Night Work and the Risk of Depression
A Systematic Review
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Background: Working the night shift interferes with the circadian chronobiological rhythm, causing sleep disturbances, fatigue, and diminished well-being, and increases the risk of serious disease. The question whether night work increases the risk of depression has not been adequately studied to date.
Methods: We carried out a systematic, broadly conceived literature search in the PubMed, Scopus, PsycINFO, and PSYNDEX databases and the Medpilot search portal on the topic of nighttime shift work and mental illness.
Results: The search yielded 5682 hits, which were narrowed down by predefined selection criteria to 11 high-quality longitudinal studies on the relationship between nighttime shift work and depressive illness. Only these 11 studies were subjected to further analysis. 3 of 4 studies on nighttime shift work in the health professions (almost exclusively nursing) revealed no association with depression over an observation period of two years. On the other hand, 5 studies on nighttime shift work in occupations outside the health sector, with observation periods of two or more years, yielded evidence of an elevated risk of depression after several years of nighttime shift work, but not in any uniform pattern. A supplementary meta-analysis of 5 of the studies revealed a 42% increase of the risk of depression among persons working the night shift (95% confidence interval [0.92; 2.19]). Psychosocial working conditions that have a negative influence on health partially account for these associations.
Conclusion: Although there is evidence that nighttime shift work (at least, in occupations outside the health sector) does increase the risk of depression, this evidence is not strong enough to sustain a general medical recommendation against shift work for employees with depressive conditions. It would seem appropriate to address this question on an individual basis, with strong support from physicians and close attention to the deleterious psychosocial factors associated with shift work.
Approximately 14% of the German workforce work at night—defined as between 11 p.m. and 6 a.m.—at least occasionally, and 15% work alternating shifts, e.g. early shift and late shift or early shift, late shift, and night shift (1). Shift work is associated with increased physical and psychological strain in the short term, and in the long term it increases the risk of serious illness. Although scientific debate has not yet reached a clear conclusion, particularly regarding cancers, recent meta-analyses show an increased risk of coronary heart disease (relative risk: 1.23; 95% confidence interval: [1.15; 1.31]) (2), diabetes mellitus (odds ratio: 1.09; [1.05; 1.12]) (3), metabolic syndrome (relative risk: 1.57; [1.24; 1.98]) (4), and cancer (relative risk: 1.19; [1.05; 1.35]) (5), particularly breast cancer (relative risk: 1.089; [1.016; 1.166]) (6, 7).
Those who work shifts do not work and sleep in line with their endogenous circadian rhythm, so biological and social time are misaligned. This is essentially what happens after a swift journey through multiple time zones and is therefore also known as „social jetlag“ (e1). The endogenous circadian rhythm determines the need for sleep and wakefulness, body temperature, heart rate, blood pressure, and hormone levels, particularly levels of cortisol and melatonin (e2). Exogenous influences known as zeitgebers, such as light in the light-dark cycle of a 24-hour day, synchronize the endogenous circadian rhythm with the astronomical solar day (e3). Although any change to the phases of waking and sleeping caused by shift work does affect the endogenous chronobiological rhythm, external zeitgebers are dominant. Chronobiological adaptation of the sleep-wake cycle in those who work night shifts is therefore usually inadequate (e4, e5). Total melatonin production over the 24-hour day is reduced by nocturnal light exposure; this also plays a role in the risk of breast cancer (5) (e6). Disrupted cortisol regulation is thought to play an important role in arteriosclerotic diseases such as coronary heart disease (e7). As an additional consequence of disrupted circadian rhythm, a reduction in mean length of sleep by one hour or more—particularly in those who work night shifts, but to a lesser extent also in those who work early shifts (e8)—can increase the activity of the sympathetic nervous system and systemic inflammatory precursors (e9). Independently of its biological effects, shift work is often associated with psychosocial strain (e10) or less healthy behavior (e11, e12).
