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Keywords:

  • alertness;
  • napping;
  • productivity;
  • recovery;
  • sleep

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Difficulties Faced by Shift Workers
  5. Strategies to Mitigate Damage from Shift Work
  6. Future Challenges
  7. Conclusions
  8. Acknowledgment
  9. References

Shift work poses a significant threat to multiple aspects of working life. The present review addresses the health, safety/performance, and psychosocial issues associated with shift work. Recent studies have revealed that health disorders due to working in shifts are closely related to dysfunction of sleep and the circadian timing system. Safety and performance problems such as occupational injuries and poor work performance that occur during abnormal working hours can be explained by impaired neurobehavioral function. An increased level of understanding regarding shift work problems as well as sleep and circadian biology should be translated into evidence-based strategies to assist shift workers. Although some promising countermeasures including sleep and circadian approaches have been proposed, many questions regarding better shift schedules and fatigue risk management remain unsolved. In particular, shift work has been defined differently in previous studies, leading to confusion and errors when estimating the associations between shift work and outcomes. Additionally, there are no validated, reliable indicators to represent the adaptability of shift work. Such indicators, if available, could be used for health evaluations of current shift workers and the early detection and treatment of poor adaptation to a given shift schedule. Clearly, modern society exists with individuals who work in shifts. Therefore, effective countermeasures must be developed to reduce the risks arising from shift work. Sleep and circadian biology are expected to direct these processes.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Difficulties Faced by Shift Workers
  5. Strategies to Mitigate Damage from Shift Work
  6. Future Challenges
  7. Conclusions
  8. Acknowledgment
  9. References

Most individuals believe that a pattern of life in which humans sleep at night and work in the daytime should be considered the only “normal” pattern. However, a fraction of the population, night and shift workers, considers this pattern “unusual or even abnormal”. Globally, large numbers of individuals work during the night and/or during shifts, including 27.3% of workers in Japan,[1] 28.7% in the United States,[2] and 17.0% in the European Union.[3] Modern society is functioning with significant efforts from these shift workers. Simultaneously, sleeping and working at abnormal hours exert considerable impacts on the shift workers' health, safety, and well-being.

Rapid advances have been achieved in the field of sleep and circadian rhythm.[4, 5] In this field, generating sufficient evidence might only be the first step toward the final goal of enhancing an individual's quality of life. A more essential question is about the optimization of the available scientific knowledge in order to help shift workers. In other words, sleep and circadian biology are expected to be the principal domains associated with better understanding and management of shift-related problems.

This review shows an overview of several aspects of health, safety, and well-being that have been associated with shift work, and is prepared according to recently published data. Additionally, the present review evaluates possible countermeasures against these shift-related disturbances. Generally, increased options for shift work situations will be welcomed and, consequently, we must identify the costs and benefits of each countermeasure to ensure the appropriate implementation.

Difficulties Faced by Shift Workers

  1. Top of page
  2. Abstract
  3. Introduction
  4. Difficulties Faced by Shift Workers
  5. Strategies to Mitigate Damage from Shift Work
  6. Future Challenges
  7. Conclusions
  8. Acknowledgment
  9. References

Various problems in multiple aspects of life occur during shift work. Herein, we address the health, safety/performance, and psychosocial domains and focus on the recent research findings within each domain.

Health concerns

Sleep

Most shift workers complain of poor and reduced sleep despite a desire for good sleep. The workers are also adversely affected by excessive sleepiness during night shifts. Therefore, sleep disturbances are considered representative of shift work-related occupational problems.[6] In turn, inadequate sleep might be an important risk factor for health dysfunction in shift workers.[7] Furthermore, recent studies have shown that impaired sleep and health are observed even after retirement, suggesting that shift work can induce life-long issues.[8, 9]

Extensive efforts have been made to classify shift work-associated sleep problems as a sleep disorder. Indeed, the International Classification of Sleep Disorders: Diagnostic and Coding Manual second edition (ICSD-II) has included shift work type (shift work disorder, SWD).[10] Despite this inclusion, insufficient data are available for the prevalence of SWD. Table 1 summarizes the currently available SWD prevalence data. The values vary considerably according to the occupation, shift schedule, and definition of insomnia or excessive sleepiness. ICSD-II has provided diagnostic criteria for SWD but does not specify the intensity/severity of either insomnia or excessive sleepiness. This limitation could cause confusion among researchers, and therefore explicit criteria are critical to a more accurate estimation of workers with SWD.

