January 11, 2011— A new study suggests that light at night, specifically in the bedroom, increases the risk for breast cancer. The finding comes from an Israeli case–control study of 1679 women, published in the February issue of Chronobiology International.
This is the third study to suggest that women who have lights on in the bedroom are at greater risk for breast cancer than those who sleep in total darkness. (The previous 2 studies were by Davis et al in J Natl Cancer Inst. 2001;93;1557-1562 and O'Leary et al in Am J Epidemiol. 2006;164:358-366.)
However, all 3 of these studies have the same serious potential flaw, according to an expert in the field. Richard Stevens, PhD, professor of cancer epidemiology at the University of Connecticut in Farmington, told Medscape Medical News that in each of these studies, researchers asked women to recall whether they slept with the lights on years earlier, and how often during the night they switched on the lights. In addition, the breast cancer patients were asked these questions after they had been diagnosed the disease, he pointed out.
"This introduces the potentially serious error of recall bias," he said.
However, Dr. Stevens, who was the first to suggest that there might be a link between electrical light and breast cancer, said that there is mounting evidence of an association between light at night and breast cancer. There has also been a sea change in attitudes toward the idea. "When I first proposed this theory 23 years ago, cancer researchers just smiled and dismissed it; now they are saying that it's interesting and plausible."
In the latest study, researchers from the University of Haifa, Israel, used data that had been collected using questionnaires in the Breast Cancer in Northern Israel study.
The information on light-at-night exposure was obtained from individual interviews (conducted from 2006 to 2008), and women were asked about bedroom light levels, light coming in the room outside the bedroom, the availability of shutters in the bedroom, and sleeping with the television on. Women were asked to rate their night-time bedroom light levels from 1 to 4, where 1 represented complete darkness, 2 was low light, 3 was average light, and 4 was very strong light (with all the lights on).
The intensity of bedroom light emerged as the strongest predictor of breast cancer, the researchers report. After controlling for education, ethnicity, fertility, and alcohol consumption, light at night in the sleeping habitat significantly increased the risk for breast cancer (odds ratio, 1.22; 95% confidence interval, 1.118 to 1.311; P < .001).
This is the "first study to have identified an unequivocal positive association between bedroom light intensity and breast cancer risk," the researchers conclude.
But 2 experts in the field took issue with the word "unequivocal."
"I think that is going too far," said Dr. Stevens. This is a strong, well-done epidemiological study, and this is a solid group of researchers, he said, but there is a serious potential for recall bias, he emphasized.
Echoing this concern was Johnni Hansen, PhD, senior epidemiologist at the Danish Cancer Society's Institute of Cancer Epidemiology in Copenhagen. He told Medscape Medical News that he would hesitate to describe this as "unequivocal," and he emphasized the potential for recall bias. In addition, he noted that there was a large difference is the response rates — only 52% of controls, compared with 86% of cases, agreed to answer the questions on light in the bedroom. He said this is "worrying" and that "we would prefer there to be a similar response rate for the 2 groups."
Evidence Is Mounting
There is mounting interest in, and mounting evidence for, his theory that light at night increases the risk for breast cancer, Dr. Stevens maintained.
Breast cancer is the one major cancer for which there is no consensus on the main causes, he pointed out. The risk factors that are known so far (reproductive history, diet, exercise, etc.) account for less than 50% of the international variation that is seen in the incidence of this disease. In addition, the incidence is increasing globally for unknown reasons, he added.
The light-at-night theory proposes that the increasing use of electricity to light the night leads to circadian disruption, which accounts for part of the breast cancer burden in the modern world and the rising risk in developing countries. This, in turn, has led to predictions that nightshift workers are at increased risk, blind women are at lower risk, long sleep duration means lower risk, and that community night-time light level would codistribute with breast cancer incidence on the population level. There is some evidence for all of these, as Dr. Stephens outlined in a recent review (Int J Epidemiol. 2009;38:963-970).
The issue has already moved from the realms of research into the real world. In 2007, the International Agency for Research on Cancer classified shift work that involves circadian disruption as "probably carcinogenic to humans." On the basis of that ruling, compensation was paid to women who developed breast cancer after long spells of working at night. The women had worked as nurses and flight attendants, and compensation was paid out by the Danish National Board of Industrial Injuries, as reported by Medscape Medical News at the time.
Dr. Hansen, who has been involved in some of the studies suggesting that long-term shift work increases the risk for breast cancer, said there is a lot of research now suggesting that there is a link. "This is not causal, but it is probable," he said. There is a good biologic theory to explain how it could happen, with disruption of the circadian rhythms affecting the synchronization of hormone secretion, he said.
The research on light at night, specifically in the bedroom, increasing the risk for breast cancer is "very limited," and he is more "skeptical" about this, he said. But it is supportive of shift work increasing the risk for breast cancer, he added.
Chronobiol Int. 2011;28:76-80. Abstract
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Cite this: Lights in the Bedroom May Increase Breast Cancer Risk - Medscape - Jan 11, 2011.