Total darkness at night could be key to the success of breast cancer therapy, scientists say.

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.

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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.