Table 1. Summary of reported prevalence of shift work disorder (SWD)Thumbnail image of
Mental health

Depression, anxiety, and other mental disorders along with sleep disturbances are believed to be associated with shift work. Depression, anxiety, and burnout have been identified as important outcomes of SWD.[12] The hypothesis that shift work could be a predictive factor for future mental illness, however, has not necessarily been supported.[18, 19] Possible reasons might include the influences of other relevant factors (e.g., the psychosocial work environment) and an early transition to permanent day work when a depressive mood is experienced.[19] The latter would shift the association between shift work and mental health outcomes toward null.

Cardiovascular and cerebrovascular function

Coronary heart disease has been studied most extensively in the field of shift work. A couple of pioneering studies have shown an increased risk of coronary heart disease approximately 10 years after beginning shift work.[20, 21] The timing at which this risk would increase should be considered essential, so that occupational health staff at the worksite would implement appropriate preventive measures by that time.

Hypertension is an important and common condition that is closely associated with shift work.[22, 23] In a historical cohort study, 63%, 29%, and 10% of shift workers exhibited 10%, 20%, and 30% increase in their systolic blood pressures from the baseline (126 mm Hg), respectively, over a 14-year period.[24] The corresponding data indicated that 51%, 21%, and 6% of the day-working counterparts exhibited 10%, 20%, and 30% increase in their systolic blood pressures from the baseline (127 mm Hg). Considering the number of observed years per person, it might take approximately 10 years to develop hypertension.

The available evidence regarding cerebrovascular disorders among shift workers is quite limited. A case-control study with a small number of participants (shift workers, n = 138; day workers, n = 469) did not reveal any significant association between shift work and ischemic stroke.[25] A prospective study of approximately 70 000 female nurses demonstrated an elevated risk of ischemic stroke after performing shift work for ≥15 years.[26]

Metabolism

Impaired glucose metabolism has been reported among shift workers,[27] and diabetes mellitus has also been a research focus.[28] A recent study suggested that shift work duration of ≥15 years was associated with the development of impaired glucose metabolism.[29] A finding of another prospective study indicated that the risk of diabetes mellitus increased slightly (adjusted hazard ratio, 1.10; 95% confidence interval [CI], 1.02–1.18) in participating nurses who worked for 10–19 years, and increased moderately (adjusted hazard ratio, 1.24; 95% CI, 1.13–1.37) in nurses who worked for ≥20 years.[30] These time intervals are principally consistent with those from a recent survey of elderly retired workers.[31]

Lipid metabolism is likely to be affected because of shift work.[32] A retrospective analysis suggested a minimum requirement of 20 years of shift work for a 5% increase in the total cholesterol level among middle-aged men.[33] A subsequent study found that this association depended on the body mass index at enrollment and was more evident in non-overweight men (body mass index <25 kg/m2).[34]

Body weight

Weight gain can provide an “infrastructure” for health disorders. Therefore, weight control has been targeted to protect shift workers' health.[35] Among middle-aged women in the City of Helsinki, shift work was found to prospectively associate with major weight gain (a 5 or more kg increase during the follow-up period), with adjusted odds ratios (aOR) of 1.07 (95% CI, 0.89–1.29) in those who did shift work without night shifts and 1.37 (95% CI, 1.08–1.74) in those who did night shifts, compared with the permanent day workers.[36] For their male counterparts, the results were similar but not statistically significant; the aORs were 1.24 (95% CI, 0.69–1.93) for those who did shift work without night shifts and 1.25 (95% CI, 0.81–1.82) for those who did night shifts. It should be noted that according to both the prospective and retrospective data, the risk of obesity became apparent 10 years after beginning shift work.[37, 38]

Gastrointestinal function

Eating at odd hours might considerably burden the gastrointestinal system. The evidence is limited; however, a recent study showed that although functional dyspepsia was not significantly associated with shift work, irritable bowel syndrome was observed among nurses and nursing assistants (aOR, 2.36; 95% CI, 1.01–5.47).[39] Another study of nurses reported a significant association between rotating shift work and irritable bowel syndrome (aOR, 2.14; 95% CI, 1.14–3.03) and a non-significant association between permanent night shift work and irritable bowel syndrome (aOR, 1.18; 95% CI, 0.67–2.07).[40] Currently, the findings regarding the role of Helicobacter pylori infections are controversial.[41, 42]

