This article is from the WSSF 2016 AFRMA Rat & Mouse Tales news-magazine.

In studies done at the Tulane University School of Medicine in New Orleans, rats with human breast tumors that were exposed to dim light at night made the tumors resistant to the breast cancer drug tamoxifen. Night light exposure also suppresses the nocturnal production of melatonin that inhibits breast cancer growth. However, the negative effects of dim light exposure on tamoxifen treatment were overcome by giving the rats a melatonin supplement during the night and the tumors regressed. Tumor growth was 2.6-fold faster in rats living in the dim night light conditions compared with tumor growth in rats living in normal 12-hour light/12-hour dark cycles. Melatonin levels are not determined by sleep, but by the darkness.

From article Exposure to Dim Light at Night May Make Breast Cancers Resistant to Tamoxifen. Study Circadian and Melatonin Disruption by Exposure to Light at Night Drives Intrinsic Resistance to Tamoxifen Therapy in Breast Cancer published in Cancer Research August 2014, Volume 74, Issue 15, 4099–110.

One breeder in England found that when she introduced an artificial long day regime in the winter to try and get her rats to breed, she had a sudden spate of mammary tumors in unrelated, different age females. (Pro-Rat-A #204 Nov./Dec. 2014).