Night Troubles

WHILE WORKING THE NIGHT SHIFT IN VIENNA’S KAISER FRANZ JOSEF Hospital years ago, Austrian-born DR. EVA SCHERNHAMMER was disturbed by an unusually high cancer rate among her late-night colleagues. “Two of the nurses that worked in my department got cancer at a particularly young age,” she says. “The people who worked there were being treated alongside the patients.” Schernhammer wondered: Could disruptions to the body’s circadian rhythm—the 24-hour cycle that governs everything from sleeping and waking to digestion—somehow contribute to increased cancer rates?

In 1999, Schernhammer enrolled at Harvard to pursue a doctorate in public health, but she continued to ponder the health effects of working the night shift. The stakes were considerable: According to the most recent data from the Bureau of Labor Statistics, almost 15 percent of American workers— about 45 million people—work evening, night, or rotating shifts. Schernhammer began poring over data from Harvard’s Nurses’ Health Study, a long-term investigation begun in 1976 to study the impact of oral contraceptives on women’s health. More than 200,000 women have participated in the Nurses’ Health Study, and Schernhammer’s review of the data suggested that women who worked rotating night shifts—particularly those who had done so for more than 30 years—were indeed at a moderately increased risk for breast cancer. The question was why.

Schernhammer wasn’t the first to consider the relationship between circadian rhythms and cancer. In the late eighties, Dr. Richard Stevens, an epidemiologist at the University of Connecticut, had posed a similar question: Why, when oncology had made such great advances, did rates of certain cancers, especially breast cancer, continue to increase? Stevens hypothesized that Thomas Edison’s electric revolution was to blame: The introduction of artificial light had interfered with our production of melatonin, a hormone secreted by the pineal gland in the brain. Melatonin is thought to help neutralize linoleic acid, a polyunsaturated fat which, in small quantities, promotes normal cell growth. But with the considerable quantities we now consume in processed foods, linoleic acid is like an all-you-can-eat buffet for cancer cells.

Was it possible, Schernhammer wondered, that something about the night shift was lowering workers' melatonin levels? "Melatonin is produced in a very specific way," says Schernhammer, now an assistant epidemologist at Brigham and Women's Hospital. "It follows a rhythm determined by the biological clock we all possess." Break the rhythm, and you interrupt the production of melatonin.

The problem with night shifts, according to Schernhammer and her colleagues, is not the absence of sleep, but the fact that most night-workers revert to a daytime schedule on their days off, constantly interrupting the body's carcadian rhythm. "If you stay up every night until three and that's your rhythm, that's fine," she says. "The problem is introducing irregularity." Frequent disruptions to a person's natural sleep cycle, Schernhammer stresses, are far more dangerous than the occasional all-nighter. "If you are exposed to bright light at a time when you normally are making melatonin, then you run into trouble," she says.

For now, she sees no quick fixes for night workers. She does not, for instance, advocate melatonin supplements, which the FDA does not regulate. "It's premature to recommend specific changes [in policy]," she says. "After all, we've only had artificial light for about 100 years. It's only fair that the research is just now picking up."

Schernhammer came to the U.S. thinking she would stay only long enough to complete her Ph.D., but she has continued to find new areas of investigation that are keeping her here. She is now examining the type of light night-workers are exposed to: Some scientists believe that shorter wavelength lights, such as fluorescent and halogen, are particularly detrimental to melatonin production. Meanwhile, she frequently flies to Vienna to visit her parents--and worries about the effects of frequent jetlag. "In an ideal world, I would try to live regularly," she says. "I often envy people who have children, because they keep you on a regular schedule."