Cancer

The debate regarding the role of circadian rhythm-disrupting shift work as a probable human carcinogen has accelerated since the International Agency for Research on Cancer's announcement in 2007[43] and the Monograph in 2010.[44] Figure 1 depicts the significantly increased risk of various types of cancer as a function of the shift work duration. A relatively shorter duration of (15 years) shift work was found to be associated with an elevated risk of colorectal cancer, whereas a longer duration (40 years) was reported to be more strongly associated with a risk of prostate cancer. Breast cancer, the most investigated cancer linking to shift work, is shown to occur following 30 years of working under shifts. Several questions must be addressed in order to support this proposed association. From a methodological perspective, the critical role of exposure assessment should be evaluated, as will be addressed later in this paper. In practical terms, however, shift workers and their employers should at least address anti-cancer measures within a 15-year period. As mentioned earlier, several types of health disorders are likely to occur during this 15-year period. Health maintenance during the first half of the shift-working life could help to prevent both non-cancerous diseases and certain types of cancer.

figure

Figure 1. Cancer risk (hazard ratio, HR) associated with duration of shift work: non-shift work [Reference] (○), colorectal (▲),[45] lung [current smokers] (■),[46] endometrial (♦),[47] breast (●),[48] prostate (×).[49] Error bars show 95% confidence intervals.

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Safety and performance concerns

Individuals who work night and/or rotating shifts must struggle to maintain alertness while on the job.[50] At a physiological level, excessive sleepiness and poor performance are very natural consequences of a variety of factors such as circadian misalignment, long periods of prior wakefulness, and an accumulated sleep debt.

Neurobehavioral function

Sleep and circadian research has revealed degraded neurobehavioral function in association with shift work, and has accordingly presented recommendations to prevent unfavorable results in a timely manner.[51-53] In the laboratory, slower responses and greater lapses in psychomotor vigilance tasks are found to be characteristic features during extended wakefulness and the biological night.[54, 55] Although reaction-time tasks are sometimes criticized in light of real-world relevance, we can recognize that the stimulus-response sequence is a principal component of actual work activities (e.g., braking in response to a red signal on the wheel, operating machines following a feedback response in the factory, and treating patients according to their vital signs in hospitals).

Work performance

Reduced levels of neurobehavioral functioning can lead to tragic events in actual work settings.[56] Health care workers are asked to attend to patients at any time of the day, regardless of the prior sleep duration. Therefore, residents working during extended shifts have been reported to make serious medical errors, particularly with respect to diagnosis and medication.[57] Nurses have also been reported to be more likely to experience an increased occurrence of needlestick injuries when working for >12 h per day, shifts other than day shifts, and frequent weekend shifts.[58] Moreover, public safety professionals engage in their duties while shift working. Among police officers, the age-adjusted incidence rate ratio of occupational injuries was found to be significantly elevated during night shifts, and this association increased if the work intensity was high (e.g., very busy districts with frequent complaints or high crime rates).[59] When focusing on injuries leading to long-term leave (≥90 days), night shifts had incidence rate ratios that were 3-fold and 2-fold higher than those of the day and evening shifts, respectively, after controlling for age and sex.[60]

As noted, health care and public safety workers have gained intense attention with respect to shift work. However, we should also consider other occupational groups in order to reduce the overall risk associated with working during non-standard hours. Information regarding the time at which an on-duty injury occurred is very important with respect to accurate risk assessments. This information also should be preferred over objective (recorded) information. A recent study of lost-time compensation claims in Canada estimated the risk of work injury according to the time of day.[61] As shown in Figure 2(a), increases in work injuries were found to be concentrated within a time window from 22.00 to 04.00 hours. This temporal pattern, which was more evident in manual and mixed occupations, was generally consistent between men and women. Interestingly, the injury profile presented herein agrees with the time-of-day period during which fatal truck crashes were found to occur most frequently in Japan (Fig. 2b).[62]

figure

Figure 2. (a) Upper and middle panels: Rate of work injury per 200 000 h worked by time of day and occupation according to lost-time claims in Ontario (2004–2008).[61] Occupation: manual (image), mixed (image), and non-manual (image). (b) Lower panel: Percentage of fatal truck crashes relative to all truck crashes (2000–2010) by time of day in Japan.[62] Error bars show SD.

