In the U.S., about 8.6 million people perform shift work, whether they have a night job or rotate shifts during the week. For many, it's a rite of passage in their careers; for others, it's a financial necessity. But there's a growing sense that shift work could be taking a serious toll on their health. "There is strong evidence that shift work is related to a number of serious health conditions, like cardiovascular disease, diabetes, and obesity," says Frank Scheer PhD, a neuroscientist at Harvard Medical School and Brigham and Women's Hospital in Boston. "These differences we're seeing can't just be explained by lifestyle or socioeconomic status." Shift work is also linked to stomach problems and ulcers, depression, and an increased risk of accidents or injury.

The Many Faces of Shift Workers According to the National Sleep Foundation, a shift worker is not just someone who works nights, but anyone who works outside a steady 9 to 5 schedule. The millions of shift workers in the U.S. include police officers, firefighters, nurses, doctors, pilots, waitresses, truck drivers, and many more professionals. Even a personal trainer who works out at the gym with clients in the early mornings and evenings is a shift worker. As shift work has become more widespread in the U.S., the health risks have become a focus both for researchers and for the businesses that employ shift workers. How serious are those dangers -- and can they be reduced? Unfortunately, we don't have all the answers yet.