, the associations became not significant when demographic variables were adjusted.4 A number of psychosocial work hazards including workplace violence, high job demands, lack of job control, job insecurity and poor workplace justice have been found to correlate with poor mental health outcomes.15 ,16 Yet whether or not these adverse work characteristics may contribute to greater mental health risks in shift workers has not been evaluated.
To the best of our knowledge, large-scale population-based surveys looking at the independent effects of night shifts and rotating shifts are limited, especially in non-western populations. In this study, we used data from a national survey of representative employees in Taiwan to examine the associations of two specific features of shift work, that is, night work and rotating shift, with the risks of short sleep duration, insomnia, burnout and minor mental disorder after adjusting for selected psychosocial work conditions. Gender-stratified analyses were carried out because we anticipated that shift work may affect men and women in a different manner. Since previous studies suggested that sleep disturbances, rather than shift work per se, increase the health risks in shift workers,11 ,17 ,18 we also examined the associations of shift work with burnout and minor mental disorders after controlling for insomnia, to evaluate the potential mediating effects of insomnia.
Methods
Study populations
In this cross-sectional study, we used data from a national survey of representative employees conducted in 2013. The Ministry of Labor of Taiwan has conducted nationwide surveys of the working population every 3–5 years since 1988, to understand occupational safety and health issues and concerns. A two-stage random sampling process was adopted. In the first stage, all districts and villages were grouped into strata according to the levels of urbanisation, and a random sample of districts and villages was chosen from each stratum. In the second stage, a random sample of households was selected within each district or village, and residents of the sampled households who were currently working at the time of the survey were identified and invited to participate. Detailed information with regard to the sampling scheme of the survey can be found elsewhere.2 ,15 ,19 Self-administered questionnaires were delivered to the selected households by trained interviewers. After 1 week, completed questionnaires were collected and on-site checking was performed by the same interviewer.
For the survey of 2013, a total of 28 677 participants were sampled, and 25 480 participants completed and returned questionnaires, with a response rate of 89%. After excluding employers and the self-employed, only 19 654 employees were included. Those who were aged younger than 25 years or older than 65 years, and those who had no work shift data, were excluded. This resulted in a total sample of 16 440 participants. Information concerning demographic characteristics including gender, age and education attainment was obtained by the questionnaire. Educational attainment was divided into three categories: primary school (6 years or less), secondary school (7–12 years), and university or above (more than 12 years).
Sleep and mental health status
Information with regard to sleep duration, presence of insomnia, burnout status and minor mental disorder was obtained by a self-administered questionnaire. Participants were asked if they had poor sleep quality or difficulty in falling asleep within 1 week prior to the interview, and the response to this single-item question was recorded on a scale ranging from none, seldom, occasionally to usually. Those who answered ‘occasionally’ and ‘usually’ were classified as having insomnia. Participants were also asked to provide information concerning the length of sleep. In this study population, the mean sleep time was 7.1±1.1 hours; thus, we classified those with sleep time shorter than 7 hours per day as ‘short sleep time’.
Burnout status was assessed by the five-item scale for personal burnout from the Chinese version of the Copenhagen Burnout Inventory.20 The responses were recorded on a five-point scale: ‘always’ (score 100), ‘often’ (75), ‘at times’ (50), ‘not often’ (25) and ‘never’ (0), and a mean score of 50 or above was classified as having burnout in this study.
Furthermore, the five-item Brief Symptom Rating Scale (BSRS-5) comprising the dimensions of anxiety, depression, hostility, interpersonal sensitivity and additional symptoms was used to assess participants' mental health status.21 The five items assessed the following conditions over the past week: (1) feeling tense or keyed up, (2) feeling depressed or in a low mood, (3) feeling easily annoyed or irritated, (4) feeling inferior to others, and (5) having trouble falling asleep. For each item, the response was recorded on a Likert-type scale, ranging from 0 (not at all), 1 (a little bit), 2 (moderately), 3 (quite a bit) to 4 (extremely). A sum score of 6 or above based on the BSRS-5 has been known to identify minor mental disorder with good sensitivity and specificity.21 ,22
Work characteristics
Study participants were asked to provide information regarding their total working hours and the status of shift work during the past week prior to the survey. They were first asked if their work shifts were fixed or on a rotating/non-fixed basis, and then asked if their work shifts involved night work, which was defined by having a work schedule encompassing midnight. Work shifts that started in the morning, afternoon or evening but did not extend to midnight were all classified as day shift. Study participants were accordingly categorised into the following four types of shift work: fixed day shift, rotating day shift, fixed night shift and rotating night shift.
