Case–control study of shift-work and breast cancer risk in Danish nurses: Impact of shift systems
Introduction
From ancient times all species ranging from cyanobacteria to humans have been organised around the 24 h rhythm given by the rotation of the earth around the sun. In mammals, including humans, many biochemical and physiological processes, e.g. hormone secretion, cell cycle, apoptosis and gene expression, are entrained to the regular 24 h rhythm by daily periods of light exposure from dawn to dusk, and darkness in the period in between.1 Such rhythms are disrupted during periods of shift-work,2 including work during evening, night and early morning, and thereby rapid change of timing of light exposure, dietary pattern and sleep/wake cycle, some of which may be involved in carcinogenicity.1 Shift-work is increasing due to the demands of the 24-h society and night-shifts occur now in 10% to 20% of the workforce.3
In 2007, the International Agency for Research on Cancer concluded: ‘shift-work that involves circadian disruption is probably carcinogenic to humans (group 2A)’.4 This finding was based on evidence that was deemed ‘sufficient’ in animals and ‘limited’ for human breast cancer. Earlier epidemiologic studies supported an association,5 and there is an emerging understanding of biological mechanisms that could account for a causal relationship,3, 6 however, the epidemiology is limited in two ways: there are fewer than 20 pertinent studies,6 and the previously used definitions of shift work are crude and not directly comparable.3
More high-quality studies in humans are required before a more definite conclusion can be reached about whether shift-work causes breast cancer and, if so, which aspects of shift-work are most problematic. We report here the results of a nested case–control study of nurses in Denmark in which detailed information was collected on the length and frequency of work in various shift systems, including day, permanent night and rotating between day, evening and night.
Section snippets
Methods
An interview-based case–control study was nested within an established national cohort of 91,140 female members of the Danish Nurses Association, covering over 95% of nurses in Denmark.7 Information on all nurses is computerised on the basis of their unique personal identification number, which includes the date of birth and sex, and is assigned to all residents of Denmark by the Central Person Register.8 This number can be used to obtain updated computerised information on e.g. vital status,
Results
The characteristics of the control women by after-midnight shift status are shown in Table 1. Nurses who worked on after-midnight ‘graveyard’ shifts, in comparison with nurses on day shifts, had a significantly longer working life, were younger at menarche and menopause, had fewer children, were older at the birth of their first child, spent fewer hours weekly on sport, had slept fewer hours per night 10 years before the interview, and were more frequently exposed to electromagnetic fields. No
Discussion
In this nationwide case–control study of Danish nurses, we observed a significantly increased odds ratio for breast cancer when work after midnight was compared with permanent day work. Further, the odds ratio tended to increase with duration and cumulative overnight shifts. In most analyses, we observed only minor confounding. Each of these tendencies is roughly in line with the results of most previous studies of shift-work and breast cancer risk (reviewed in Ref. 6).
Increased risk for breast
Role of the funding source
The funding sources did not involve in the data collection, data analysis, manuscript writing or publication. The corresponding author had full access to all data in the study and had final responsibility for the decision to submit for publication.
Contributors
JH and RGS were responsible for the study design and funding. JH supervised data collection, analysed data and wrote the draft report. Both authors critically revised the final report.
Conflict of interest statement
None declared.
Acknowledgements
We received unrestricted grants from the Danish Cancer Society and from the National Programme of Environmental Health Research.
References (28)
- et al.
Carcinogenicity of shiftwork, painting, and firefighting
Lancet Oncol
(2007) Electric light causes cancer? Surely you’re joking, Mr. Stevens
Mutat Res
(2009)- et al.
Circadian rhythms, sleep, and the menstrual cycle
Sleep Med
(2007) Light-at-night, circadian disruption and breast cancer: assessment of existing evidence
Int J Epidemiol
(2009)- et al.
Biological clocks and shift work: circadian dysregulation and potential long-term effects
Cancer Causes Control
(2006) - et al.
Considerations of circadian impact for defining ‘shift work’ in cancer studies: IARC Working Group Report
Occup Environ Med
(2001) Light at night, shiftwork, and breast cancer risk
J Natl Cancer Inst
(2001)- et al.
Shift work and cancer – considerations on rationale, mechanisms, and epidemiology
Scand J Work Environ Health
(2010) - et al.
Cancer incidence among a large cohort of female Danish registered nurses
Scand J Work Environ Health
(2009) The Danish Civil Registration system
Scand J Public Health Suppl
(2011)
The Danish Cancer Registry
Scand J Public Health Suppl
An incidence density sampling program for nested case–control analyses
Occup Environ Med
Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans
J Clin Endocrinol Metab
Do permanent night workers show circadian adjustment? A review based on the endogenous melatonin rhythm
Chronobiol Int
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