Dim Lighting May Weaken Breast-Cancer Drug’s Effects

Rats taking tamoxifen who were exposed to dim lighting at night didn’t produce normal amounts of melatonin, and their tumours grew quickly as a result. But when they were given a melatonin supplement, their tumors shrank.

People taking tamoxifen should make sure their bedroom is free of light, suggests this new research.

The rats in the light and dark conditions had an increase in melatonin as the night got darker and a decrease as the light got brighter. But the rats in the light and dim conditions had consistently low melatonin levels throughout.

This had serious implications: Tumours in the rats exposed to dim lighting grew 2.6 times faster than those in the rats exposed to darkness. But when rats in the dim lighting were given a melatonin supplement, their tumours shrank.

“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 tumours to become resistant to the drug by suppressing melatonin production,” Hill said in a statement.

He said that the amount of lighting that could cause the drug to falter could be as weak as that of a street light coming through a bedroom window. He and his colleagues plan to conduct more studies to answer exactly how much light is harmful.