From an excellent article about light pollution (not online) by David Owen in the 20 Aug 2007 issue of The New Yorker:
Richard Stevens, a cancer epidemiologists at the University of Connecticut Health Center, in Farmington, has suggested a link between cancer and the “circadian disruption” of hormones caused by artificial lighting. Early in his career, Stevens was one of many researchers struck by the markedly high incidence of breast cancer among women in the industrial world, in comparison with those in developing countries, and he at first supported the most common early hypothesis, which was that the cause must be dietary. Yet repeated studies found no clear link to food. In the early eighties, Stevens told me recently, “I literally woke up in the middle of the night — there was a street lamp outside the window, and it was so bright that I could almost read in my bedroom — and I thought, Could it be that? A few years later, he persuaded the [directors] of the Nurses’ Health Study . . . to add questions about nighttime employment, and the study subsequently revealed a strong association between working the night shift and an increased risk of breast cancer. [The researchers] wrote, “We hypothesize that the potential primary culprit for this observed association is the lack of melatonin, a cancer-protective agent whose production is severely diminished in people exposed to light at night.”
Exposure to strong light at night reduces the amplitude of your circadian rhythms. That causes a thousand changes. To decide that one of them (“lack of melatonin”) is the one that matters is highly premature. If reducing circadian amplitude increases cancer, it follows that getting more light during the day — which surely increases circadian amplitude — will reduce cancer.
The article also says:
Growing numbers of us pass most of our waking hours “in a box, looking at a box,” as Dave Crawford put it . . . Fewer and fewer of us spend much time outside at all, except in automobiles.
I have measured the light inside cars (front seats) several times and found it is quite strong (you are close to a big window). If the article is arguing that night light is bad and causes cancer, I am unconvinced. Night light exposure and daylight exposure are confounded — people who work night shifts get more night light and less daylight.
The Nurses Study paper: E Schernhammer, K Schulmeister. Light at night and cancer risk. Photochemistry and Photobiology, 2004, Vol 79, Iss 4, pp 316-318.