Women who live in neighborhoods with higher levels of outdoor light during the night may have a greater risk of developing invasive breast cancer than women living in areas where light levels are lower, according to a recent study from Harvard University.
This particular study found that the increased risk appeared to be limited to premenopausal women and those who were current or former smokers. It also found the association was stronger among women who worked night shifts.
The study suggests — as other studies have done previously — that exposure to light at night (referred to as LAN by researchers) may have a synergistic effect on other breast cancer risks, possibly by disrupting the body’s circadian (daily) biological rhythms.
The study also supports findings from earlier research that night-shift work is associated with an increased risk of breast cancer.
As the authors of the Harvard study point out in their paper, LAN triggers the pineal gland in the brain to stop the release of melatonin, a hormone that helps control the sleep/wake cycle. Animal studies have suggested that decreased levels of melatonin increase cancer risk.
“In our modern industrialized society, artificial lighting is nearly ubiquitous,” says Peter James, the study’s lead author and an environmental health epidemiologist at Harvard, in a released statement. “Our results suggest that this widespread exposure to outdoor lights during nighttime hours could represent a novel risk factor for breast cancer.”
For the study, James and his colleagues used health and demographic data collected during the years 1989-2013 from 109,672 nurses participating in the ongoing Nurses’ Health Study.
The homes of the nurses were geocoded, and satellite images taken by the Defense Meteorological Satellite Program were used to estimate the average LAN in the immediate neighborhood of each address. The researchers then divided the women into five groups, depending on those LAN averages.
During the 22 years of the study, 3,549 of the women developed invasive breast cancer. The researchers found that the relative risk of receiving that diagnosis was 14 percent higher among the women in the top fifth of exposure to LAN compared to those in lowest fifth.
The finding held even after adjusting for other factors that affect breast cancer risk, such as age, weight, number of pregnancies and use of hormone medications.
Subgroup analyses revealed, however, that the effect occurred only among women under the age of 50 and those who were current or past smokers. And it was greater among women with a history of working the night shift. (Given that nurses often work nights, a large number of the women in the study — 42 percent — did have such a history.)
Interestingly, the study also found the association between LAN and breast cancer was limited to estrogen receptor-positive (ER-positive) tumors, which is the most common type of invasive breast cancer.
“This finding is consistent with the hypothesis that LAN acts through estrogen receptor signaling-mediated pathways to increase breast cancer risk,” the authors write.
Plenty of caveats
This study has many limitations. Most notably, it’s observational, which means it can show only an association, not a direct causal relationship, between outdoor LAN and increased risk of breast cancer. Other factors in the most highly lit neighborhoods — ones not among those adjusted for in this study — may explain the link.
In addition, the study did not account for exposure to indoor LAN, which can also interfere with the sleep/wake cycle. Nor is it clear that satellite images of women’s neighborhoods are accurate tools for assessing the women’ actual exposure to outdoor LAN.
Still, the findings are provocative, particularly given last year’s warning from the American Medical Association (AMA) regarding the potential health problems from overhauling our roads with high-intensity LED lighting systems.
“Blue-rich LED streetlights operate at a wavelength that most adversely suppresses melatonin during night,” the AMA stated. “It is estimated that white LED lamps have five times greater impact on circadian sleep rhythms than conventional street lamps. Recent large surveys found that brighter residential nighttime lighting is associated with reduced sleep times, dissatisfaction with sleep quality, excessive sleepiness, impaired daytime functioning and obesity.”
“The AMA recommends an intensity threshold for optimal LED lighting that minimizes blue-rich light,” the statement added. “The AMA also recommends all LED lighting should be properly shielded to minimize glare and detrimental human health and environmental effects, and consideration should be given to utilize the ability of LED lighting to be dimmed for off-peak time periods.”