Jennifer Ritonja & Kristan J Aronson

Department of Public Health Sciences; and Division of Cancer Care and Epidemiology, Cancer Research Institute, Queen’s University, Kingston, Ontario, Canada

https://doi.org/10.36866/pn.113.14

Shift work, often including work at night, is an increasingly prevalent work schedule around the globe. For years, disparate researchers such as epidemiologists, chronobiologists, and physiologists have each studied how night work may cause poor health outcomes from their distinct vantage points. We have learned that, acutely, shift work can cause poor sleep quality and sleep deprivation, fatigue, melatonin suppression, and cognitive impairment. Long-term, shift work raises the risk for several health problems including injury, cardiovascular disease, and cancer. But the complete picture of the pathways linking shift work to various health outcomes is not well understood, and could be more rapidly discovered with epidemiologists working in collaboration with physiologists.

The relatively recent discovery of circadian clock genes and the recognition that about 10% or more of our biological functions depend on our synchronisation to an approximately 24-hour clock (called circadian rhythm) are key to understanding the pathways from shift work to increased disease risk. Circadian rhythm refers to physiological processes and behavioural habits that follow a roughly 24-hour cycle, such as sleep-wake activity and highs and lows of metabolism and blood hormone concentrations. As a diurnal species, our physiological rhythms are naturally timed to coincide with being active during the day and sleeping at night. However, rhythms can be adjusted when we change our sleep-wake patterns, eating patterns, and light exposure. When our physiological rhythms operate outside of the natural diurnal rhythm, transient periods of circadian misalignment or disruption can occur as the system attempts to re-align to the new environmental time cues. It is hypothesised that shift work leads to circadian disruption by misaligning an individual’s physiological processes because of exposure to light at night, changed sleep-wake cycles, and altered eating patterns.

Both experimental and epidemiological studies suggest a misalignment of circadian rhythms in shift workers working nights in a rotating or permanent schedule. Night work is related to changes in the level and timing of melatonin and cortisol, disruption of body temperature cycles, changes in sex hormone levels, reduced alertness and psychomotor performance, and reduced heart rate variability. Since the purpose of the circadian rhythm is to synchronise physiological function with the environment, disruption of this rhythm by shift work can lead to sleep deprivation and poor sleep quality, dysregulation of metabolism, changes in hormonal production, as well as maladaptive changes in behaviour (e.g. diet, physical activity) that may promote the development of disease.

Following our research showing an association between long-term shift work and increased breast cancer risk, our research group is contributing to this area by exploring how shift work affects sleep quality and biomarkers such as melatonin and cortisol (Grundy et al., 2013). In observational studies, female hospital employees working day and rotating night shifts answered questionnaires including their current and past shift work exposure, and were studied over an 8-day period. Sleep and physical activity were measured using accelerometers, and urine was collected over a 48-hour period to measure melatonin and cortisol. The results suggest that working nights on a rotating pattern is associated with lower 24-hour melatonin and cortisol output, phase shifts in these patterns, poorer self-reported sleep quality, and shorter objectively measured sleep duration (Grundy et al., 2013; Hung et al., 2016; Korsiak et al., 2018; Lajoie et al., 2015; Leung et al., 2016). In addition, our research suggests that cortisol and sleep duration may be intermediates in
the pathway linking shift work to cardiovascular risk, including metabolic syndrome (Korsiak et al., 2018; Ritonja et al., 2018). Currently we are assessing differences in light exposure at night, objectively measured by accelerometers, for night and day workers, and investigating the relationship between melatonin patterns and methylation of clock genes.

While our research and other evidence supports a link between shift work and circadian disruption, there are still gaps in knowledge. It is unclear how exactly different aspects of shift work, such as the duration of night work, and the frequency and pattern of night shifts induce chronic circadian disruption. This is made even more challenging by the lack of tools to clearly measure and define circadian disruption and a lack of consideration of chronotype in many observational studies.

In addition, studies are lacking on factors related to circadian adaptation in shift workers (Ritonja et al., 2018): while there is evidence that many shift workers show varying degrees of circadian adaptation to their work schedules, there is no evidence that complete adaptation to shift work is possible. It may be possible for some individuals to adapt to night shift work, by adjusting shift lengths and schedules, changing exposure to blue and bright light, and allowing napping during shifts. However, current evidence on these strategies to promote circadian adaptation is limited.

In order to fully understand the complex pathways linking shift work to circadian disruption and associations with adverse health outcomes, interdisciplinary knowledge from multiple fields is needed to understand the causal mechanisms. We hope that physiologists can collaborate with epidemiologists to investigate how the biological effects of different aspects of shift work, light exposure, changes in sleep times, and changes in eating patterns can impact physiological rhythms.

Most research has focused on melatonin, necessitating the need for studies addressing how cortisol and metabolic biomarkers change in response to circadian disruption. In addition, physiologists can help inform strategies for circadian adaptation by exploring how factors such as chronotype and varying shift schedules may induce circadian misalignment. Finally, new measurement techniques for precise measurement of circadian disruption are needed. By working together, we hope that new knowledge will be generated to understand causal mechanisms, identify risk factors for shift workers, and inform future workplace interventions and policies.

In conclusion, prevalent shift work poses many challenges to the health and well-being of workers in today’s society. We recommend greater interdisciplinary collaboration between physiologists and epidemiologists in this exciting and important research area, a collaboration that is essential for fully understanding the impact of shift work and chronic circadian disruption on disease risk.

References

Grundy A, Richardson H, Burstyn I, et al. (2013). Increased risk of breast cancer associated with long-term shift work in Canada. Occupational and Environmental Medicine 70(12), 831–838.

Grundy A, Tranmer J, Richardson H, et al. (2011). The influence of light at night exposure on melatonin levels among Canadian rotating shift nurses. Cancer Epidemiology, Biomarkers & Prevention 20(11), 2404–2412.

Hung EWM, Aronson KJ, Leung M, et al. (2016). Shift work parameters and disruption of diurnal cortisol production in female hospital employees. Chronobiology International 33(8), 1045–1055.

Korsiak J, Tranmer J, Day A, et al. (2018). Sleep duration as a mediator between an alternating day and night shift work schedule and metabolic syndrome among female hospital employees. Occupational and Environmental Medicine 75(2), 132–138.

Korsiak J, Tranmer J, Leung M, et al. (2018). Actigraph measures of sleep among female hospital employees working day or alternating day and night shifts. Journal of Sleep Research 27(4), e12579.

Lajoie P, Aronson KJ, Day A, et al. (2015). A cross-sectional study of shift work, sleep quality and cardiometabolic risk in female hospital employees.
BMJ Open 5(3), e007327.

Leung M, Tranmer J, Hung E, et al. (2016). Shift work, chronotype, and melatonin patterns among female hospital employees on day and night shifts. Cancer Epidemiology, Biomarkers & Prevention 25(5), 830–838.

Ritonja J, Aronson KJ, Day AG, et al. (2018). Investigating cortisol production and pattern as mediators in the relationship between shift work and cardiometabolic risk. Canadian Journal of Cardiology 34(5), 683–689.

Ritonja J, Aronson K, Matthews R, et al. (2018). Individual differences in shift work tolerance and recommendations for research and practice. Industrial Health (In Press).