Downloaded from http://oem.bmj.com/ on July 12, 2015 - Published by group.bmj.com
PostScript
LETTER
Research into ‘night shift work’ and cancer: on the evolution of ‘exposure’ classification Grundy et al1 provided a diligent study into possible links between long-term shift work and incident breast cancer. Their investigation contributes to the evolution of research in a field of considerable relevance to occupational and environmental health. This is particularly true with regard to reducing ‘exposure’ classification errors by capturing diverse shift patterns and temporal details of the latter. Intriguingly, from a chronobiological perspective, there may be room for further improvements of classifying ‘exposure’; to examine this notion, we must answer a key question: what is the exposure-ofinterest in this and in similar studies? The authors start out from the 2007 evaluation by the International Agency for Research on Cancer (IARC): “shift-work that involves circadian disruption [CD] is probably carcinogenic to humans”.2 Clearly, not shiftwork per se but exposure to shiftwork involving CD has been identified as a probable cancer culprit. CD, while neither being defined in the general literature nor by IARC,2 presumably corresponds to a relevant disturbance of the temporal organisation of physiology, endocrinology, metabolism and behaviour.3 Equally clearly, since “among the many different patterns of shift-work, those including night work are the most
78
disruptive for the circadian clock”,2 Grundy et al will have captured some gradients of CD when comparing histories of ‘night shift’ with histories of ‘non-night shift’ work. Additional room to comprehensively assess relevant ‘exposures’ to3 or ‘doses’ of,4 CD may be provided by considering the chronotype,5 which conveys differences regarding when workers tend to be asleep (‘biological night’2) relative to local time. When Hansen and Lassen6 used cumulative number of years of night shifts as an exposure metric and diurnal preference for stratification, breast cancer risks were almost quadrupled in morning types and doubled in evening types with at least 884 cumulative night shifts. Notably, Grundy et al1 themselves see value in considering chronotype: ‘unfortunately, chronotype was not included in this study questionnaire’. Since ‘start and end times for each shift type’ were collected, the authors may want to examine whether chronotype of cases and controls can be assessed in retrospect. Comparing temporal details of shifts worked with the time window of the genetically (co-)determined biological nights may serve as a valid basis for approximating CD at the individual level.3–5 Overall, epidemiological studies in this important area of research are evolving at an accelerating pace. Grundy and colleagues contribute a valuable improvement of how we capture ‘exposure’ to day, evening and night shifts over a worker’s lifetime. To ultimately understand how work at biologically unusual times may be associated with disease in man, we should strive to improve how we specifically
classify ‘exposure’ to CD at the individual level as well. Thomas C Erren Correspondence to Professor Thomas Erren, Institute and Policlinic for Occupational Medicine, Environmental Medicine and Prevention Research, UNIKLINIK KÖLN; University of Cologne, Kerpener Straße 62, Lindenthal 50937, Germany; [email protected] Competing interests None. Provenance and peer review Not commissioned; internally peer reviewed. To cite Erren TC. Occup Environ Med 2014;71:78. Received 20 July 2013 Revised 18 August 2013 Accepted 19 August 2013 Published Online First 18 September 2013 Occup Environ Med 2014;71:78. doi:10.1136/oemed-2013-101747
REFERENCES 1
2
3
4
5
6
Grundy A, Richardson H, Burstyn I, et al. Increased risk of breast cancer associated with long-term shift work in Canada. Occup Environ Med 2013;70: 831–83. Straif K, Baan R, Grosse Y, et al. Carcinogenicity of shift-work, painting, and fire-fighting. Lancet Oncol 2007;8:1065–6. Erren TC, Reiter RJ. Revisiting chronodisruption: when the physiological nexus between internal and external times splits in humans. Naturwissenschaften 2013;100:291–8. Erren TC, Morfeld P. Shift work and cancer research: A thought experiment into a potential chronobiological fallacy of past and perspectives for future epidemiological studies. Neuro Endocrinol Lett 2013;34:282–6. Erren TC. Shift work and cancer research: can chronotype predict susceptibility in night-shift and rotating-shift workers? Occup Environ Med 2013;70:283–4. Hansen J, Lassen CF. Nested case-control study of night shift work and breast cancer risk among women in the Danish military. Occup Environ Med 2012;69:551–6.
Occup Environ Med January 2014 Vol 71 No 1
Downloaded from http://oem.bmj.com/ on July 12, 2015 - Published by group.bmj.com
Research into 'night shift work' and cancer: on the evolution of 'exposure' classification Thomas C Erren Occup Environ Med 2014 71: 78 originally published online September 18, 2013
doi: 10.1136/oemed-2013-101747 Updated information and services can be found at: http://oem.bmj.com/content/71/1/78
These include:
References Email alerting service
This article cites 5 articles, 2 of which you can access for free at: http://oem.bmj.com/content/71/1/78#BIBL Receive free email alerts when new articles cite this article. Sign up in the box at the top right corner of the online article.
