According to a recent article in the Washington Post, a new breast cancer study has revealed a correlation between levels of exposure to light in the nighttime and rates of breast cancer. Scientists mapped satellite imagery of nighttime light distribution onto cancer registries and found that geographic areas with an average amount of nighttime illumination show up to 37 higher rates of breast cancer than communities with the lowest light.

Scientists hypothesize that light may lower levels of melatonin, a tumor-suppressing hormone produced by the brain at night.

Many breast-cancer researchers have suspected for years that there is a link between exposure to light in the nighttime and breast cancer. Past studies have demonstrated that rats sleeping in lit up cages have higher cancer rates than rats allowed to sleep in darkness. Also, women who work night shifts have been shown to have a 60 percent above normal risk of breast cancer. These findings have led the World Health Organization to classify night shift work as a probable carcinogen.

This study controlled for other factors that can affect health, including ethnicity and wealth. However, it did not measure indoor lighting or the types of light bulbs being used -- a factor that can affect melatonin levels.