Sage Advice Blog

Melatonin, Tamoxifen, and Nighttime Light

November 3, 2017

Alex Speers ND, MS


When we sleep, our pineal gland, located near the center of our brain, produces the hormone melatonin. Melatonin helps to regulate our sleep-wake cycle, reaching its highest levels between midnight and 8:00 am and dropping to undetectable levels during the day. Melatonin’s sedative effect is well-known and it has become a popular supplement for those with insomnia. Recent research has shown that melatonin also has antioxidant, anti-inflammatory, and immune-stimulating effects. In fact, we now know that melatonin plays a protective role in the development of cancer. Therefore, it is important that we support our body’s natural production of this powerful hormone.


The most significant factor affecting our body’s ability to produce melatonin is nighttime light exposure. Light exposure at night can suppress melatonin production, disrupting our normal circadian rhythm. Several observational studies of women who work night shifts have found an increased risk of breast cancer development, leading the World Health Organization to classify night shift work as a “probable carcinogen.” It makes sense that being exposed to full light at nighttime could disrupt our circadian rhythm, potentially encouraging cancer growth, but what about low levels of light?


A recent rat study investigated whether nighttime exposure to dim light affected breast tumor growth and induced tamoxifen resistance. Tamoxifen is a type of drug called a selective estrogen receptor modulator (SERM) that is used in estrogen-receptor positive (ER+) breast cancer. 60-75% of breast cancers are ER+, meaning that they can use the hormone estrogen to help them grow. Tamoxifen acts in part by binding to estrogen receptors on the surface of breast cancer cells, preventing estrogen from binding. Unfortunately, 30-50% of ER+ patients are resistant to tamoxifen and many patients who are initially responsive to tamoxifen eventually become resistant.


In this study, researchers implanted rats with ER+ breast tumors and separated them into two groups: one group on a 12-hour light/12-hour dark cycle and one group on a 12-hour light/12-hour dim light cycle. Dim light was defined as 0.2 lux, which the researchers compared to a crack of light underneath a door or the light from a television that you experience when your eyes are closed. In other words, not that much light!


Interestingly, the researchers found that even this small amount of light was enough to almost completely suppress the natural production of melatonin in the rats. Not surprisingly, the rats who were exposed to dim light had significantly higher tumor growth compared to the rats on a light/dark schedule. In addition, the rats exposed to dim light had complete resistance to tamoxifen whereas tamoxifen effectively inhibited tumor growth in the rats on a normal light/dark schedule. In a second part of the study, the researchers found that supplementing the rats with melatonin effectively decreased tamoxifen resistance in the rats exposed to dim light.


Good sleep hygiene, which includes sleeping in a dark room at night, is a common recommendation made by naturopathic doctors. This study supports that recommendation, while also highlighting the important point that any amount of light could potentially disrupt our body’s normal rhythms and therefore, it is necessary to emphasize sleeping in a completely dark room. Without complete darkness, our brains may not be able to adequately produce melatonin, which is beneficial not only for its anti-cancer effects, but also, as shown in this study, for its potential role in decreasing tamoxifen resistance in patients with breast cancer.   


Dauchy R, Xiang S, Mao L, et al. Circadian and melatonin disruption by exposure to light at night drives intrinsic resistance to tamoxifen therapy in breast cancer. Cancer Res. 2014;75(15):4099-4110.


DISCLAIMER: These statements have not been evaluated by the Food and Drug Administration. There are no financial ties to any supplement companies, pharmaceutical companies, or to any of the products mentioned in this post. This post is not meant to treat, cure, prevent, or diagnose conditions or diseases and is meant for educational purposes. As always, please consult your doctor before trying any new treatments or supplements.


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