First study to measure the influence of night-shift work on hormone levels that may affect breast-cancer risk

Three years ago, investigators at Fred Hutchinson Cancer Research Center made headlines nationwide when they published the results of a population-based study that suggested women who work the graveyard shift have an increased risk of breast cancer as opposed to women who work the day shift.

Now, the researchers are taking their investigation one step further by launching the first study of its kind to measure the influence of night-shift work on hormone levels that may affect breast-cancer risk.

More than 300 female Seattle-area hospital and laboratory shift workers are needed for this National Cancer Institute-funded study, led by Scott Davis, Ph.D., a member of Fred Hutchinson's Public Health Sciences Division.

"There is evidence that shift work may increase the risk of developing breast cancer, and perhaps have other impacts on health as well," Davis said. "It is important to identify the biological mechanisms that may be responsible for such effects so that strategies might be developed to reduce risk in the future."

Davis and colleagues hypothesize that the increased risk in breast cancer they've observed among night-shift workers is most likely due to disruptions in the sleep/wake cycle and exposure to light at night, both of which may affect endocrine function and the regulation of reproductive hormones implicated in the development of breast cancer.

One theory, Davis said, is that nighttime sleep deprivation or exposure to light at night somehow interrupts the production of melatonin, a hormone produced at night by the brain's pineal gland. Melatonin production in turn prompts the ovaries to make extra estrogen — a known hormonal promoter of breast cancer.

To better gauge the impact of shift work on the body's hormones, Davis and colleagues designed this follow-up study to look at the effects of circadian disruption on the production of melatonin and estrogen, among other biological factors.

"We need to measure the specific biological effects of shift work, because right now, the connection between working nights and decreased melatonin/increased estrogen has yet to be shown in humans," said Davis, also chairman of the Department of Epidemiology at the University of Washington School of Public Health and Community Medicine.

Indirect support of the melatonin/breast-cancer connection comes from an earlier study of blind women, who were found to have a 20 percent to 50 percent reduced risk of breast cancer as compared to a group of sighted women diagnosed with stroke and cardiovascular disease. The theory behind the reduced risk, Davis said, is that blind women do not perceive light and are therefore not sensitive to fluctuations in light so their melatonin levels remain constant, which also keeps their circulating estrogen levels in check.