Being exposed to light at night makes breast cancer resistant to the widely used drug tamoxifen, researchers at Tulane University in New Orleans found.
Such exposure shuts off night-time production of the hormone melatonin, and the study suggests this is vital to the success of the drug.
The researchers studied the effect of tamoxifen on human breast-cancer cells implanted in rats. They found melatonin delayed and slowed tumor growth.
But tamoxifen caused "dramatic" regression of tumours in animals with high night-time melatonin during complete darkness or those given melatonin supplements in dim light at night.