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Nearly 20 % of the working population in Europe and North America is engaged in shift work, and it is quite obvious that for technical, cultural and economic reasons shift work can not simply be avoided in a modern society. Shift work involving night work (for the sake of simplicity only referred to as “shift work” in the following) poses several challenges to those who perform it, by interfering both with biological and social circadian rhythms. While the latter fact can usually be adjusted for through appropriate organizational measures, the former has been found much more difficult to address. This so-called “chronodisruption” has been associated with a wide range of adverse health effects, where shift workers may be at a higher risk if compared to day workers (Garaulet and Madrid 2009, Straif et al. 2007, Erren et al. 2009).
What are the main health effects possibly associated with shift work?
The short-term effects of performing shift work are rather straight-forward and can easily be compared to those of a jet lag after a flight across several time zones. They include, above all, sleep disturbances, fatigue and gastrointestinal malfunction. It has further been demonstrated that a jet lag can temporarily affect immune functions. An elevated risk of accidents, both at work and while commuting, has also been described and attributed to sleep deficits. Regarding the long-term health effects of shift work, more than half a century of research has provided a wealth of observations, however, also yielded a considerable variability of findings across studies (Knutsson 2003). Gastrointestinal diseases, namely peptic ulcers, have been found more frequently in shift as compared to day workers, but this has also been the case for Helicobacter pylori infection as a major risk factor for these ulcers (Zober et al. 1998). Small to moderate excess risks for cardiovascular disease and hypertension have been found in some, but not all studies, and failure to demonstrate an elevated risk has sometimes been attributed to a “healthy survivor” effect. A number of, mostly small and cross-sectional, studies indicate that shift work may also increase the risk for metabolic disorders including hypertriglyceridemia and diabetes, thus providing a possible link to the purported cardiovascular effects, and well in accordance with the often reported higher prevalence of obesity in shift workers (Garaulet and Madrid 2009). Finally, in 2007 an expert Working Group convened by the IARC Monographs programme has concluded on the basis of “limited evidence in humans for the carcinogenicity of shift work that involves night work”, and “sufficient evidence in experimental animals for the carcinogenicity of light during the daily dark period (biological night)” that shift work that involves circadian disruption is probably carcinogenic to humans (Group 2A)” (Straif et al. 2007).
Since the IARC assessment, several new studies have been published, with inconclusive results. Heterogeneity of study exposures and outcomes and emphasis on positive but non-significant results make it difficult to draw general conclusions. Further data are needed on additional disease endpoints and from different study populations. Also, several reviews and commentaries, which have been published meanwhile, came to equivocal results. Published evidence is widely seen as suggestive but not conclusive for an adverse association between night work and breast cancer, and limited and inconsistent for cancers at other sites and all cancers combined (Wang et al. 2011).
Challenges in studying shift effects
There exists a large variability of shift work organization schemes across countries and across industries. Shift patterns may be clockwise or counter-clockwise rotating and involve few or many night shifts in a row or even permanent night shift. Shift duration may be anything between four and twelve hours. Almost needless to say that, depending on type of work and industry, other exposures may be linked to shift work which themselves can pose health risks on their own. Notorious examples from two widely studied occupational groups in shift work research may be cosmic radiation in flight attendants and handling of chemotherapeutic drugs by nurses. Shift workers may differ from day workers because of selection effects that may occur not only at entry to the respective occupational groups but also throughout later working life, specifically if shift work is discontinued for health reasons. The direction of the resulting bias is usually expected to tend towards a “healthy worker effect” in shift workers. On the other hand, life style and dietary factors have often been described as substantially different between shift and day workers, with the less favorable habits being more prevalent in the former group. These potential sources of bias make the interpretation of shift study findings, and even more comparisons across studies in different populations, challenging.
The solution may not only lie in the shift work system
More than half a century of shift work research has provided some insights into advantages and disadvantages associated with different shift schedules. It is mostly agreed that shift patterns should be fast forward rotating, with shift duration inversely related to physical and psychological work load, and involving not more than three night shifts in a row. In addition to an improved shift organization, regular occupational medical surveillance, counseling and health promotion activities have been shown to be effective in avoiding excessive adverse health effects of shift work. In our own studies we observed a lower accident risk and neither an elevated risk of total mortality nor an increased incidence of cancers in a cohort of more than 14,000 BASF shift workers compared to 17,000 day workers (Ott et al. 2009, Oberlinner et al. 2009). Additional analyses provided evidence that this “lack of effect” was indeed at least partially attributable to the health prevention programs performed at our site over the past decades (Ott et al. 2010).
Conclusion
There is evidence and biological plausibility that shift work including night work may adversely affect the health of those who perform it. However, there is also evidence that shift schedules can be organized in ways that minimize the associated health risks, and the risks may be further reduced or even completely avoided through the implementation of structured and sustained health promotion programs specifically tailored to the needs of shift workers. They may thus see the graveyard not earlier than their day-working colleagues.
References
Erren TC, Morfeld P, Stork J, Knauth P, von Mülmann MJ, Breitstadt R, Müller U, Emmerich M, Piekarski C (2009) Shift work, chronodisruption and cancer? - The IARC 2007 challenge for research and prevention and 10 theses from the Cologne Colloquium 2008. Scand J Work Environ Health 35:74-79
Garaulet M, Madrid JA (2009) Chronobiology, genetics and metabolic syndrome. Curr Opin Lipidol 20:127-134
Knutsson A (2003) Health disorders of shift workers. Occup Med 53:103-108
Oberlinner C, Ott MG, Nasterlack M, Yong M, Messerer P, Zober A, Lang S (2009) Medical program for shift workers - impacts on chronic disease and mortality outcomes. Scand J Work Environ Health 35:309-318
Ott MG, Oberlinner C, Lang S, Hoffmann G, Nasterlack M, Pluto R-P, Trauth B, Messerer P, Zober A (2009) Health and safety protection for chemical industry employees in a rotating shift system: program design and acute injury and illness experience at work. J Occup Environ Med 51:221-231
Ott MG, Yong M, Zober A, Nasterlack M, Messerer P, Pluto R-P, Lang S, Oberlinner C (2010) Impact of an occupational health promotion program on subsequent illness and mortality experience. Int Arch Occup Environ Health 83:887-894
Straif K, Baan R, Grosse Y, Secretan B, El Ghissassi F, Bouvard V, Altieri A, Benbrahim-Tallaa L, Cogliano V (2007) Carcinogenicity of shift-work, painting, and fire-fighting. Lancet oncology 8:1065-1066
Wang X-S, Armstrong MEG, Cairns BJ, Key TJ, Travis RC (2011) Shift work and chronic disease: the epidemiological evidence. Occup Med 61:78-89
Zober A, Schilling D, Ott MG, Schauwecker P, Riemann JF, Messerer P (1998) Helicobacter pylory infection: prevalence and clinical relevance in a large company. J Occup Environ Med 40:586-594