Breast Cancer Risk Linked to Sleep Patterns

One of the most effective ways to reduce the risk of breast cancer may be to regularly get a good night's sleep -- in the dark.

A new study shows that women with the highest levels of melatonin -- a hormone the body produces only when a person is sleeping at night, in the dark -- have a breast cancer risk that is 40 per cent lower than those with low levels of melatonin.

Dr. Eva Schernhammer, an epidemiologist at Brigham and Women's Hospital in Boston, said the research suggests that "melatonin secretion may play an important role in breast cancer development."

She said that when and how well a woman sleeps may also influence whether she develops breast cancer, and that sleep patterns could also have an impact on tumour development and, by extension, on the effectiveness of treatment.

The research, published in today's edition of the Journal of the National Cancer Institute, seems to confirm the long-held hypothesis about the cause of sharply higher breast cancer rates among shift workers.

A number of studies have shown that workers who regularly toil on the late-night shift, such as nurses, are about twice as likely to develop breast cancer as those who work day shifts.

Disruption of melatonin production was long suspected as the culprit, but it was only a theory, based on a retrospective look at the work habits of cancer patients.

The new study by Dr. Schernhammer and a team at Harvard University is different in that the researchers actually measured levels of melatonin in the urine of women before and after they developed breast cancer.

The research is an offshoot of the massive Harvard Nurses Study, in which the health of almost 120,000 nurses has been tracked since 1989. As part of that project, more than 30,000 women have provided regular urine samples.

The new study by Dr. Schernhammer focused on 147 women who developed breast cancer; they were compared with 291 women of similar background who did not develop it.

Melatonin production peaks at night, and exposure to light at night interrupts production of the hormone. When this occurs, it also stimulates a women's ovaries to produce extra estrogen; excess production of the female sex hormone is a known risk for breast cancer.

The idea that too much exposure to light can raise a woman's cancer risk derives from earlier research on blind women, who are half as likely to develop breast cancer as sighted women. In blind women, melatonin levels do not fluctuate and, as a result, their estrogen levels are more stable.

In the new study, researchers found that melatonin levels were sharply lower in women who developed breast cancer, even well before their diagnosis. Among the 25 per cent of women with the lowest levels of melatonin, 50 developed breast cancer; by comparison, among the 25 per cent with the highest levels of melatonin, 23 developed breast cancer.

Dr. Schernhammer said the results suggest that the melatonin is influencing risk, not the shift work itself.

This year, an estimated 21,600 women and 150 men will be diagnosed with breast cancer, according to the Canadian Cancer Society, and an estimated 5,300 women and 45 men will die of the disease.