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Investigating causal relations between sleep traits and risk of breast cancer in women: mendelian randomisation study

BMJ 2019; 365 doi: (Published 26 June 2019) Cite this as: BMJ 2019;365:l2327

Linked Editorial

Larks, owls, and breast cancer

Rapid Response:

Chronobiology-based epidemiology: Towards Zeitgeber hygiene in all walks of life

The focus on ‘work-at-night’ as an exclusively targeted source of disturbed chronobiology recently led the International Agency for Research on Cancer [IARC] to classify “night shift work” as probably carcinogenic to humans.[1] By using UK Biobank’s rich information on a population scale,[2] Richmond and colleagues (doi:10.1136/bmj.l2327) appropriately broaden the scope of links between chronobiology facets [chronotype, insomnia symptoms, sleep duration, genetic variants] and cancer.[3] That results conveying morning preference to be inversely associated with breast cancer risk were similar after excluding night workers [3] could empirically imply that epidemiology into ‘work-at-night’ alone is reductionist. Possibly analogous, had smoking researchers in the 1940s only asked for “how much do you smoke at work?” could have masked this strong risk factor by ignoring smoking at play.[4, 5]

Importantly, beyond much-needed less reductionist epidemiology into effects of living against chronobiology in occupational + environmental settings, experimental evidence for the carcinogenicity of disturbed chronobiology should already gear studies towards Schernhammer’s call for “exploring how the stresses on our biological clock can be reduced” (doi:10.1136/bmj.l4267).[6] One (chronobio)logical lever could be Zeitgeber hygiene. Combining Zeitgebers to promote health can be a complex (interacting candidates may include light, food,[7] exercise/physical activity, [8] noise, social interactions, and possibly even the Moon [9]) but also rewarding, objective.

Overall, since we may strain chronobiology in all walks of life, research into, and prevention of, disturbed chronobiology as a conceivably ubiquitous (co-)determinant of disease [4, 5] is imperative.


1. IARC. Monographs Vol 124 group. Carcinogenicity of night shift work. Lancet Oncol 2019;Jul 4.
2. UK Biobank data on 500,000 people paves way to precision medicine. Nature 2018;562(7726):163-64.
3. Richmond RC, Anderson EL, Dashti HS, et al. Investigating causal relations between sleep traits and risk of breast cancer in women: mendelian randomisation study. Bmj 2019;365:l2327.
4. Erren TC, Lewis P. Can yesterday's smoking research inform today's shiftwork research? Epistemological consequences for exposures and doses due to circadian disruption at and off work. Journal of occupational medicine and toxicology (London, England) 2017;12:29.
5. Erren TC, Lewis P. Hypothesis: ubiquitous circadian disruption can cause cancer. European journal of epidemiology 2019;34(1):1-4.
6. Schernhammer ES. Larks, owls, and breast cancer. Bmj 2019;365:l4267.
7. Stokkan KA, Yamazaki S, Tei H, et al. Entrainment of the circadian clock in the liver by feeding. Science 2001;291(5503):490-3.
8. Lewis P, Korf HW, Kuffer L, et al. Exercise time cues (zeitgebers) for human circadian systems can foster health and improve performance: a systematic review. BMJ Open Sport Exerc Med 2018;4(1):e000443.
9. Erren TC, Schmiedehausen S, Groß JV, et al. What if …. the Moon provides zeitgeber signals to humans? Molecular psychiatry 2018.

Erren TC & Lewis P

Competing interests: No competing interests

26 July 2019
Thomas C Erren
Chair & Director of Institute and Policlinic of Occupational Medicine, Environmental Medicine and Prevention Research
Philip Lewis;
University Of Cologne, Germany
Kerpener Straße 62 , 50937 Köln