Light and Dark exposure changes cancer risks
Subject: Constant illumination increases risk for certain cancers in mice
Our long term readers know that I have a specific interest in the effect of light and darkness on cancer development. Although most people are quick to blame a toxic environment for causing cancer, few consider light as a pollutant. Yet light is a pollutant which is having long term toxic effects on our health.
A new study has come out adding another piece to this story. Data from mice subjected to constant illumination suggest that artificial light may increase risks of lung and liver cancers and leukemia.
Exposure to light at night reduces production of melatonin, a hormone that calibrates the body's biological clock and its secretion of estrogen. This may explain why working the graveyard shift appears to increase a woman's risk of breast cancer and possibly other cancers associated with estrogen.
Scientists in St. Petersburg , Russia , subjected 50 female mice to alternating in 12- hour periods of light and dark and exposed 50 similar mice to constant light. Of the light exposed mice, 17 developed lung or liver tumors, or leukemia. Just one mouse in the light-dark group developed any of these cancers. 
The mice exposed to constant light also had more regular menstrual cycles and ate less. Both group gained weight during the experiment.
This isn't the first study on light and dark cycles affecting cancer development in mice.
A prior study published in 2001 showed that Light and dark exposure compared to 24 hour a day lighting “ decreased the incidence and size of mammary adenocarcinomas, and the incidence of lung metastases.”  A more recent study published in 2003, showed that, “ Constant treatment with melatonin decreased incidence and size of mammary adenocarcinomas, and incidence of lung metastases, compared to control” 
In another study of varying light and dark exposure, pregnant rats were injected with a carcinogen. One group of rats were then exposed to continual light through their pregnancies, one third were left in continual darkness, and the last group lived in alternating dark and light at 12 hour intervals. Once the rats delivered, they and their offspring lived out their natural lives with regular alternating light and dark at 12 hour intervals. The offspring of rats kept in perpetual daytime during pregnancy were more than 2.5 times as likely to develop tumors as the offspring of the rats who had lived in alternating light and dark. The offspring of the rats who lived completely in darkness during pregnancy were even less likely to get cancer. They were 2.4 times less likely to develop tumors than the light/dark group. 
Somewhat perplexing results occurred when mice were given melatonin for 5 days per month. They lived longer but were more likely to get tumors.  The moral here is that if you take supplemental melatonin, you take it consistently.
Of course all these studies are talking about rodents. Rats and mice tend to live in dark, very dark holes and tunnels. Does this light and dark effect only apply to them and not to humans? One would hope so, but the epidemiological evidence suggests that humans also are affected by light and dark exposure. Women who work night shifts are more likely to get breast cancer. Women who are blind and use less night time lighting are protected from breast cancer. Rats and mice may live in dark holes; once upon a time people lived in caves. Perhaps we are equally sensitive to light.
Compared to other forms of pollution, one would think light would be easier to clean up. Try turning off your lights at home when the sun sets. See how long that idea lasts. You can though make sure your room is dark when you are asleep. As mentioned in prior articles, even small amounts of light during sleep are enough to suppress melatonin production. So the least you can do is sleep in the dark.
 Int J Cancer 2004;111(4): 475-9
Effect of exposure to light-at-night on life span and spontaneouscarcinogenesis in female CBA mice.
VN Anisimov, DA Baturin, IG Popovich, MA Zabezhinski, KG Manton, AV Semenchenko, and AI Yashin
 Neuro Endocrinol Lett. 2001 Dec;22(6):441-7.
The effect of light regimen and melatonin on the development of spontaneous mammary tumors in HER-2/neu transgenic mice is related to a downregulation of HER-2/neu gene expression.Baturin DA, Alimova IN, Anisimov VN, Popovich IG, Zabezhinski MA, Provinciali M, Mancini R, Franceschi C.
 Int J Cancer. 2003 Jan 20;103(3):300-5
The effect of melatonin treatment regimen on mammary adenocarcinoma development in HER-2/neu transgenic mice.
Anisimov VN, Alimova IN, Baturin DA, Popovich IG, Zabezhinski MA, Manton KG, Semenchenko AV, Yashin AI.
 Cancer Lett. 2001 Feb 10;163(1):51-7.
Effect of light/dark regimen on N-nitrosoethylurea-induced transplacental carcinogenesis in rats.Beniashvili DS, Benjamin S, Baturin DA, Anisimov VN.
 J Gerontol A Biol Sci Med Sci. 2001 Jul;56(7):B311-23.
Melatonin increases both life span and tumor incidence in female CBA mice.
Anisimov VN, Zavarzina NY, Zabezhinski MA, Popovich IG, Zimina OA, Shtylick AV, Arutjunyan AV, Oparina TI, Prokopenko VM, Mikhalski AI, Yashin AI.