Results of the study, the first population-based investigation of the relationship between breast-cancer risk and exposure to light at night as determined by sleep habits, bedroom lighting and graveyard-shift work, will be published tomorrow in the Journal of the National Cancer Institute. The NCI funded the study.
Led by Scott Davis, Ph.D., a member of the Hutchinson Center's Public Health Sciences Division, the study is one of two independent investigations to be published in tomorrow's JNCI that conclude sleep interruption, especially among women who work the night shift, is linked to a significant increase in breast-cancer risk. "Although there have been a number of studies looking at the health effects of night-shift work, from heart disease to stomach ailments, this is the first that has looked comprehensively at both graveyard-shift work and light at night as independent risk factors for breast cancer," Davis said.
The study involved in-person interviews with more than 800 Seattle-area women who had been diagnosed with breast cancer, as well as a similar number of age-matched control subjects. The study assessed exposure to light at night and history of shift work, among other factors, during the 10 years prior to breast-cancer diagnosis.
Davis and colleagues found that women who worked the graveyard shift at least once during the decade before breast-cancer diagnosis were at approximately 60 percent increased risk for breast cancer compared with those who did not work the graveyard shift. In addition, the risk of breast cancer significantly increased with each additional hour per week of graveyard-shift work.
The link between sleep, light at night and breast cancer may involve melatonin, a hormone produced by the brain's pineal gland. Production of melatonin peaks at night during sleep. One theory is that nighttime sleep deprivation or exposure to light at night somehow interrupts melatonin production, which in turn stimulates the ovaries to kick out extra estrogen - a known hormonal promoter of breast cancer.
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 comparison group of women diagnosed with stroke and cardiovascular disease. The theory behind this reduced risk: Because blind women are immune to fluctuations in light, their melatonin levels remain constant, which also keeps their circulating estrogen levels in check.
Davis and colleagues found that breast-cancer risk was associated with the number of years of shift work and the number of hours per week of working the graveyard shift. For example, women who logged at least four and a half years of frequent night-shift work - during which they were awake at least three nights per week between 1 and 2 a.m., when noctural melatonin levels are typically at their peak - experienced a twofold increased risk of breast cancer. Similarly, cancer risk increased by 14 percent for each night per week of wakefulness during this crucial time period.
There was also some indication - although not statistically significant - of increased breast-cancer risk among the women who had the brightest bedrooms. The degree of ambient bedroom light was based on self-reported recall, such as whether the room was so dark a woman could not see her hand in front of her face or whether she could see to the end of her bed. There was, however, no increased cancer risk associated with getting up at night and turning on a light, such as when using the bathroom.
So what's the upshot of these findings regarding light at night, graveyard-shift work and breast cancer?
"We'd be premature to make recommendations about specific interventions or behavior modifications for people with average light-at-night exposure," said Davis, also professor and chairman of the Department of Epidemiology at the University of Washington School of Public Health and Community Medicine in Seattle. "More important from a research point of view is to focus on the general notion that anything that disrupts the normal circadian biology of the body might affect hormones that influence cancer risk." Take stress, for example. Could the inherent stress of shift work - independent of nocturnal light exposure - also cause the fluctuation of certain key hormones?
"Among shift workers, quite a number of factors could affect the mechanisms that control the body's circadian rhythms," Davis said. "One is stress, considering the kinds of jobs that require shift work, from police, fire and rescue work to nuclear-power-plant monitoring to factory work, medicine and nursing."
To better gauge the impact of shift work on the body's hormones, Davis and colleagues are planning a study to look at the effects of circadian disruption on the production of melatonin and estrogen.
"Our next step will be to compare these hormone levels among night-shift nurses and day-shift nurses. 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."