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Exposure to light at night, which shuts off nighttime production of the hormone melatonin, renders breast cancer completely resistant to tamoxifen, a widely used breast cancer drug, says a new study. Melatonin by itself delayed the formation of tumors and significantly slowed their growth, researchers report, 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.