Thursday Sept 6, 2016 (foodconsumer.org) -- A review-type of study funded by Cancer Research UK (CRUK) and other organizations claims that night work or night shift work does not increase risk for breast cancer.   The data used for the study came from selected 10 studies conducted  in the UK, USA, China and other countries.

The results from the current review are surprising because prior to this study, many studies provided solid evidence to indicate that night shift work is associated with elevated risk of breast cancer.   In fact, evidence that shift work increases cancer risk has been so overwhelming that the International Agency for Research on Cancer (IARC), an organization under World Health Organization, classified shift work as a probable carcinogen in 2007. 

From a biological viewpoint, it is possible that night work increases risk of breast cancer.   Night shift work or exposure to artificial light at night has been known to disrupt the body clock or so called circadian rhymth and suppress the production of an essential compound known as melatonin.   A recently published report in European Journal of Epidemiology explains why circadian disruption affects the risk of breast cancer. 

Melatonin is known to play a role in the protection against cancer.   Studies conducted recently have also suggested that melatonin as a dietary supplement may be used to prevent or even treat breast cancer. 

In a word, the new study sponsored by Cancer Research UK and other organizations may not have a final say about how night shift work may or may not affect the risk of breast cancer.   The CRUK study could not overrule the classification of shift work as a probable carcinogen by IACR.

A case-control study conducted in French and reported in April 2016 in Chronobiology International is another study that supports the notion that night shift work may increase risk of breast cancer.

The study involving 975 cases of breast cancer and 1317 controls shows that among premenopausal women, those who worked at night were 103% more likely to be diagnosed with hormone receptor positive breast cancer. 

The hormone receptor positive breast cancers include estrogen-receptor (ER) positive breast cancer, progesterone-receptor (PR) positive breast cancer, and human epidermal growth factor-receptor 2 (HER2) positive breast cancer.

However, this study does not find any significant associations between shift work and risk of breast cancer among postmenopausal women.  Even among premenopausal women, night shift work does not seem to affect the risk of ER, PR or HER2-negative tumors.

The study concludes that "This study provides evidence that working at night may increase risk of ER, PR and HER2-positive subtypes of breast cancer particularly among premenopausal women."

The study was conducted by Pascal Guenel and colleagues at the Center for Research in Epidemiology and Population Health in France.

Emilie Cordina-Duverger, Yves Koudou, Thérèse Truong, Patrick Arveux, Pierre Kerbrat, Florence Menegaux & Pascal Guénel, Night work and breast cancer risk defined by human epidermal growth factor receptor-2 (HER2) and hormone receptor status: A population-based case–control study in France, Chronobiology International, Volume 33, 2016 - Issue 6, Pages 783-787 | Published online: 14 Apr 2016

Many more new studies provide evidence to suggest that shiftwork can increase risk of breast cancer.  The following are some published in 2016 related to shift work and breast cancer.

1) Light at night and breast cancer incidence in Connecticut: An ecological study of age group effects -- The study shows that long exposure to night light can increase risk of breast cancer by more than 64%.  The study seems to be consistent with the one led by Pascal Guenel et al. in that older women are less sensitive to the influence of night light exposure, which means that the link between nightlight levels and risk of breast cancer is not as significant in older women.

2) Circadian clocks and breast cancer -- The report explains how disruption of circadian rhythm can increase risk of breast cancer

3) Sleep duration and breast cancer risk among black and white women -- The increase in the breast cancer risk in women working nights may not be solely affected by circadian rhythm disruption.   Night shift work often results in short sleep duration, which this study finds related to increased risk for hormone receptor (ER+ and PR+) positive breast cancers in black women.  (Two thirds of the study population were black).

4) The effect of melatonin on sleep and quality of life in patients with advanced breast cancer -- This study indicates that melatonin may be used to help improve sleep and quality of life in women with advanced breast cancer.

5) Chapter Three – Circadian Genes in Breast Cancer -- The report says that light at night can cause circadian gene alterations which are linked to risk of breast cancer, particularly those hormone-receptor positive breast cancers.

6) Can Avoiding Light at Night Reduce the Risk of Breast Cancer? -- Exposure to artificial light at night increases risk of breast cancer

7) High Incidence of Breast Cancer in Light-Polluted Areas with Spatial Effects in Korea -- The study from South Korea also indicates that exposure to artificial light at night may increase risk of breast cancer.  (David Liu)