”Graveyard shift” may increase cancer risk

More and more evidence suggests that women who work night shifts have an increased risk of developing breast cancer, particularly women on the so-called “graveyard shift” from midnight until the early morning hours. Besides two newly published American studies showing a correlation between nocturnal work shifts and increased breast cancer risk, a Danish study suggests that there is a connection.

Exposure to artificial light at night, experts explain, suppresses the body’s natural secretion of melatonin, a hormone which controls the circadian rhythm, and this is believed to be what causes the problem According to Johnni Hansen of the Danish Cancer Society, disrupting the body’s melatonin production leads to an increased estrogen release in the ovaries, which may contribute to an increased breast cancer risk. At the same time, melatonin is a powerful antioxidant, which in lab tests has been shown to inhibit certain cancer forms, such as breast cancer, cancer of the uterus and lung cancer.

The two recent American studies, which are published in the Journal of the National Cancer Institute, document a worrisome risk increase in women with night work. Especially the ”graveyard shift” appears to be the problem. Since the body secretes the largest amount of melatonin in the early morning hours, provided the person is sleeping, people who work during this period have the largest increase in cancer. In one of the two recent studies researchers observed a 60% increase in cancer. The risk increases by 14% for every night of the week where the women is not sleeping during these hours.

In the second study, which is carried out on 78,000 nurses, researchers discovered an eight per cent increase in breast cancer in women who had been working shifts for as long as 29 years. Women who had been working shifts for more than 30 years had a 36% increase.

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