Exposure To Nighttime Light Could Be Dangerous for Breast-Cancer Patients

By Jane Farrell

The breast-cancer drug tamoxifen is useless if a patient is exposed to even dim overnight light, according to a new study. But it can become effective during the same period in combination with the hormone melatonin.

Researchers from Tulane University School of Medicine published the study in the journal Cancer Research. It is the first study to show that melatonin, which regulates sleep and wake cycles, is essential to tamoxifen’s success in treating breast cancer. But melatonin doesn’t work if it’s shut off by exposure to light at night.

The principal investigators and co-leaders of the Tulane University Circadian Cancer Biology Group, Steven Hill and David Blask, along with team members Robert Dauchy and Shulin Xiang, analyzed the role of melatonin in the effectiveness of tamoxifen by examining 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,” says Hill. “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 researchers found that melatonin by itself delayed the formation of tumors and significantly slowed their growth. But tamoxifen caused a dramatic regression of tumors in animals with either high nighttime levels of melatonin during complete darkness or those receiving melatonin supplementation during dim light at night exposure.

These findings have potentially enormous implications for women are being treated with tamoxifen and are also regularly exposed to light at night due to sleep problems, working night shifts or 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,” Blask says.

The study could make light at night a new and serious risk factor for developing resistance to tamoxifen tand make the use of melatonin, administered at optimal times, standard treatment for patients who are already taking tamoxifen.