Jul 28, 2014 08:12 AM EDT By Staff Reporter, UniversityHerald Reporter

Night light exposure makes breast cancer patients resistant to tamoxifen drug, according to a new study by the Tulane University School of Medicine. The researchers said that light at night stops the production of the hormone melatonin.

Several claims have been made regarding the benefits of melatonin. Due to melatonin's alleged antioxidant properties, some believe it quashes the growth of some types of cancer cells, especially in combination with certain anti-cancer drugs. The hormone is also claimed to stimulate a type of white blood cell called natural killer cells, which attacks tumors. Melatonin may decrease the toxic effects of radiation therapy and chemotherapy.

This is the first study to demonstrate the importance of melatonin in breast cancer treatments along with the combination of tamoxifen drug.  

For the study, the researchers implanted human breast cancer cells in rats and analysed the role of melatonin on the success of tamoxifen. The rats were either exposed to normal light/dark cycle (12 hours of light followed by 12 hours of darkness) or in conditions where 12 hours of light was followed by 12 hours of dim light - roughly equivalent to faint light coming under a door.

The researchers found that higher levels of melatonin in the blood was observed in rats associated with the normal day/night cycle, whereas lower levels were found in those subjected to dim light.

They also found that melatonin hindered tumour formations and slowed their growth. Tamoxifen reduced cancer growth in rats that lived in 12 hours of darkness or those receiving melatonin supplementation.

"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," Principal investigator and co-leader of Tulane's Circadian Cancer Biology Group, David Blask, said in a press release.

The study, "Circadian and Melatonin Disruption by Exposure to Light at Night Drives Intrinsic Resistance to Tamoxifen Therapy in Breast Cancer," is published in the journal Cancer Research.

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