Since the International Agency for Research on Cancer (IARC) classified night work as a "probable human carcinogen" in 2007, the search for the biological mechanisms involved has been intense.  Disruption of human circadian rhythms (our 24 hour ‘body clock’) is a prime suspect.  Circadian rhythms occur in sleep, hunger, hormone levels, body temperature, and a host of other physiological aspects of health.

Researchers at Yale University and the Danish Cancer Society have now shown epigenetic alterations – biological alterations that affect the expression of inherited genes --  in two of the core circadian genes found in the body’s 24-hour biological timekeeping system, CLOCKand CRY2, in night working women in Denmark that are the same alterations previously shown in women with breast cancer.  In addition, a wider analysis of the genetic make-up of the human biology has identified hundreds of other genes that are epigenetically affected by long-term night work.  The study appears in the October issue of Chronobiology International.

Lead author Yong Zhu at the Yale School of Public Health explains:  "We previously found epigenetic changes in two circadian clock genes in women with breast cancer, and we published those findings last year.  We then wondered if night work could cause the same epigenetic changes in these same circadian genes as well as other cancer-related genes, which could be a molecular mechanism accounting for the previously observed link between night work and breast cancer risk."

As opposed to genetic polymorphisms (i.e., different inherited forms of a given gene, that are inherited from birth such as BRCA1 mutations that are linked to breast cancer risk in women), epigenetic changes, such as DNA methylation, can be influenced by the environment and change over time.  These epigenetic changes can alter the function of genes in ways that could increase or decrease risk of disease.

Breast cancer is the leading cause of cancer death in women globally, and the reasons are not clear.  The major causes for most common cancers, such as of lung, liver, cervix, and stomach, are known, yet for breast cancer it is a mystery what exposures or lifestyles are major causes of the disease.  It is largely unknown why breast cancer risk is so high in the developed world, and increases so dramatically as developing societies industrialize.  The idea that electric lighting at night might be part of the explanation was stated over 20 years ago, and led to the studies of breast cancer in night workers as well as studies of breast cancer risk in blind women and women who sleep longer than normal.

Richard Stevens at the University of Connecticut Health Center, a coauthor on the report, says "The implications of this new study are vast.  As we begin to identify environmental exposures - such as light at night exposure - that change risk and the biological mechanisms at work, we can then figure out effective interventions and mitigation that would lower risk.  Changing our lighted environment to be more friendly to our circadian health might provide a very fruitful option."

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