Doing activities and working during the night affects the circadian system, causing sleeping disorders thus severely affecting health.
A large prospective study has been published this month on the American Journal of Preventive Medicine using data from the Nurses’ Health Study (NHS), linking night shift rotation, cardiovascular diseases (CVDs), and some types of cancer mortality1. The NHS was established in 1976 enrolling more than 100,000 nurses aged 30–55 years. Women with previous CVD or cancer or nurses who did not provide information about their shift were excluded from the study; therefore only the information on 74,862 nurses was used during 22 years of follow-up.
The nurses were classified in four groups according to the years of rotating night shift work: never, 1–5, 6–14, and >15 years. Working rotating night shifts for more than five years significantly correlated with all-cause, CVD, in particular ischemic heart disease, and some type of cancer mortality. Lung cancer was the most significantly correlated cause of cancer mortality, followed by colorectal and breast cancers, but overall cancer mortality was not significantly increased in nurses who worked rotating night shift for more than five years. Also lifestyle factors—such as physical activity, dietary habits, and smoking—body mass index, diabetes, cholesterol were considered, but no significant correlations were identified. Women working rotating night shifts tended to be older, be more physically active, drink less alcohol, eat less cereal, and were more likely to have diabetes, hypertension, and hypercholesterolemia than women without night shift work.
Undoubtedly working rotating night shifts disrupts circadian rhythms, causing not only sleeping pattern disorders, but also affecting health. Many studies have correlated night work and exposure to light during nighttime with cardiovascular diseases and cancer, thus the WHO classified night work as a probable carcinogen in 19972,3,4. The molecular mechanism underlying this correlation is not fully known. Circadian rhythms are controlled by melatonin, the hormone produced during nighttime by the pineal gland in the brain. Any disturbance in sleeping pattern, or exposure to light during night affects melatonin production and the pathways controlled by this hormone. Melatonin has been involved in several processes (metabolism, immune response, reproduction, etc.); therefore it is not surprising that this hormone can affect several diseases.
Although in this study only nurses were considered and no other professions, this is the largest prospective cohort available worldwide with a high proportion of women working rotating night shift and with a very long follow-up period, thus making it a reliable collection of data without confounding occupation-related diseases.
Moral of the story: Don’t work during the night!
1Total and Cause-Specific Mortality of U.S. Nurses Working Rotating Night Shifts. Gu F, Han J, Laden F, Pan A, Caporaso NE, Stampfer MJ, Kawachi I, Rexrode KM, Willett WC, Hankinson SE, Speizer FE, Schernhammer ES. Am J Prev Med. 2015 Jan 6. pii: S0749-3797(14)00623-0. doi: 10.1016/j.amepre.2014.10.018
2Melatonin, sleep disturbance and cancer risk. Blask DE.Sleep Med Rev. 2009 Aug;13(4):257-64. doi: 10.1016/j.smrv.2008.07.007. Epub 2008 Dec 17. Review.
3Rotating night shifts and risk of breast cancer in women participating in the nurses’ health study.Schernhammer ES, Laden F, Speizer FE, Willett WC, Hunter DJ, Kawachi I, Colditz GA.J Natl Cancer Inst. 2001 Oct 17;93(20):1563-8.
4Carcinogenicity of shift-work, painting, and fire-fighting. Straif K, Baan R, Grosse Y, Secretan B, El Ghissassi F, Bouvard V, Altieri A, Benbrahim-Tallaa L, Cogliano V. Lancet Oncol. 2007 Dec;8(12):1065-6.
See also http://damianodemaria.scienceblog.com/80/light-exposure-at-night-induces-resistance-to-therapy-in-breast-cancer/