More Evidence Ties Night-Shift Work to Breast Cancer

Fran Lowry

July 09, 2013

Yet another study, this time from Canada, suggests that night-shift work increases the risk for breast cancer.

The researchers found that the length of time working the night shift is the important risk factor. A duration of 30 years or more was associated with a 2-fold increased risk, whereas a duration of less than 30 years was not associated with increased risk.

Results from the case–control study were published online July 1 in Occupational and Environmental Medicine.

"This work adds to the body of evidence that is becoming more persuasive in suggesting that long-term shift work increases the risk of breast cancer," said lead author Anne Grundy, PhD, a postdoctoral fellow at Cancer Care Ontario in Toronto.

"But we still don't know why," she told Medscape Medical News. "We would like to know what it is about shift work that's causing this increased risk. If we did, we might be able to design shift schedules that are better for health."

It has been suggested that night-shift work is a risk factor for several cancers. In fact, in 2007, the International Agency for Research on Cancer deemed shift work involving circadian disruption a "probable carcinogen."

One study (Occup Environ Med. 2012;69:551-556) showed that Danish female military workers who preferred mornings (larks) were at greater risk of developing breast cancer when they worked night shifts than women who preferred evenings (owls), as reported by Medscape Medical News.

However, several previous studies have been restricted to nurses, and few have considered the impact of tumor hormone-receptor status, Dr. Grundy explained.

She and her group examined the relation between night-shift work and breast cancer in 1134 patients with breast cancer and 1179 control subjects from 2005 to 2010 at 2 locations: Vancouver, British Columbia and Kingston, Ontario.

The women, who were employed in various capacities over the years, were asked about shift-work patterns over their entire work history, and hospital records were used to determine tumor type.

Night-shift work was stratified by duration: 0 to 14 years, 15 to 29 years, and 30 years or more.

In both the case and control groups, one third of the women had a history of night-shift work.

"The women were asked to list all the jobs they had held over their lifetime for 6 months or more, what the job was, the job title, the industry, how long they held the job, and dates so that we could tell the length of employment," Dr. Grundy explained.

We also asked about "the proportion of time spent on the day, evening, and night shift in each of their jobs, so we weren't using just 1 representative job, like nursing, which has been done in other studies. We were getting a full occupational history, which is a strength of this work," she said.

No association between night-shift work and breast cancer risk was observed for either the 0- to 14-year duration or the 15- to 29-year duration. However, women who worked night shifts for 30 years or more had a greater than 2-fold increased risk for breast cancer (odds ratio, 2.21; 95% confidence interval, 1.14 - 4.31).

The greatest proportion of night-shift work was in the health field; these jobs accounted for 41% of jobs that were 15 to 29 years in duration and 44% of those 30 years or more in duration.

Risk was higher in women whose tumors were sensitive to estrogen and progesterone.

It has been hypothesized that light at night and melatonin are a cause of the link between breast cancer and shift work, but sleep disturbances, clock gene dysregulation, and lifestyle differences should also be considered, Dr. Grundy said.

"There are a number of proposed theories for the increased risk, but we weren't able to evaluate these in our study." Also, because the data were collected from 2005 and 2010, we did not know to ask about thinks like vitamin D or lifestyle differences, she explained.

Light at Night Affects All Women

Richard Stevens, PhD, a cancer epidemiologist at the University of Connecticut Health Center in Farmington and an expert on circadian rhythm disruption, light at night, and breast cancer risk, told Medscape Medical News that this study was well conducted, although he questions the lack of association seen with shorter durations of night work.

"It must be kept in mind that the original reason for examining nonday-shift work was as a surrogate for circadian disruption from electric light at night," said Dr. Stevens, who was not a part of this study.

"Nonday workers were presumed to have more of this exposure than day workers. However, all women in the modern world are exposed to electric light to some extent during the period from dusk to dawn," he said.

This means that "in all the epidemiologic studies of shift work, the comparison group of day-working women has a history of exposure," Dr. Stevens noted. Therefore, if light at night does increase the risk of breast cancer, shift-work studies only capture a small portion of the societal impact."

The study was funded by the Canadian Institutes of Health Research. Dr. Grundy and Dr. Stevens have disclosed no relevant financial relationships.

Occup Environ Med. Published online July 1, 2013. Abstract

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