The most recent review on the effects of shift work on mental health and affective symptoms is a scoping review performed by Germany‘s Federal Institute for Occupational Safety and Health (Bundesanstalt für Arbeitsschutz und Arbeitsmedizin) (8), which is based on reviews and original articles, especially cross-sectional studies. The authors of the scoping review conclude that findings vary, particularly regarding alternating shifts. According to the review, there seems to be a tendency for night work to be associated with adverse consequences for mental health, most commonly manifesting as acute or chronic exhaustion (8). Although there are clear correlations between disrupted chronobiology and mental illness (9), as yet the effect of shift work that includes night shifts on the onset or exacerbation of specific mental illnesses, particularly depression, remains unclear.
This systematic review therefore focuses on how shift work affects the onset and progression of depression. It is based on prospective studies, as only these allow conclusions to be drawn on potential causes and effects.
Literature search and selection
In order to include both medical and psychological literature, the literature search in October 2015 involved the databases PubMed, Scopus, PsycINFO, and PSYNDEX, as well as the search platform Medpilot. No limit was placed on year of publication. Terms such as „burnout,“ „common mental disorders,“ and „mental illness“ in connection with shift work were included. There were 5682 hits in total (eTable 1). These were selected according to the following criteria, on the basis of their titles and, where appropriate, abstracts:
- Population: working individuals
- Exposure: shift work that included night work between 11 p.m. and 6 a.m.
- Comparison group: individuals working during the day or with a varying frequency of night shifts
- Outcome: depression measured using a valid method or another valid indicator of a depressive disorder (eTable 1)
In addition, studies in which shift work could not be separated from other working conditions that were harmful to health, those in which multiple mental illnesses were target parameters, those in which depressive disorders could not be clearly defined, and those with no statistical analysis on the correlation between shift work and depression were not included. Selection was always performed by 2 of the authors.
The bibliographies of the 35 original papers, the scoping review (8), and the 3 earlier relevant reviews on shift work and mood disorders (10–12) that remained after several stages of selection were examined to see if any other studies could be identified from them. Of the 47 suitable fulltexts, 11 original papers were included (Figure). Data was extracted by 2 authors independently of each other and according to a pre-established evaluation plan (Table 1, extended version in eTable 2) and presented in Table 1. No subgroup analysis was initially planned. Multiple evaluations or publications based on the same cohort but examining different aspects were selected (Table 1, extended version in eTable 2) but were included in the evaluation only once. The studies were rated for risk of bias according to the criteria stated in the Quality in Prognosis Studies (QUIPS) checklist and in line with published measures (13, 14) that had already been used in reviews on shift work that examined other target parameters (15). As recommended (14), scoring was not used (Table 2).
In addition, 5 of the studies for which the necessary information was available (Table 1, extended version in eTable 2) were summarized in a meta-analysis (eBox, eFigure).
A total of 11 publications, based on 9 prospective studies, met the inclusion criteria for this review.
Because the studies included in the analysis varied in terms of country, occupational group, case number, length of follow-up, and adjustment for potential confounding factors (Table 1, extended version in eTable 2), their findings also differed.
Five publications find a correlation between shift work and an increase in depressive symptoms or onset of depression during the observation period, at least in some subgroups (16–20). Three (21–23) found no evidence of a correlation. Out of 3 publications based on the same cohort of Norwegian nurses, only the publication with the longest follow-up period was included in this analysis, representing all 3 (21). The findings of the other 2 (24, 25) are in line with those of the included publication. Another study reports a correlation between shift work and reduced depressive symptoms in a subgroup of workers who stated that they had more autonomy in their schedule planning (26).
To summarize by investigated population and length of follow-up, studies that investigated nurses or those in related professions for a maximum of 2 years mostly find that there is either no correlation or a favorable association between shift work and depression (21, 22, 26). An exception to this is the oldest study—also the one with the smallest case number—which is quasi-experimental in design and does find evidence of an increase in depressive symptoms (16). In contrast, studies investigating large samples of a more or less unselected population with a follow-up lasting between 4 and 10 years do find evidence of an increased risk of depression in a subgroup of shift workers: in one case, there was a correlation between shift work and risk of depression in women only. In men, the correlation disappeared after adjustment for psychosocial working conditions (17). In another case, the association between shift work and depression is found only for men working night shifts and women working alternating shifts (18), for white-collar but not blue-collar workers (19), or not at all after adjustment for psychosocial working conditions (20). However, a supplementary meta-analysis also indicates an increased risk (risk estimate: 1.42; [0.92, 2.42]) (eFigure).
