From 1992 to 1999, Eva Schernhammer, MD, worked rotating night
shifts in a cancer ward in Vienna, Austria. Her shifts included
10 all-nighters a month in addition to her regular hours; she
labored under banks of flickering fluorescent lights through the
darkest hours. "I didn't think much of it," she says, "until two
of my colleagues developed cancer. These were healthy women in
their 30s. They really had no risk factors, no family history."
She couldn't help wondering: Could working late nights be linked
to cancer? Three years later, Schernhammer landed at Harvard
Medical School in the Channing Laboratory--the perfect place to
find an answer to that question. The lab is home to the Nurses'
Health Study, one of the largest data banks of women's health
information ever amassed. She tapped into medical, work, and
lifestyle records gleaned from 78,562 nurses. The end result,
published in 2001, was startling: Nurses who'd worked 30 or more
years on night shifts had a 36% higher rate of breast cancer,
compared wit