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.