The shift workers and daytime workers included in these studies differed from each other in terms of a number of characteristics, only some of which are shown explicitly: in particular, there are statistically significant differences regarding age (20, 22, 23), sex (20, 26), school and occupational education (23, 27), weekly working hours (23, 26), family situation (22, 26), baseline psychological wellbeing (20, 26), psychosocial working conditions (20, 25, 26), and professional position (23). However, differences in these characteristics that themselves affect mental health and might therefore lead to bias were adjusted for in all the studies (Table 1, extended version in eTable 2).
Several studies investigated the effect of differences in a wide range of psychosocial working conditions on the correlation between shift work and depressive disorders. In 3 studies, the findings regarding the effect of shift work on depression did not change after adjustment for psychosocial working conditions (16, 19, 25). In contrast, in 3 other studies the correlation did change when the following working conditions were adjusted for (17, 20, 26): decision latitude, social support by superiors and colleagues, demands of work, pride in work, and control over working hours.
None of the studies investigated whether selection caused by excluding sensitive or sick employees from shift work may have led to a healthy worker effect resulting in no differences being found between shift workers and daytime workers. Thun et al. (21) did find evidence of selection: depressive mood improved in individuals who switched from night shifts to day shifts during the observation period. Driesen et al. (20) show that depressive mood increases the probability of switching from night shifts to day shifts. In a sensitivity analysis, Norder et al. (23) conclude that their findings do not change when only individuals who did not switch their shift plans during the observation period are included.
To the best of our knowledge, we have put together the first systematic review on the correlation between shift work that includes night shifts and depression, which is the group of mental illnesses that plays a dominant role in the workplace (28). The studies evaluated in this review, all of which are prospective, yield evidence both for and against an increased risk of depression in those who work shifts. These correlations show patterns.
The finding that there is no such association among nurses is essentially based on 2 studies conducted in Norway (21, 24, 25) and Denmark (26). It can be speculated that favorable working conditions in these countries, such as opportunities to determine one‘s own shift plan (26), reduce the damaging effects of shift work. Other psychosocial factors may also play a dominant role among nurses, mask the effects of shift work, and explain why depression is a greater burden in this occupation than in the general population (29). Finally, it is possible that shift work only increases the risk of depression after several years.
In contrast, studies that drew their samples from the general population and thus from a wide variety of occupations did find an increased risk of depressive disorders in some subgroups over a long observation period (more than 4 years) (17–20). These studies examined workers who differed from each other in terms of country and culture, training and profession, psychosocial working conditions, socioeconomic status, and other characteristics that have a wide range of effects on the risk of depressive disorders. As they nevertheless concurred in finding that shift work affected depression, although in different subgroups and with differing definitions of exposure, it seems more likely that there truly is a correlation. The findings of the supplementary meta-analysis essentially confirm the conclusion of the systematic review: prospective, high-quality studies indicate that in all occupational groups shift work that includes night work can increase the risk of depression. However, this evidence cannot be statistically confirmed at this time. The statistical power of the meta-analysis is limited by the fact that so few studies could be included, and by the great variation between them in terms of case number, length of observation period, occupation, country, and measurement of target parameters.
The lack of a correlation between shift work and depression may be due to sick individuals switching from shift work to daytime work (a healthy worker effect). Exhaustion, need for rest, sleep disorders, poorer overall health, or conflict between work-related and private responsibilities increase the probability of switching to day shifts (30). Workers react to the first symptoms of worsening mental health by switching from shift work (31).
