We know that artificial light at night appears to affect breast cancer risk: studies have shown that the incidence of the disease is higher among women who are working night shifts. This may be because the body's reaction to light suppresses the normal nighttime production of melatonin, which has the effect of boosting estrogen levels that can in turn increase breast cancer risk.
Now Israeli researchers have found that the amount of light in your bedroom at night can also increase breast cancer risk. A team at the Israeli Center for Interdisciplinary Research in Chronobiology at the University of Haifa has learned that sleeping with light in the bedroom - from a night light, as a result of leaving a television on or from street lights outside - can raise the risk of breast cancer by 22 percent compared to the risk of women sleeping in completely dark rooms. This study, the first to look at the effect of bedroom light on breast cancer risk, involved more than 1,670 women who were followed for 10 years. The investigators suggested that the solution might be as simple as blocking outdoor light with window blinds, wearing a sleeping mask, or turning off the TV.
My take? Proper regulation of hormones requires us to be bathed in full light for at least part of the day, and to experience a natural level of darkness at night. It makes perfect sense to me that spending our days in relatively dark interiors and our nights under artificial lights can disrupt our normal hormonal patters and predispose us to disease.
More on lowering breast cancer risk.