The case control study of more than 1,670 Israeli women, found that those routinely exposed to higher intensity light in their sleeping environment had 22% higher odds of developing breast cancer than women who slept in full darkness (P<0.001), according to Abraham Haim, PhD, of the University of Haifa, and colleagues.
It may be that leaving the lights on while sleeping interferes with melatonin production, which, in turn, modulates endogenous estrogen levels. Several studies have also associated the circadian rhythm-regulating hormone with a protective effect against beast cancer, Haim and co-authors reported in the February issue of Chronobiology International.
Animal research has also linked even dim light exposure during normal sleeping hours to increased weight gain and loss of glucose tolerance.
"Fortunately, this is an environmental variable that can easily be controlled," the researchers wrote in the paper.
Simple solutions may include installing light-blocking window shades or blinds in the bedroom and the use of sleep masks, they suggested.
Keeping the sleeping area dark is part of good sleep hygiene -- regardless of a possible connection with breast cancer, commented Donna Arand, PhD, of Kettering Hospital in Dayton, Ohio, and a spokesperson for the American Academy of Sleep Medicine.
But the study findings may be more hypothesis-generating than a basis for clinical recommendations, she cautioned.
"I don't think it is the definitive word in that we should start being worried about this at the present time," she told MedPage Today.
Despite the large number of women involved in the study, the use of questionnaire data weakened any conclusions that could be drawn, Arand noted.
The study analyzed questions on light exposure at night added to a broader questionnaire in the Breast Cancer in Northern Israel study, designed to look at the molecular and environmental etiology of breast cancer in the region.
Along with 794 women with breast cancer, the study included 885 controls randomly selected from members of the largest healthcare provider in Israel.
The largest proportion of women reported that their bedroom was completely dark at night (46.5% cases and 54.1% controls). Next most common was a high level of light exposure, equivalent to having all the lights switched on (29.5% and 22.6%, respectively).
A higher level of light exposure at night was the strongest predictor of breast cancer in this cohort, with a 22% elevated odds ratio after controlling for education, ethnicity, fertility, and alcohol consumption.
Sleeping with open window shutters, presumably admitting outside light, or sleeping with the television on did not significantly predict cancer in the study, Haim and colleagues noted.
The results were similar when restricting cases only to the Jewish population in the Israeli cohort, they added.
Haim's group acknowledged that the study was limited by a lack of precise measurements of light levels in the women's "habitat," which would have been a more accurate way of measuring nighttime light exposure in the bedroom compared to questionnaire-based data. Also, the they noted that observational results could not prove causality.
"Nevertheless, more precise measurements of light levels in the 'sleeping habitat' should be investigated in follow-up studies," they wrote.
The researchers reported having no conflicts of interest to disclose.
Arand reported having no conflicts of interest to disclose.
- Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, RN, BC-ADM, CDE, Nurse Planner
Chronobiology InternationalSource Reference: Kloog I, et al "Does the modern urbanized sleeping habitat pose a breast cancer risk?" Chronobiol Int 2011; 28: 76–80.