The observation that in 2 of the 4 studies (17, 20) correlations disappeared after adjustment for psychosocial working conditions according to the job demand-control-support model (work stress) suggests that the correlation between shift work and depression is caused at least in part by differences in work-related stress between night and other shifts, and thus only partly by chronobiological strain due to work being misaligned with the body clock. Differences in psychosocial working conditions between shift workers and daytime workers can also be found where there was no correlation with depression (23). That fact that psychosocial working conditions partly determine the risk of depressive disorders has been definitively confirmed (32, 33). That would shift the practical focus of prevention from night shift work or shift planning to psychosocial working conditions that are typically less favorable for night shift work than for daytime work. This includes, for example, reduced social support by superiors (20, 26, 34), less freedom to make decisions and less control over one‘s work (34, 35), increased conflicts at work (35), and more experience of violence (34).
There is little doubt that shift work can have short-term affective consequences, such as worsened mood, which are similar to depressive symptoms. However, there is no evidence that these acute complaints increase the risk of developing a clinical depressive disorder in the longer term (10). In order to provide an at least plausible explanation of how shift work might cause depression, the direct consequences of disruption to the chronobiological rhythm must be considered: reduced sleep time (36, 37, e8), reduced sensitivity of peripheral cortisol receptors and functional hypercortisolism (38, 39), and reduced melatonin production (40).
In addition to the scoping review mentioned above (8), all earlier reviews of which we are aware included mostly cross-sectional studies, with only a few of the 11 truly longitudinal studies evaluated here. Most of these publications did find an association between working night shifts and exhaustion, sleep disorders, and reduced wellbeing (11, 12). Regarding an association between working night shifts and mental illness, particularly depression, Tucker and Knowles conclude that the hypothesis of a correlation is at least supported (10). Vogel et al. rate the evidence provided by studies as lacking solid proof of a correlation (12) but also indicate the role of psychosocial factors (12).
Interpretation of our findings is limited by the fact that although we performed a comprehensive search of the literature we were able to find only a very small number of studies that could be included in the analysis. Because correlations were found only in varying, non-comparable subgroups, the power of the meta-analysis was limited. Similarly, it was not worthwhile to use a scoring system to provide a quantitative evaluation of the evidence (15); moreover, the QUIPS criteria do not recommend this (14). Our findings cannot be extrapolated to other psychiatric illnesses. If the findings were extrapolated to other shift systems that did not include night work, the effects found would probably tend to be weaker.
Even after decades of research into the impact of shift work on health, it remains unclear whether shift work—in its chronobiologically most marked form, shift work that includes night work—can increase the risk of developing or exacerbating depression. While studies in nurses do not confirm such a correlation, there are multiple findings from high-quality studies conducted in the working population in general that do suggest correlations. However, no definitive conclusion can be drawn because the findings differ concerning specific groups and circumstances in which the risk is increased. It seems that, in addition to the direct effects of disrupted chronobiological rhythm on sleep, hormone regulation, and many other biological functions, psychosocial working conditions with adverse effects on health, which are qualitatively and quantitatively different for shift workers and those who work during the day, play a role in mediating certain effects. There do not seem to be grounds for medical professionals to advise all individuals who are at increased risk of depression or have suffered depression to avoid shift work. Instead, it may be beneficial to adopt a highly personalized approach, closely observing work-related complaints and noting psychosocial working conditions that entail psychological stress. The best way to achieve further advances in knowledge is by conducting large cohort studies in which shift systems, working conditions, individual characteristics, and illness are differentiated and documented using a valid procedure.
The authors would like to thank Prof. Dr. Bernd Richter of the Cochrane Metabolic and Endocrine Disorders Group at the Institute of General Practice and Family Medicine (Institut für Allgemeinmedizin), Centre for Health and Society at the University of Düsseldorf, for his critical examination of the manuscript of this article.
Conflict of interest statement
The authors declare that no conflict of interest exists.
Manuscript received on 10 September 2016, revised version accepted on
15 March 2017.
Translated from the original German by Caroline Shimakawa-Devitt, M.A.
Prof. Dr. med. Peter Angerer
Institut für Arbeits-, Sozial- und Umweltmedizin
Centre for Health and Society
Heinrich Heine University Düsseldorf
40225 Düsseldorf, Germany
For eReferences please refer to:
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Medical Faculty Heinrich Heine University Düsseldorf: Prof. Dr. med. Angerer, Dr. phil. Schmook,
Irina Elfantel, Dr. rer. sec. Li, MD, MPH, PhD
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