No link between “light and Night” and breast cancer?

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."