In a study that sheds new light on the body’s wake-sleep cycle, Tulane University researchers have found dim light at night can compromise the effectiveness of the breast cancer drug tamoxifen, while sleeping in total darkness can enhance its impact.
The study, published in the journal Cancer Research, found that exposure to light at night shuts down production of the hormone melatonin, rendering breast cancer completely resistant to tamoxifen.
The findings, based on studies involving mice implanted with human breast cancer cells, are the first to show that melatonin is vital to the success of tamoxifen in treating breast cancer.
"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," Steven Hill who helped conduct the study.
"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."
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 supplements during dim light at night.
These findings have enormous implications for women being treated with tamoxifen 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," said co-researcher David Blask.
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