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Psychosocial concerns

Mismatch in timing between shift workers and their spouse might be an important source of the problems related to psychosocial health. The psychosocial satisfaction level seems to determine the quality of the working life and the long-term quality of life. Although we generally view shift work as a cause of negative influences on spouses and children,[63, 64] this viewpoint is not necessarily supported.[65, 66] The latest research, however, suggests that a high level of work/non-work conflict might induce the cessation of shift work.[67]

Strategies to Mitigate Damage from Shift Work

  1. Top of page
  2. Abstract
  3. Introduction
  4. Difficulties Faced by Shift Workers
  5. Strategies to Mitigate Damage from Shift Work
  6. Future Challenges
  7. Conclusions
  8. Acknowledgment
  9. References

If shift work is required, effective countermeasures should be developed and implemented in worksites. Several options for such countermeasures have been suggested from different perspectives. Some of the main strategies are explored in the following sections, with the schematic outline (Fig. 3).

figure

Figure 3. Outline of strategies to mitigate damage from shift work.

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Sleep management

Given the fact that shift work dysregulates sleep and wakefulness, the primary effort should be directed toward either minimizing the unfavorable effects on sleep or managing sleep and naps.[68, 69] Sufficient prior sleep duration can serve as the core; therefore, shift schedules must allow workers sufficient opportunities to sleep during protected intervals between shifts.[15, 70, 71] Another promising technique involves napping. In particular, naps taken before and during night shifts, if possible, have been shown to yield beneficial effects with respect to sleepiness and fatigue.[69, 72] Recent studies have tested the hypothesis that napping during night shifts might help to reduce melatonin suppression, and reducing the possibility of negative consequences.[73, 74] Furthermore, treatment of sleep disorders (e.g., sleep-disordered breathing) should be provided to shift workers in a timely manner.[75] Otherwise, those workers will be unable to obtain adequate sleep even if an opportunity for sleep is available.

Circadian approaches

Previous studies demonstrated that exposures to bright light and darkness (light blocking) could act as powerful countermeasures to change the circadian timing system according to the shift schedule.[76] However, most shift workers operate under rotating schedules, which might limit this strategy of light/dark phase shifting.[77] Alternatively, partial rather than complete alignment to day-oriented schedules (e.g., day shifts and days off) seems to yield sufficient positive performance and sleep outcomes.[78]

Health surveillance

Regular health evaluations are recommended for shift workers. In Japan, night shift workers must undergo such evaluations twice every year. Nevertheless, the parameters measured on these evaluations are the same as those measured in permanent day workers. This fact does not devalue the current health evaluation system but suggests that most industrial physicians and occupational health nurses need indicator(s) to assess the extent to which a worker has adapted to his/her shift schedule.

Pharmacological and non-pharmacological treatments

Pharmacological advances have produced a small number of drugs to treat SWD, which primarily promote alertness while on the job.[79, 80] Given the many uncertainties related to shift-related problems, sleep experts and non-sleep clinicians should exercise considerable caution when prescribing such drugs to individuals with suspected SWD. However, cognitive behavioral therapy for insomnia has attracted great attention with respect to treating sleep disturbances among shift workers.[81, 82]

Fatigue risk management system

As fatigue due to demanding work schedules has been recognized to impose substantial costs on both workers and workplaces, fatigue-related risk management has become an essential topic in the field of occupational health and safety. A fatigue risk management system (FRMS) requires the following multifaceted processes to ensure good health, safety, and well-being standards: policies, risk management procedures (data collection, analysis, and risk reduction), reporting systems, incident investigations, training and education, sleep disorder management, and internal and external auditing.[83, 84] It should be noted that the FRMS is formulated largely with sleep and circadian biological evidence. Currently, the FRMS is in its initial stages and therefore, we must attempt to develop, implement, promote, and improve this self-regulated system through the active participation of employees, employers, occupational health and safety staff, and scientists in relevant fields.

