In 2007 the International Agency for Research on Cancer (IARC) classified night work as a "probable human carcinogen," since then researchers have been searching for the biological mechanisms involved. The investigators believe that a prime suspect is the disruption of human circadian rhythms (24 hour 'body clock'). Circadian rhythms occur in hormone levels, hunger, sleep, body temperature as well as in a variety of other physiological aspects of health. The investigation appears in the October issue of Chronobiology International.

Investigators at Yale University and the Danish Cancer Society have revealed epigenetic alterations - biological changes that affect the expression of DNA genes - in two of the most important circadian genes found in body's 24-hour biological timekeeping system, CLOCK and CRY2. They discovered that women in Denmark who work at night have the same changes previously observed in women with breast cancer. Furthermore, hundreds of other genes that are epigeneticaly affected by long-term night work were identified in a larger examination of genetic make-up of the human biology.

Lead author Yong Zhu at the Yale School of Public Health said:

"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."

Unlike genetic polymorphisms (i.e., different inherited forms of a given gene, that are inherited from birth, such as BRCA1 mutations that are associated to breast cancer risk in women), epigenetic changes, like DNA methylation, can be affected by environmental conditions and change over time. These epigenetic alterations can change the way in which genes function in ways that may increase or decrease risk of disease.

In 2010, nearly 1.5 million people were diagnosed with breast cancer, making it the most prevalent cancer in women. It is also the leading cause of cancer death in women worldwide, although the reasons are not clear. Although the most major causes for the most prevalent cancers, such as liver, lung, stomach and cervix are known, researchers are still unsure which exposures or lifestyles are major causes of breast cancer, why the risk is so high in the developed world and why the risk increases so significantly as developing societies industrialize. The theory that electrical lighting at night might be part of the reason was stated over two decades ago. This idea led to investigations of breast cancer in individuals who work at night in addition to investigations on breast cancer risk in blind women or those who sleep longer than recommended.

Richard Stevens at the University of Connecticut Health Center, a coauthor on the report, explains:

"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."

Written by Grace Rattue