Tamoxifen is a standard treatment option for patients fighting breast cancer. But many patients with advanced carcinomas fail to respond to the drug. While the mechanisms that lead to this resistance are not fully understood, a new research suggests that night time exposure to light halts the production of the hormone melatonin thus making breast cancer tumors completely resistant to tamoxifen.

The study titled "Circadian and Melatonin Disruption by Exposure to Light at Night Drives Intrinsic Resistance to Tamoxifen Therapy in Breast Cancer," is the first ever to show that melatonin production is necessary for tamoxifen to work in breast cancer therapy.   The study was conducted by researchers at Tulane University School of Medicine cancer researchers and published in the journal Cancer Research.

The effectiveness of melatonin in aiding the cancer killing effect of tamoxifen was demonstrated by researchers on rats which had been implanted with human breast cancer cells. Steven Hill, one of the investigators said, "In the first phase of the study, we kept animals in a daily light/dark cycle of 12 hours of light followed by 12 hours of total darkness (melatonin is elevated during the dark phase) for several weeks. In the second study, we exposed them to the same daily light/dark cycle; however, during the 12 hour dark phase, animals were exposed to extremely dim light at night (melatonin levels are suppressed), roughly equivalent to faint light coming under a door."

The hormone melatonin controls our wake and sleep cycles. It is a photosensitive hormone, which means light affects how it is produced in the body. Melatonin levels start rising in the evening and drop again in the daytime. Recent research suggests that melatonin may play a role in curbing tumor growth. The idea is that the powerful antioxidant property of the hormone when coupled with anticancer drugs suppresses the growth of certain types of cancers.

Since melatonin is produced more at night, some researchers suggest that people who work night shifts may be more prone to cancers due to decreased melatonin production in their bodies. In this study, researchers found that there was a dramatic regression of tumors in animals who had been given tamoxifen and had high levels of melatonin in their bodies, either due to supplements or due to limited light exposure at night. 

These findings further corroborate studies that show a link between tamoxifen and melatonin. The research can also aid in designing therapeutics for women who are regularly exposed to light at night due to sleep problems, working night shifts, or exposed to light from computer and TV screens. 

"High melatonin levels at night put breast cancer cells to 'sleep' by turning off key growth mechanisms. These cells are vulnerable to tamoxifen. But when the lights are on and melatonin is suppressed, breast cancer cells 'wake up' and ignore tamoxifen," says coresearcher David Blask, in a statement

More research is needed in this area to study the positive effects of melatonin in cancer till then, therapies involving tamoxifen can be centered around when the melatonin levels in the body are highest to increase its effectiveness. 

Source- Hill S, David B, Dauchy R, Xiang S, Circadian and Melatonin Disruption by Exposure to Light at Night Drives Intrinsic Resistance to Tamoxifen Therapy in Breast Cancer,Cancer Research, 2014.