Psychosocial job demands and job control were assessed by the Chinese version of the Job Content Questionnaire (C-JCQ) based on Karasek's Job Strain Model.23 ,24 In the survey, there were seven items for the demand scale and nine items for the job control scale. The latter consisted of two subcomponents: six items for the skill discretion subscale (learning new things, high level of skills, non-repetitive work, creative work, various tasks, developing one's abilities) and three items for the decision authority subscale (allowed to make own decisions, freedom to make decision, opinions influential). The Job Strain Model postulated that the combination of high psychological job demands and low job control causes job strain and stress-related illness. Many studies classified the study population into four job strain categories by the median values or other arbitrarily defined scores of job demands and job control. In this study, however, we chose to use job demands and job control scores independently, because several studies have suggested that the mental health effects of high job demands and low job control might be different and the associations might not be linear.25 ,26
A seven-item scale for the assessment of workplace justice (trust, information reliable, work arranged fairly, rewards arranged fairly, performance evaluated fairly, information during decision-making process, respect) was also adopted, which was modified from the original standard questionnaire and showed good psychometric properties.16 All items aforementioned were listed as a statement with response recorded on a four-point Likert scale ranging from 1 (strongly disagree) to 4 (strongly agree). Items stated oppositely were reversely coded and the mean scores were then calculated and standardised. These scores were ranked and divided into tertiles (low, medium and high) in regression models.
Also included in this study was one item for job insecurity (my job is secure), which measures the study participant’s perceived likelihood of not being able to maintain the desired continuity of his or her current employment, and the response was coded dichotomously (agree/disagree).
Study participants were also asked if they have experienced any of the following four types of workplace violence, namely physical violence, verbal violence, psychological bullying and sexual harassment over the past 12 months prior to the survey, and responses were dichotomised (yes/no).
Statistical analysis
Descriptive analyses were performed to compare demographic characteristics, psychosocial work conditions and sleep and mental health status across four shift-type groups. Differences were tested with a χ2 test for categorical variables and analysis of variance for continuous variables. Tukey's test for categorical variables and Bonferroni correction for continuous variables were used for post hoc analysis.
Multivariate logistic regression models were used to examine the associations between shift schedules (independent variable) and sleep and mental health status (outcome), separately for men and women. Age, educational level and psychosocial work conditions were included in the regression models as confounders. Insomnia was further included in these regression models to examine its intermediating effects in the association between shift schedules and mental health status. The interaction effects of night work and rotating shift (both dichotomised) on health outcomes were also examined. SAS V.9.4 (SAS Institute, Cary, North Carolina, USA) was used for all of the analyses and the significant level set at p<0.05.
Results
As shown in table 1, workers engaged in night shifts were younger, more likely to be male than female, and had significantly higher percentages of long working hours defined by having weekly working hours of 60 or more. It is worth noticing that the percentage of weekly working hours of 60 hours or more was 16.9% for night workers with a fixed schedule, while the percentage was 3.8% in day workers with a fixed work schedule. Workers with fixed night shifts also reported significantly lower job control, lower workplace justice, and higher prevalence of workplace violence and job insecurity. Among day workers, those with rotating work schedules had a greater percentage of women, a higher percentage of short working hours defined by weekly working hours of <40 hours, and a higher level of job insecurity than those with a fixed schedule. These findings suggested that day workers with irregular work shifts were more likely to be temporary part-time workers.
Demographic characteristics, work conditions and sleep and mental health status in workers with different shift types
In terms of health outcomes, descriptive analyses showed that workers with fixed night shifts had the shortest sleep duration, highest prevalence of insomnia, highest level of burnout and highest prevalence of minor mental disorder across all the four shift work types.
The associations between shift work types with sleep and mental health outcomes were examined with adjustment of age, education and work characteristics. As shown in tables 2 and 3, male and female workers with a fixed night shift had 2.09-fold and 4.33-fold increased risks for short sleep duration, and 1.61-fold and 2.22-fold increased risks for insomnia, respectively, as compared with workers with a fixed day shift. Fixed night shifts were also associated with increased risks for burnout and minor mental disorders in female workers, but the associations were not significant in male workers. Among female workers, when insomnia was further controlled in the regression models, the association between fixed night shift and the risks for burnout (OR=1.59, 95% CI 0.82 to 3.11) and minor mental disorder (OR=1.30, 95% CI 0.59 to 2.89) becomes insignificant (data not shown).
Multivariate logistic regression models of short sleep duration, insomnia, burnout and minor mental disorder in men with adjustment of age and education (n=9165)
Multivariate logistic regression models of short sleep duration, insomnia, burnout and minor mental disorder in women with adjustment of age and education (n=7275)
Discussion
The findings of this study showed that workers engaging in night shifts experienced poor mental health and reported more sleeping problems as compared with day workers. Gender-stratified regression analyses with adjustment of age, education and psychosocial work conditions showed that both in male and female workers, a fixed night shift was associated with the greatest risks for short sleep duration and insomnia among the four shift types. In female workers, fixed night shifts were also associated with increased risks for burnout and mental disorders. However, after adjusting for insomnia, the associations between fixed night shifts and poor mental health disappeared, suggesting that insomnia might play a mediating role in the associations between shift work and poor mental health.
