Exposure to light at night is the most powerful factor in breast cancer besides genetic defects, according to a new University of Haifa study. Although one can't easily move to a dark neighborhood, stop using computers and watching TV late at night, or refuse to do shift work, it is advisable to close the shutters when you go to bed, wear eye shades if you can't darken the bedroom, avoid night lights and lower lights in working environments after sunset. These recommendations come from new Israeli research just published in Chronobiology International. Women who live in neighborhoods and streets with strong outdoor lighting at night are more likely to contract breast cancer than those who have minimal nocturnal lighting, according to the research performed by Prof. Avraham Haim, a chronobiologist and expert in evolutionary and environmental biology at the University of Haifa; Itai Kloog, a doctoral student in the natural resources and environmental management department; and Prof. Boris Portnov. There is also some unpublished evidence that men in well-lit surroundings are at higher risk for prostate cancer, but there was no link between excess light at night and lung cancer, which is caused almost exclusively by smoking. Kloog, Haim and Portnov overlaid satellite images produced by NASA with geographical data on breast cancer from Israel's National Cancer Registry. They also processed questionnaires filled out by 100 women with breast cancer and 100 healthy women about a wide variety of socioeconomic, environmental, genetic and other factors and their exposure to light at night. They found that the breast cancer rate in neighborhoods with average night lighting was 37 percent higher than in those with the darkest streets, while the rate was an additional 27% higher in areas with the highest amount of light. Prof. Tamar Peretz, chairman of the Sharett Institute of Oncology at Hadassah-University Medical Center in Jerusalem's Ein Kerem neighborhood and one of the country's leading experts on breast cancer, told The Jerusalem Post that the study was "very interesting and may explain some of the causes of breast tumors. There are many causes, and light at night may be one, but we don't know how important it is. I would recommend reducing lighting at night. It certainly can't hurt." The findings bolster theories, promoted by studies of night-shift workers such as nurses, that exposure to light when one should be sleeping increases the risk of cancers. Haim told the Post that scientists have suggested the hormone melatonin - produced by the pineal gland in the brain during sleep in darkness - was involved. Melatonin, he said on Wednesday, is a powerful antioxidant that promotes the suppression and minimizes the expression of cancer genes such as BRCA1, BRCA2 and P53. "Thomas Edison's invention of the electric lightbulb was great and changed the world," said Haim, "but what does it do to health? In recent years, there has been an investigation of seasonality of animals. When rodents in the field were exposed to strong light at night, they died after it disrupted their seasonal acclimatization. Separate studies have shown that blind women have significantly lower breast cancer rates than sighted women. Clinical research on night-shift workers has also shown a higher risk of cancer." A part of the World Health Organization announced two months ago that it had recognized night-shift work as a "probable carcinogen" and thus as risky as exposure to certain toxic substances. A study a few years ago found that young children who had a light on all night in their rooms were at higher risk for developing nearsightedness, and it may also slow their sexual development. Non-image-forming photo receptors in the eye's retina affect the pineal gland, which makes more melatonin nocturnally if there is little or no light. Intensive blue-tinged light, such as that produced by fluorescent bulbs, reduces melatonin production more than other bulbs, although short-waved fluorescent lights are recommended to save energy. More studies on this subject should be conducted, said Haim, who noted that "light is not only a source of pollution, but also a carcinogen, and this should be taken into consideration." But he added that it was not the only risk, nor the most important one.