A drug widely used to treat breast cancer may stop working at night unless a patient sleeps in complete darkness, as even low light levels can make tumors develop a resistance to the drug, researchers say.

Light entering a bedroom affects sleep hormones, which in turn alter cancer cell function, making tumors resistant to the cancer drug Tamoxifen, they say.

Any light at night, even at low levels, can cause the creation of the sleep hormone melatonin to shut off, researchers from Tulane University have reported in the journal Cancer Research.

Melatonin is a factor in slowing or delaying the growth of tumors, and in combination with Tamoxifen is even more effective.

That's why anything interfering with melatonin production is a cause for concern, researchers say.

"Our study does not identify how much light exposure is needed to suppress nighttime melatonin production, and potentially drive tamoxifen resistance in humans, but we think that it could be as a little as the amount of light that comes in the bedroom window from a street light," says Tulane cellular biology Professor Steven J. Hills.

Tamoxifen works by stopping production of the female hormone estrogen, which can drive the spread of tumors, thereby letting melatonin do its work.

The drug has been proven effective at increasing survival times although tumor cells can eventually develop a resistance to it.

The Tulane researchers used rats to study the circadian rhythm -- the body's internal clock -- and its effect on melatonin production during periods of complete darkness as compared to nighttime periods with low light levels.

In dim light environments at night, the rats' levels of melatonin were found to drop, the tumors became larger and more resistant to the Tamoxifen, they say.

"Our data, although they were generated in rats, have potential implications for the large number of patients with breast cancer who are being treated with Tamoxifen, because they suggest that night time exposure to light, even dim light, could cause their tumors to become resistant to the drug by suppressing melatonin production," Hill says.

Hill points out that melatonin levels are regulated by sleep, as a lot of people think, but rather but by our internal body clodk, and that darkness is important.

At nighttime, sleeping in a brightly lit bedroom can cause your melatonin levels to be reduced, he says.

In total darkness -- even if you can't sleep -- your melatonin levels will increase normally, he explained.