Results from a large cohort study provide no evidence to support the “intriguing but unproven” theory that circadian rhythm disturbance caused by exposure to light at night (LAN) increases breast cancer risk, say investigators.
“Our study suggests that light levels at night do not materially increase a woman’s risk of breast cancer, but further research will be needed before a definitive conclusion can be reached,” lead author, Anthony J. Swerdlow, MD, PhD, professor of epidemiology at the Institute of Cancer Research in London, United Kingdom, said in a statement.
However, a US expert on circadian rhythm disruption said the study is weak because exposure to light was misclassified, as well as self-recalled.
The study analyzed prospective data from 105,866 women recruited to the ongoing Breast Cancer Now Generations Study shows no association between LAN and increased overall risk for breast cancer (hazard ratio [HR], 1.01).
There was also no evidence of an association between LAN and risk for different breast cancer tumors.
These subtypes included invasive (HR, 0.98), in situ (HR, 0.96), estrogen receptor (ER)–positive (HR, 0.98), and ER-negative (HR, 1.16) breast cancers, the investigators say in a report published online January 23 in the British Journal of Cancer.
These findings didn’t change after adjustment for menopausal status, sleep duration, sleeping at unusual times, and a history of night work, the study authors note.
Night waking with exposure to light around age 20 years was associated with a reduced risk for premenopausal breast cancer and “may warrant further investigation,” the researchers say. The HR for breast cancer overall in this group was 0.74, with an HR of 0.69 for ER-positive breast cancer.
“Although our findings raise the possibility of a protective effect in pre-menopausal women who reported night waking with exposure to light at age 20, this was a subset analysis with modest statistical significance, based on relatively few breast cancer cases, has not been reported elsewhere, and has no plausible mechanism, so cannot be taken as strong evidence unless confirmed independently,” they write.
Intriguing Hypothesis
Previous studies have proposed that with the advent of industrialization, exposure to artificial light at night disrupts circadian rhythms, reducing melatonin secretion, raising levels of circulating estrogen, and suppressing tumor antiproliferative processes.
“The theory that exposure to light at night might affect a woman’s risk of breast cancer dates back 30 years,” Dr Swerdlow acknowledged. “It remains an intriguing but unproven hypothesis.”
Study Is Weak, With “Massive Misclassification”
When asked to comment, however, Richard G. Stevens, PhD, professor of cancer epidemiology at the University of Connecticut Health Center in Farmington, said the results of the current analysis are nothing more than a “massive case of exposure misclassification.”
As previously reported by Medscape Medical News, Dr Stevens is an expert on circadian rhythm disruption who first hypothesized in 1987 that suppression of nocturnal pineal melatonin production in response to LAN might explain the rise in breast cancer rates following industrialization.
“This study is exceedingly weak and does not provide good evidence,” Dr Stevens said in an interview. “Despite it being very large, it has very little power. It tells us nothing about participants’ light exposure in the evening, from dusk until they went to bed.”
Dr Stevens also emphasized that “there is more support for the LAN and breast cancer idea than lack of support. We get almost weekly epidemiologic reports on this.”
“I think this study has no implications for your clinicians at all,” he told Medscape Medical News. “I would put virtually no weight on the results, which have little or no ability to identify a true effect should one exist. The study authors allude to this and then dismiss it.”
The investigators acknowledge that the self-reported LAN exposure information is a potential limitation of the study. “However, since this information was ascertained before breast cancer occurrence, it should not have biased the results,” Dr Swerdlow and colleagues say. “Misclassification would be likely to have diluted any true relation, but the lack of any sign of raised risk does not suggest a relationship.”
Study Details
The Generations study recruited 113,207 women between June 2003 and June 2012 and is following them for 40 years. Mean age of the participants at recruitment was 46.5 years.
Questionnaires were used to determine breast cancer risk factors, such as obesity, physical activity, alcohol consumption, family history of breast cancer, age at first period and at menopause, number of children and maternal age at childbirth, and duration of breastfeeding.
For their analysis, Dr Swerdlow and colleagues excluded 6581 participants with a history of breast cancer.
Information on sleeping patterns and bedroom light levels at age 20 years was also recorded, with follow-up every 3 years by postal or online questionnaires. With an average follow-up of 6.1 years, 1775 cases of breast cancer have been diagnosed.
Women were asked to report their level of exposure to light at night in the room in which they slept for the year before joining the study and at age 20 years.
All sources of artificial and natural light both inside and outside the bedroom were considered, as was the extent to which window coverings blocked light.
A total of 37% of participants reported waking at night and turning on lights or going into a bright room during the year before recruitment; 10% reported this at age 20.
Although women from earlier birth cohorts reported somewhat lower LAN levels, 49% of participants reported “medium” levels of LAN in the year before the study and 47% of participants for age 20. The categories of light exposure included enough to read; enough to see across the room but not read; enough to see your hand in front of you, but not to see across the room; and too dark to see your hand.
“Even under controlled laboratory conditions, this wouldn’t be sufficient light to suppress melatonin,” Dr Stevens said.
“The exposure of interest is chronic excessive light exposure in the evening and at night,” he noted. “The ideal study, which is completely impossible to conduct, of course, would be to put a light meter on the foreheads of 10,000 women for 10 weeks and then follow them for 10 years.”
Still, for clinicians counseling patients, there is “adequate evidence to dim the light at night,” said Dr Stevens, who uses a 40- or 60-watt incandescent bulb with a shade for reading in bed.
Before the advent of electric light, the transition to nighttime physiology in humans began at dusk, he noted, pointing to a recent study looking at the impact on circadian rhythms in a group of campers after 1 week of exposure only to natural light.
“With no electric light, the transition to nighttime physiology began at sundown,” Dr Stevens pointed out.
The Generations study was funded by Breast Cancer Now and the Institute of Cancer Research. Dr Swerdlow and coauthors and Dr Stevens have disclosed no relevant financial relationships.
Br J Cancer. Published online January 23, 2018. Abstract
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