A new study published in the journal Cancer Research examined how light and dark influence the effectiveness of tamoxifen therapy for breast cancer. The researchers found that disruptions to circadian rhythms and melatonin production can increase resistance to tamoxifen. Resistance to this therapy can mean the difference between life and death for breast cancer patients.

According to the Journal of Neuroendocrinology, melatonin is an internal facilitator of sleep in humans. It is important for the regulation of circadian rhythms, or the internal body clock, and can help treat sleep disorders such as insomnia. Where melatonin production or the circadian rhythm is disturbed, it can have ramifications to health stemming from adverse effects on sleep patterns.

In the current study, CBS Local Detroit explains that exposure to light at night shuts off the production of the hormone melatonin, making the breast cancer resistant to the drug tamoxifen. High melatonin levels at night were able to put the breast cancer cells to sleep and make them vulnerable to tamoxifen. However, even with dim lights on, melatonin production was impaired and the breast cancer cells stayed active. Tamoxifen was unable to combat the cells in these circumstances, making the cancer resistant.

WebMD describes breast cancer as the growth of abnormal tissue in the breast tissue. Most often, it is found in the milk ducts, which is known as ductal carcinoma, or the glands, which is known as lobular carcinoma. Even if the cancer has traveled to other parts of the body, it is still called breast cancer. Overall, breast cancer has a 90 percent rate of survival after five years.