(WWJ) — New research shows how light and dark could mean the difference between life and death for some breast cancer patients.

Tulane University researchers have found exposure to light at night, which shuts off production of the hormone melatonin, makes breast cancer completely resistant to the drug tamoxifen.

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Researchers exposed rats implanted with human breast cancer cells to either 12 hours of light followed by 12 hours of total darkness, or, exposed the animals to very dim light during the 12 hour dark phase.

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They found high melatonin levels at night put breast cancer cells to sleep, by stopping their growth and making them vulnerable to tamoxifen. But when even dim lights were on and melatonin was suppressed, breast cancer cells woke up and ignored tamoxifen.

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The study appears in the journal Cancer Research.