A new retrospective study has reported that light in the bedroom at night is associated with increased risk of breast cancer. Night shift work is recognized as a risk factor for breast cancer because of its disruptive effects on circadian rhythms. Completely blind women have lower levels of breast cancer than partially blind women and sighted women, which is thought to be the result of mechanisms influenced by the perception of light by the eye, such as melatonin or circadian synchronization.

The study included 1,679 women in the Breast Cancer in Northern Israel study. Participants were interviewed from 2006 to 2008 regarding bedroom light levels in previous years, including light coming into the room from outside the bedroom and sleeping with the television on. The women were asked to rank their bedroom light levels at night on a scale of 1 to 4 (1 = complete darkness, 2 = low light, 3 = average light, and 4 = very strong light (all the lights on)).

High exposure to light at night in the bedroom was found to be associated with a 22% increased risk of breast cancer after controlling for breast cancer risk factors such as level of education, ethnicity, history of pregnancies, and alcohol consumption. To the best of the authors' knowledge, this is the first study to have identified a strong positive association between bedroom light intensity and risk of breast cancer. The authors conclude that not only should night shift work be considered a potential risk factor for breast cancer, but also light at night in the bedroom.