Melatonin is a hormone that communicates information concerning external light conditions to various parts of the body. Melatonin is synthesized and secreted by the pineal gland in the brain. It is known primarily for its involvement in the regulation of circadian rhythms (the body clock), however it has also been shown to have antioxidant and anticarcinogenic properties, particularly in breast cancer. Low levels of melatonin due to excessive exposure to light or as natural part of the aging process is associated with increased risk of breast cancer. Now a new study has reported that exposure to dim light at night induces tamoxifen resistance in an animal model of hormone receptor positive (ER+/PR+) breast cancer.

Melatonin and circadian regulation

Circadian synchronization depends on an internal clock that is synchronized to light-dark cycles. Circadian rhythms are popularly thought of as being driven solely by the master clock in the brain, which receives information directly concerning light-dark conditions through the eyes. However, individual cells in other organs also have clocks. Circadian rhythm disruption is not a misalignment of the master clock in the brain and the earth's 24-hour light-dark cycle (although such misalignment may contribute to it). Rather, it refers to when the master clock in the brain becomes out of sync with local cellular clocks in other organs, including the breasts. Although the process is not well understood, repeated desynchronization appears to result in degradation of the circadian cell cycle, which in turn facilitates uncontrolled growth.

Melatonin and light at night

Melatonin is produced at night (or in conditions of darkness) and nocturnal light disrupts melatonin synthesis. When women work at night or are otherwise exposed to light at night, their risk of breast cancer increases. Perhaps the most dramatic evidence of this phenomenon is the finding that blind women have a lower risk of breast cancer than sighted women. In addition, total visual blindness provides significantly higher protection against breast cancer than partial blindness (i.e., with some perception of light).

Habitually sleeping in the presence of artificial light could increase breast cancer risk by inhibiting melatonin production. Blue light wavelengths appear to have a far greater suppressive effect on melatonin production than red wavelengths. In an Israeli study, 1,679 breast cancer patients were interviewed regarding bedroom light levels, including light coming in the room outside the bedroom and sleeping with the television on. "Sleeping habitat" light intensity was found to be linked to breast cancer risk.

Ensuring adequate melatonin levels

Women can maximize their melatonin production by minimizing sources of light in the bedroom (use a red light when light is necessary), avoiding extended periods of night shift work, and getting enough sleep. Bright light therapy is sometimes successful in resetting disturbed circadian rhythm.

Melatonin levels can also be increased by consuming foods that incorporate melatonin. Common foods that are very good dietary sources of melatonin include almonds, cherries (especially sour or tart cherries), and tomatoes. Walnuts, oats, and sweet corn also contain some melatonin. Other very good sources of melatonin include white and black mustard seeds, sunflower seeds, fennel seed, red, brown and green algae, and flaxseed. Red meat consumption has been shown reduce circulating melatonin.

Melatonin supplements are typically taken to aid sleep or as part of an anti-aging regimen. Although such supplements have been shown to be effective in increasing circulating melatonin levels, there have been few studies concerning the impact of such supplementation on breast cancer risk or breast cancer recurrence. Safe and effective dosages for breast cancer survivors have not been established.

Latest research finds exposure to light at night causes tamoxifen resistance

The study referenced at the beginning of this news story was designed to investigate the effects of disturbed light/dark cycles in an animal model of tamoxifen-treated breast cancer. Exposure to light at night reduces the nocturnal production of melatonin (which in turn inhibits breast cancer growth). In the study, the authors implanted ER+/PR+ MCF-7 human breast cancer cells in rats and observed the growth of the tumor xenografts under various conditions.

Altering normal light/dark cycles by adding dim light exposure at night was found to accelerate the development of tumors (speeding their metabolism and growth) and cause tamoxifen resistance. These effects were absent in animals not exposed to dim light at night and in animals subjected to dim light at night but also administered melatonin. Importantly, the authors demonstrated that melatonin acted not only to inhibit tumor metabolism and induce tumor regression, but also to reestablish the sensitivity of breast tumors to treatment with tamoxifen. The authors conclude that disturbances in nocturnal melatonin production caused by exposure to dim light at night can render tumors insensitive to tamoxifen.

Please see our article on what to eat during tamoxifen treatment for more information.