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.