College researchers are studying whether electric light changes hormone levels in women and makes breast cancer more prevalent in developed countries.

Richard Stevens, a cancer epidemiologist at the University of Connecticut (search), said there is no scientific consensus on why there is a higher rate of breast cancer in the developed world.

He said literature on breast cancer (search) includes many discredited theories on how the environment and lifestyles may contribute to the onset of the disease.

"We knew more about the cause of breast cancer 20 years ago than we do today," Stevens said. "What we do know is that it must have something to do with industrialized society."

Researchers are looking for new explanations, and Stevens and other researchers are focusing on electric light.

Their theory is that prolonged periods of exposure to artificial light disrupt the body's circadian rhythms (search) — the inner biological clocks honed over thousands of years of evolution to regulate behaviors such as sleep and wakefulness.

They are looking into whether that disruption affects levels of hormones such as melatonin (search) and the workings of cellular machinery, and whether it triggers breast cancer.

"Mankind has only been exposed to these light sources for 150 years or so," Stevens said.

So far, the theory is based largely on suggestive, but inconclusive, observational studies. For instance, night-shift workers such as nurses tend to be more prone to develop breast cancer than day-shift workers, and blind women are less likely to have breast cancer than women with sight.

In a recent study, Stevens and scientists at Yale University School of Medicine (search) identified a possible genetic mechanism that could help explain how artificial light could trigger breast cancer.

Pre-menopausal women with a variation of a "clock gene," which helps govern the regulation of the body's response to night and day, tend to have a higher risk of cancer.

"I'm not saying this is a cause, but that the evidence shows it is worth investigating," Stevens said.

Scientists believe that environmental and lifestyle factors, not inherited risks, are the cause of nine out of 10 breast cancer cases.

While smoking is linked to lung cancer and the human papillomavirus to cervical cancer, breast cancer researchers are not sure what lifestyle or environmental factors women should worry about.

Antiperspirants and wire bras are included in some widely circulated but largely rejected theories. And in 2003, the National Cancer Institute (search) convened 100 breast cancer experts who concluded there is no evidence that miscarriages or abortions increase the risk of breast cancer.

Yet epidemiologists such as Stevens say other risk factors must exist and they urge that more studies be conducted.

"We absolutely need studies," said Deborah Winn, chief of the clinical and genetic epidemiology research branch of the National Cancer Institute. "If we have those answers, we might have the potential to improve prevention."