Notes
To request permissions go to: http://group.bmj.com/group/rights-licensing/permissions To order reprints go to: http://journals.bmj.com/cgi/reprintform To subscribe to BMJ go to: http://group.bmj.com/subscribe/
PostScript
LETTER
Research into ‘night shift work’ and cancer: on the evolution of ‘exposure’ classification Grundy et al1 provided a diligent study into possible links between long-term shift work and incident breast cancer. Their investigation contributes to the evolution of research in a field of considerable relevance to occupational and environmental health. This is particularly true with regard to reducing ‘exposure’ classification errors by capturing diverse shift patterns and temporal details of the latter. Intriguingly, from a chronobiological perspective, there may be room for further improvements of classifying ‘exposure’; to examine this notion, we must answer a key question: what is the exposure-ofinterest in this and in similar studies? The authors start out from the 2007 evaluation by the International Agency for Research on Cancer (IARC): “shift-work that involves circadian disruption [CD] is probably carcinogenic to humans”.2 Clearly, not shiftwork per se but exposure to shiftwork involving CD has been identified as a probable cancer culprit. CD, while neither being defined in the general literature nor by IARC,2 presumably corresponds to a relevant disturbance of the temporal organisation of physiology, endocrinology, metabolism and behaviour.3 Equally clearly, since “among the many different patterns of shift-work, those including night work are the most
78
disruptive for the circadian clock”,2 Grundy et al will have captured some gradients of CD when comparing histories of ‘night shift’ with histories of ‘non-night shift’ work. Additional room to comprehensively assess relevant ‘exposures’ to3 or ‘doses’ of,4 CD may be provided by considering the chronotype,5 which conveys differences regarding when workers tend to be asleep (‘biological night’2) relative to local time. When Hansen and Lassen6 used cumulative number of years of night shifts as an exposure metric and diurnal preference for stratification, breast cancer risks were almost quadrupled in morning types and doubled in evening types with at least 884 cumulative night shifts. Notably, Grundy et al1 themselves see value in considering chronotype: ‘unfortunately, chronotype was not included in this study questionnaire’. Since ‘start and end times for each shift type’ were collected, the authors may want to examine whether chronotype of cases and controls can be assessed in retrospect. Comparing temporal details of shifts worked with the time window of the genetically (co-)determined biological nights may serve as a valid basis for approximating CD at the individual level.3–5 Overall, epidemiological studies in this important area of research are evolving at an accelerating pace. Grundy and colleagues contribute a valuable improvement of how we capture ‘exposure’ to day, evening and night shifts over a worker’s lifetime. To ultimately understand how work at biologically unusual times may be associated with disease in man, we should strive to improve how we specifically
classify ‘exposure’ to CD at the individual level as well. Thomas C Erren Correspondence to Professor Thomas Erren, Institute and Policlinic for Occupational Medicine, Environmental Medicine and Prevention Research, UNIKLINIK KÖLN; University of Cologne, Kerpener Straße 62, Lindenthal 50937, Germany; [email protected] Competing interests None. Provenance and peer review Not commissioned; internally peer reviewed. To cite Erren TC. Occup Environ Med 2014;71:78. Received 20 July 2013 Revised 18 August 2013 Accepted 19 August 2013 Published Online First 18 September 2013 Occup Environ Med 2014;71:78. doi:10.1136/oemed-2013-101747
REFERENCES 1
2
3
4
5
6
Grundy A, Richardson H, Burstyn I, et al. Increased risk of breast cancer associated with long-term shift work in Canada. Occup Environ Med 2013;70: 831–83. Straif K, Baan R, Grosse Y, et al. Carcinogenicity of shift-work, painting, and fire-fighting. Lancet Oncol 2007;8:1065–6. Erren TC, Reiter RJ. Revisiting chronodisruption: when the physiological nexus between internal and external times splits in humans. Naturwissenschaften 2013;100:291–8. Erren TC, Morfeld P. Shift work and cancer research: A thought experiment into a potential chronobiological fallacy of past and perspectives for future epidemiological studies. Neuro Endocrinol Lett 2013;34:282–6. Erren TC. Shift work and cancer research: can chronotype predict susceptibility in night-shift and rotating-shift workers? Occup Environ Med 2013;70:283–4. Hansen J, Lassen CF. Nested case-control study of night shift work and breast cancer risk among women in the Danish military. Occup Environ Med 2012;69:551–6.
Occup Environ Med January 2014 Vol 71 No 1
Downloaded from http://oem.bmj.com/ on July 12, 2015 - Published by group.bmj.com
Research into 'night shift work' and cancer: on the evolution of 'exposure' classification Thomas C Erren Occup Environ Med 2014 71: 78 originally published online September 18, 2013
doi: 10.1136/oemed-2013-101747 Updated information and services can be found at: http://oem.bmj.com/content/71/1/78
These include:
References Email alerting service
This article cites 5 articles, 2 of which you can access for free at: http://oem.bmj.com/content/71/1/78#BIBL Receive free email alerts when new articles cite this article. Sign up in the box at the top right corner of the online article.
Notes
To request permissions go to: http://group.bmj.com/group/rights-licensing/permissions To order reprints go to: http://journals.bmj.com/cgi/reprintform To subscribe to BMJ go to: http://group.bmj.com/subscribe/
