A recent study conducted by the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute found that satellite images of outdoor lighting are unrelated to actual light levels reaching the eye. These results challenge previous studies linking satellite images of outdoor lighting with increased incidences of breast cancer. Light entering the eye regulates our body’s circadian rhythms, and disruptions in these rhythms may lead to serious health problems such as cancer. However, according to the LRC study, the spectrum, quantity, and duration of light exposure reaching the eye must be specified in order to determine the effects of light on human health, and satellite photometry cannot do this.
“After shift work was identified as a probable carcinogen by the World Health Organization, some studies were published that claimed a statistical association between light at night and the incidence of breast cancer. However, these studies relied on satellite photometry and subjects’ self-reports of bedroom brightness as measures of light exposure. None of these studies employed actual light measurements at the eye,” said LRC Director and principal investigator Mark Rea, Ph.D. “Before statistical associations between light at night and disease can graduate to a cause and effect relationship, it is necessary to measure the light as a potential causal agent. Our study showed no relationship between the measured light on the ground and the measured light in space.”
Scientists at the LRC are among many groups who have been working to identify factors, such as circadian disruption, that may lead to the higher incidence of breast cancer among rotating shift nurses. Patterns of light and dark are the main cues for synchronizing our internal biological clock with the 24-hour solar day, keeping us “in synch” and contributing to good health. These light/dark cues must, however, reach the retina, the back part of the eye, in order to have an impact on our circadian rhythms, according to Rea.
Using calibrated light meters, LRC scientists compared light levels in and outside the bedrooms of 72 female school teachers with satellite-measured light levels of ground brightness. The researchers then measured the 24-hour light/dark exposure of the teachers over the course of seven consecutive days. The subjects lived in a range of areas with both high and low amounts of sky brightness. These women were specifically selected because they did not work rotating shifts, which have been shown to place women at higher risk of breast cancer. If satellite measured levels of light were predictive of light exposures on the ground, irrespective of shift work, then those women who lived in bright areas might be at higher risk. The LRC study showed no relationship between satellite-measured light levels and actual light levels on the ground.
In order to obtain accurate light exposure measurements at the eye, each teacher wore a Daysimeter, a small, head-worn device developed by the LRC to measure an individual’s exposure to the daily and nightly levels of “circadian light.” The definition of circadian light is based upon the potential for light to suppress melatonin synthesis at night, as opposed to measuring light in terms of how it stimulates the visual system. Melatonin is a hormone produced in the evening and under conditions of darkness, which acts as a timing messenger signaling to the body when it is nighttime. The researchers found that the teachers were all in synch with a regular 24-hour cycle, regardless of the sky brightness outside their homes.
“It is important to note, however, that these findings do not undermine the foundational data using animal models that link melatonin suppression by light at night and cancer risks, nor does it contradict the statistical association between shift work and breast cancer risk in humans. Rather, these findings simply undermine the inference for a causal relationship between light at night, as measured by satellite photometry, and breast cancer incidence,” explained Rea.
Rea carried out his study with LRC researchers Mariana Figueiro, Ph.D., and Jennifer Brons, who are also co-authors on a paper detailing the results. The study, “Measurements of light at night (LAN) for a sample of female school teachers,” is published in the journal, Chronobiology International, Volume 28, Issue 8.
This project was funded by a grant from the Centers for Disease Control and Prevention; the Trans-NIH Genes, Environment and Health Initiative; and the National Electrical Manufacturers Association.