A population-based study investigated the interplay between exposure to light at night --- levels of which are affected by bedroom lighting as well as graveyard shift work -- and breast cancer risk. The researchers speculated graveyard shift workers could be boosting their risk of developing breast cancer by some 60 percent.
The study, led by Scott Davis, an epidemiologist at the University of Washington School of Public Health and Community Medicine in Seattle, is found in the Oct. 17 Journal of the National Cancer Institute.
"Either of these factors, light at night or working the graveyard shift impacts the normal circadian biology," Davis told United Press International. "And we think that behavior reduces the normal night-time rise in melatonin, which regulates reproductive hormones and estrogen. In other words, by interrupting melatonin, you may increase the level of estrogens, which can impact the risk of breast cancer."
The brain's pineal gland is itself a graveyard worker, with its peak melatonin-producing levels occurring during night hours. If light or lack of sleep interferes with that production, a woman's body signals its ovaries to work overtime, producing extra loads of estrogen, a known promoter of breast cancer.
Davis told UPI more studies are needed before any firm conclusions are drawn. Particularly, he plans to engage studies that specifically measure levels of hormones in subjects, something that wasn't done in the present study.
Davis's current work involved interviews with some 800 women who had been diagnosed with breast cancer. That group was compared to another group of women of the same age and from the same area.
Davis examined lifestyle factors of each group, focusing particularly on night and history of shift work during the decade prior to their cancer diagnosis.
Women who toiled at night proved to have a 60 percent increased risk for breast cancer. The researchers also reported the possibility of breast cancer increased with each additional night hour worked.
The study also reported a fact researchers said is not statistically significant, namely that women who reported having the brightest bedroom lights were at a slightly higher risk. The measure was whether the room was so dark the women could see her hand in front of her face or the end of her bed.
"Basically it is premature to make specific recommendations to individual women about changes in behavior," Davis said. "We don't know whether working the graveyard shift is a potential cause of breast cancer or what aspects of nighttime work are potential culprits. It hard for people who worry, but we can't really recommend anything without a definitive study."
"This is very interesting research that provides another view of how hormones can (potentially promote breast cancer)," said Dr. George N. Peters, professor of surgery at the University of Texas Southwestern in Dallas. "I feel we get a lot of these studies that present interesting facts and before they are looked into at great detail, it becomes publicly circulated and people assume it's the gospel. Again, it's an interesting fact, but if people are worried they should talk to their doctor."
(Reported by Kelly Hearn in Washington.)