Future Challenges

  1. Top of page
  2. Abstract
  3. Introduction
  4. Difficulties Faced by Shift Workers
  5. Strategies to Mitigate Damage from Shift Work
  6. Future Challenges
  7. Conclusions
  8. Acknowledgment
  9. References

Exposure assessment plays a vital role in risk assessment and management. However, this is the weakest area of shift work research.[85, 86] Currently, the level of a chemical substance can be measured by using an appropriate device and predetermined procedure. The measured level of the substance should be the same regardless of the person who collects the information. In contrast, no consensus has been reached regarding how to quantify the “toxicity” of a particular type of shift work. Some options have been suggested,[85, 87] and extensive efforts are clearly needed to upgrade the shift work-related methodology with respect to definitions, characterizations, and exposure assessments.

As mentioned above, there is an increasing need for validated and reliable measures of shift work adaptation. At present, the perceived level of adaptation might provide an alternative measure.[88, 89] Nevertheless, the utility of this measure must be tested critically. Clock-gene genotypes and in vitro circadian periods observed in biopsy-derived fibroblasts have been proposed as objective markers.[90, 91] Therefore, we must explore whether these variables could provide clues with which we can estimate adaptability to a shift schedule.

Companies collect a variety of important data regarding employee health, safety, and productivity, an example of which is the health evaluation. In other words, many companies have been conducting prospective cohort studies at their own worksites. These data, unfortunately, are not used efficiently in order to understand the status and develop preventive strategies. While employees' personal information should be strictly protected, it would be worthwhile to conduct data mining within a company to produce evidence with which to improve shift work in collaboration with sleep, circadian, and epidemiological experts.

Every effort should be directed towards facilitating intervention research at the worksite, while taking great care to minimize the common limitations of shift work studies such as healthy worker effect, socioeconomic differences, bias, and confounding. The identification of a prospective association between shift work and outcomes would be valuable. However, more importantly, testing the effects of evidenced-based interventions would produce strong data.[92] This is, of course, a difficult endeavor, and mutual, trusting discussions will be required between companies and researchers. Previous attempts have produced some essential findings regarding an understanding of the effects of shift schedule changes,[93-95] increased flexibility,[96] and self-rostering.[97, 98]

As a final note, clarifying the mechanisms by which shift work affects our health remains a challenge to be achieved. At this moment, some models are available to explain an increased risk of non-cancer diseases including cardiovascular diseases. Potential pathways causing those diseases may involve disturbances in the autonomic nervous system, the neuroendocrine system, and the immune system, coupled with poor health behavior.[99, 100] The biological mechanisms for an elevated risk of cancer are presumed to be related primarily to dysregulation of the circadian timing system due to undesirable light-dark cycles imposed by shift schedules.[101, 102] In any event, sleep deficiency and circadian disruption can be seen as two essential factors for health disadvantages among shift workers.[69, 103]

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Difficulties Faced by Shift Workers
  5. Strategies to Mitigate Damage from Shift Work
  6. Future Challenges
  7. Conclusions
  8. Acknowledgment
  9. References

Our society has exhibited an increasing requirement for shift work. However, shift work, as discussed above, creates considerable problems in many aspects of working life. If society requires a shift-based workforce, then society shares the responsibility of minimizing the adverse influences of shift work. Sleep and circadian biology research will continue to progress; the mission of such research includes an understanding of shift-related problems, clarification of the underlying mechanisms, and the development of scientifically sound strategies for optimizing the lives of shift workers. We believe that bidirectional interactions between real-world shift work and scientific disciplines will yield fruitful results.

Acknowledgment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Difficulties Faced by Shift Workers
  5. Strategies to Mitigate Damage from Shift Work
  6. Future Challenges
  7. Conclusions
  8. Acknowledgment
  9. References

This work was supported in part by an Intramural Research Grant (No. 23-3) for Neurological and Psychiatric Disorders of National Center of Neurology and Psychiatry, Japan, and in part by a Grant-in-Aid for Scientific Research (C) (No. 25350876) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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  1. Top of page
  2. Abstract
  3. Introduction
  4. Difficulties Faced by Shift Workers
  5. Strategies to Mitigate Damage from Shift Work
  6. Future Challenges
  7. Conclusions
  8. Acknowledgment
  9. References
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