Our findings were consistent with some previous studies which reported higher risks of sleep problems and depressive symptoms in night shift workers than day shift workers.27 ,28 Several previous studies also showed that night shift workers have shortened sleep time than day shift workers, and the problem was more apparent among workers with permanent night shifts.11 ,29 ,30
The mediating effects of insomnia in the association between night shift and disturbed physiological and psychological health status had been reported in previous studies.11 ,17 ,18 That suggested that the disturbance of the internal circadian rhythm and/or insufficient recovery due to sleep disorders might play an important role in mediating the effects of shift work on workers' mental health risk. Accordingly, researchers have suggested that interventions aiming to improve mental health status among shift workers should incorporate strategies for the improvement of sleep quality, including changes in individual behaviours and sleeping environment as well as organisation-level work schedule redesign.31
However, conflicting findings have been noticed. For example, in a female-dominant study population of nurses, a rotating night shift was found to be associated with increased social and health symptoms than a fixed night shift.32 Yet Vallières et al 17 did not find significant differences in the prevalence of insomnia and mental health problems between rotating night workers and fixed night workers. Inconsistencies in the existing literature might be explained by the failure in controlling for potential confounders associated with shift work such as psychosocial conditions, failure to distinguish the effects of night shift and rotating shift, and/or different definitions and classifications for shift work and night shift across studies. Reasons for less health damaging effects of a rotating night shift than a fixed night shift observed in our study deserved further investigation. More information with regard to the type of rotating patterns is also needed. In a recent systematic review of studies focusing on the effects of consecutive night shifts on workers' adaptation based on diurnal rhythms in cortisol, melatonin and heart rate variability, Jensen et al 7 found that adaptation to night work had not occurred after two consecutive night shifts in most studies, but in a few studies full adaptation of workers was documented after seven consecutive night shifts. These findings indicated that the intensity and shift work arrangement should be considered when evaluating the health impacts of night and irregular shifts.
In this study, the associations between a fixed night shift and poor sleep and mental health outcomes were stronger in women than in men, suggesting that women might have a greater difficulty to cope with fixed night work than men. Barton32 reported that men and women chose night shift and rotating shifts based on different reasons—women were more likely to modify their work patterns because of domestic responsibility, and thus were more likely to be affected by irregular and rotating shifts than men. Greater family care burdens for women than men might also contribute to the observed differences in our study.14
Several study limitations should be mentioned. First, owing to the cross-sectional nature of this study, causal inference of the shift work pattern with sleep and mental health outcomes cannot be confirmed. Since it is expected that shift work is often associated with poor work conditions and is less favourable to most workers, the observed associations between shift work and poor health might be partially related to selection effect, that is, socially disadvantaged groups with poor health might have been selected into shift work rather than standard day work. Future studies should adopt a longitudinal study design following workers' occupational paths to confirm the causal relations between shift work and health outcomes. Regardless of the direction of causation, however, the fact that shift workers had greater health risks still poses an occupational health concern.
Second, owing to the limitation of the questionnaire design, work schedules could only be classified into day versus night shift and fixed versus rotating shift, leaving other domains of shift work undetermined. The self-reported nature of this survey and the absence of register-based records for work schedule were another study limitation, leaving the validity of shift-type categorisation questionable. Future research should specify different aspects of shift work and improve the assessment of shift work patterns.
Third, individual differences in vulnerability to sleep disturbance could be expected. For instance, people with a late chronotype generally have a higher shift work tolerance.12 On the one hand, evening-type people who have better ability to cope with shift work may have self-selected into night jobs. On the other hand, workers with poor health conditions might have quit shift work, leading to a healthy worker selection bias. As a result, the prevalence of sleep problems and other health risks in shift workers might have been underestimated. Longitudinal studies following workers' occupational paths are needed to confirm the causal relations between shift work and health outcomes and to avoid selection effects.
Finally, the assessment of sleep-related problems in this study was rough. Future studies should adopt more comprehensive and ideally more objective measures to capture different aspects of sleep problems.
Despite the aforementioned limitations, the findings of this study still documented that workers engaged in night shifts were more likely to have short sleep duration, insomnia, burnout and minor mental disorder than day shift workers. Furthermore, we found that among night shift workers, those on a fixed night shift schedule were at higher risks for sleep problems than those with a rotating night shift.
The findings of this study suggested that a fixed night shift was associated with greater risks for sleep and mental health problems, and the associations appeared to be mediated by sleep disturbance. While more studies are needed to fully understand the biological and social consequences of night shift and rotating shift in different genders and different subgroups, we suggest that employers and occupational health practitioners should pay attention to the impacts of shift work on workers' health and well-being and design a suitable working schedule to minimise its health impacts.
Acknowledgments
The authors acknowledge the contributions of the Institute of Labor, Occupational Safety and Health, Ministry of Labor of Taiwan for data collection and data management. This study was supported by a research grant from the China Medical University Hospital, Taiwan (DMR-106-100).
References
Footnotes
Contributors W-JC conceived the study, conducted data analyses and drafted the manuscript. YC provided guidance in research design and data analyses and revised the manuscript. Both authors contributed equally to the refinement of the final manuscript.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.