Melatonin and Breast Cancer Risk

A new study by researchers at the Fred Hutchinson Cancer Research Center has found that women who work the night shift may face an increased risk of breast cancer. The reason: Nighttime sleep loss or light at night may suppress the production of melatonin, leading to an increase in estrogen release. Estrogen has been linked to an increased risk of breast cancer.

Writing in the Journal of the National Cancer Institute, Scott Davis, Ph.D. and colleagues reported on their investigation of the relationship between breast-cancer risk and exposure to light at night as determined by sleep habits, bedroom lighting and night-shift work.

"Although there have been a number of studies looking at the health effects of night-shift work, from heart disease to stomach ailments, this is the first that has looked comprehensively at both graveyard-shift work and light at night as independent risk factors for breast cancer," Davis said.

Davis' team interviewed more than 800 Seattle-area women who had been diagnosed with breast cancer. They assessed each woman's exposure to light at night and history of shift work, among other factors, during the 10 years prior to their breast cancer diagnosis.

They found that women who worked the night shift during the decade before their breast-cancer diagnosis had a 60 percent increased risk for breast cancer compared with those who did not work the night shift. In addition, the risk of breast cancer significantly increased with each additional hour per week of night-shift work.

The researchers believe that the link between sleep, light at night and breast cancer may involve melatonin, a hormone produced by the brain's pineal gland. Production of melatonin peaks at night during sleep. One theory is that nighttime sleep deprivation or exposure to light at night somehow interrupts melatonin production, which in turn stimulates the ovaries to kick out extra estrogen.

Another study published in the same issue of the Journal examined the relationship between rotating night-shift work and breast cancer risk in more than 78,500 women who participated in the Nurses' Health Study.

Dr. Eva Schernhammer of Harvard Medical School and colleagues found that women who worked at least three night shifts per month, in addition to their usual day and evening shifts, had a moderately increased risk of breast cancer compared to women who did not work rotating night shifts.

However, Schernhammer cautioned that "further research is needed to determine if melatonin suppression is the reason for the increased breast cancer risk."

In an accompanying editorial, Dr. Johnni Hansen of the Danish Cancer Society in Copenhagen wrote that "regardless of the underlying biological cause for the apparent increased risk of breast cancer among women who work at night ... there is an urgent need for further exploration of the relationship between exposure to light at night, shift work and cancers that may be influenced by melatonin."

SOURCE:

Journal of the National Cancer Institute, October 17, 2001; 93:1513-1515, 1557-1568

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