Nighttime Light Exposure May Weaken Breast Cancer Therapy

Night lights could cut the success rate of breast cancer therapy, according to a new study.

Total darkness at night could be the key to successfully treating breast cancer, according researchers at Tulane University School of Medicine.

Researchers explained that exposure to light at night shuts off the nighttime production of the hormone melatonin. Previous studies revealed that melatonin significantly slows the growth of tumors.

Lead researchers Steven Hill and David Blask of Tulane University's Circadian Cancer Biology Group looked at how melatonin influenced the effectiveness of tamoxifen in killing human breast cancer cells implanted in rats.

"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," Hill said in a news release. "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."

Researchers found that that melatonin itself delayed the development of tumors and significantly stalled their growth.

However, animals in both melatonin groups that received tamoxifen experienced a dramatic regression of tumors. The study found that tamoxifen caused a dramatic regression of tumors in animals with either high nighttime levels of melatonin during complete darkness or in animals given melatonin supplementation during dim light at night exposure.

In light of the latest findings, researchers recommend women being treated with tamoxifen to avoid light exposure at night, night shifts or light exposure 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," Blask said.

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