Women who spend 30 or more years working the "graveyard" shift are 50 percent more likely than others to develop breast cancer, no matter their occupation.
While a previous study suggested a heightened breast cancer risk among nurses working at night, a large study conducted in Canada from 2005 to 2010 showed similar risk associated with nightshift work in general, though only for those working 30 or more years.
Based on animal models and some evidence in humans, the International Agency for Research on Cancer in 2007 had classified nightshift work as a probable carcinogen, as a disruptor of the body's circadian rhythm suspected to lower levels of melatonin, a hormone normally produced at night that may possess several cancer-fighting properties.
However, this latest study from U.S. and Canadian researchers showed no association between hormones and elevated risk of cancer, following reason to believe melatonin influences risk by increasing production of oestrogen.
"The study demonstrated an increased breast cancer risk among women employed in nightshift work for 30 [or more] years across several occupations, with no association seen for shorter durations and no interaction with hormone receptor status apparent," the researchers reported in the journal Occupational Environmental Medicine.
Over the five-year study period, investigators led by Anne Grundy of Queens University, Kingston in Canada analyzed the medical records of more than 1,100 women who had developed breast cancer as well as nearly 1,200 who hadn't. They considered the self-reported occupational nightshift employment of the women in addition to hormone receptor status obtained through tumor pathology records.
Among the one in three women in the study with a history of nightshift work, there was no statistically significant variance across occupational categories, although other factors in play included ethnicity, household income, education, menopause status, family history of breast cancer, history of pregnancy and breastfeeding, oral contraceptive use, age of first mammogram, and lifetime history of alcohol use. However, researchers largely dismissed these other possibly confounding factors as less significant, but acknowledged study limitations including a greater proportion of Asians versus whites among those in the study who'd had breast cancer.
The study was also deemed an improvement over previous efforts for its greater examination of lifetime occupational history of participants, though women in the Kingston location versus another group in Vancouver reported self-reported through a questionnaire.
In pondering the underlying biological mechanisms by which nightshift work may impact breast cancer risk, investigators suggested that future efforts examine lifestyle differences, sleep disturbances, or genetic abnormalities relating to circadian rhythm. "While light at night and melatonin have been proposed as one pathway through which nightshift work may influence breast cancer, and data from prospective studies has generally supported a protective effect of melatonin on breast cancer, biomarker studies of night work and melatonin are less consistent," they wrote.
With a better understanding of this increased breast cancer risk, employers of longtime nightshift workers may develop healthier workplace policies — leading to a better day's rest.
Source: Grundy A, Richardson H, Burstyn I, et al. Increased Risk Of Breast Cancer Associated With Long-Term Shift Work In Canada. Occupational Environmental Medicine. 2013.