The night shift and breast cancer

Constant exposure to light comes under scrutiny

July 22, 2005|By Delthia Ricks | By Delthia Ricks,Newsday

Night shift workers who had low levels of the body's vital "sleep hormone," were significantly more likely to develop breast cancer than those who were awake during the day but got plenty of shut-eye at night, a team of scientists reported this week.

The analysis by Boston researchers goes straight to the heart of a question scientists have asked for years: Are night shift workers, because of extended exposure to light, more likely to develop cancer?

"Two or three years ago, we probably would have been reluctant to say there was an association," said Dr. Eva Schernhammer, an epidemiologist at Brigham and Women's Hospital. "But now the evidence is becoming clearer" that a relationship exists between tumor development and being exposed to light at night, she said.

"This study brings us one step closer to understanding why night shift workers may be more likely to develop breast cancer."

During daylight or exposure to bright lights, the hormone melatonin is suppressed. Switch off the lights, and melatonin, secreted by the brain's pineal gland, begins to stream into the blood. The hormone flows most copiously during sleep, thus its nickname: the sleep hormone.

A growing number of scientists theorize that melatonin also suppresses the growth of cancer cells. Laboratory studies by researchers at the University of Arizona have shown that breast cancer cells stop proliferating in melatonin's presence.

Schernhammer, who reported her results in Wednesday's Journal of the National Cancer Institute, said women exposed to light at night had measurably lower levels of a melatonin breakdown product in urine.

In her analysis, which involved participants from the Nurses Health Study, urine samples were taken from 147 nurses with invasive breast cancer and 291 matched controls who did not have the disease.

The lowest levels of melatonin's breakdown product were in night shift workers who had developed the cancer, leading Schernhammer to conclude that women with the lowest levels were 70 times more likely to develop breast cancer.

"This is an interesting bit of new information, and it will be put in context with other studies that have shown similar results and those that have shown no association," said Dr. Brian O'Hea, director of the Carol M. Baldwin Breast Care Center at Stony Brook University Hospital.

Newsday is a Tribune Publishing newspaper.

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