Light at night has already been widely linked to poorer sleep quality, but now even dim light is being shown to interfere with a breast cancer drug's ability to do its job. So says a new study on rats out of Tulane University School of Medicine, which found that even the low light of a nearby street lamp interferes with the body's natural production of the sleep hormone melatonin. And the rats (all of which had human breast tumors) exposed to dim light at night not only exhibited lower levels of melatonin but had larger, faster-growing tumors that were also completely resistant to the breast cancer drug Tamoxifen.
"Our levels of melatonin are not determined by sleep, as many people think," one researcher tells Medical News Today. "It is actually the darkness that is important." And he tells the BBC that the blue light emitted from digital devices such as TVs and tablets is particularly disruptive for melatonin production. Other studies have suggested that melatonin may help improve cancer treatments. Still, researchers warn that this study needs to be replicated and validated in humans before determining that all the same mechanisms are at play. (City lights have also been linked to higher breast cancer rates.)