I’m grateful to work the day shift. I never gave my work schedule much thought until I started working at Safety+Health and reading study after study about possible health risks associated with shift work.

I wrote an article in January summarizing some of those findings, particularly as they related to women. Now there’s another cause for concern – a study from Queen’s University in Kingston, Ontario, shows long-term shift work may increase the risk of breast cancer. Women who were employed in shift work for 30 years or more had roughly twice the cancer risk as women who had never been employed in shift work.

Although a number of shift work studies have focused on health care workers, this one found that the cancer risk existed for a variety of occupations. Study author Anne Grundy said those findings suggest cancer risk is less likely to be linked to a job characteristic – such as nurses being exposed to X-rays – and more likely related to shift work patterns.

“What it is about shift work that is increasing the cancer risk is still unclear, which is unfortunate because that’s what everybody would like to know,” said Grundy, who was a Ph.D. student at Queen’s University when she undertook the study.

She cited multiple theories as to why shift work may be hazardous. A popular one involves melatonin, a hormone that helps regulate the body’s sleep cycle and is thought to be cancer protective. Indoor light exposure inhibits melatonin production, so night workers may not be producing as much as they should. Grundy said other theories have to do with misalignment of the body’s circadian rhythm, sleep deprivation, unhealthy lifestyle due to limited food options and less time for physical activity, and lack of vitamin D exposure.

The obvious question is what can we do about it – is there a safe number of nights to work in a row, or a certain night schedule that’s best? The answer is we don’t know…yet. “Identifying which shift patterns are better is still a work in progress,” Grundy said.

As more studies illuminate the dangers associated with shift work, I’m hoping it’s not long before researchers pinpoint why so we can begin developing interventions to protect workers around the clock.

The opinions expressed in "Research Spotlight" do not necessarily reflect those of the National Safety Council or affiliated local Chapters.