"Working 9 to 5" is a catchy song lyric, but it doesn't describe the real-life experience of about 15 million Americans. That's how many shift workers—on duty evenings, nights, or in some rotating or otherwise irregular schedule—the Bureau of Labor Statistics estimates are in the workforce (or were in 2001). On top of the sleep problems this kind of off-hours schedule produces, there is plenty of evidence that it can be hazardous to your health—a review published in 2003 lists gastrointestinal problems (specifically peptic ulcers), cardiovascular disease, cancer, and diabetes. Shift work has also been linked to obesity and depression. In fact, "every physiological system has been noted to have increased problem with shift work in general," says Elizabeth Klerman, a physician in the division of sleep medicine at Brigham and Women's Hospital and an associate professor at Harvard Medical School.
What's going on? Well, the lack of sleep alone probably isn't helping anyone. "Shift workers are among the most sleep-deprived segments of the population," says Eve Van Cauter, a professor at the University of Chicago who studies circadian rhythms and their impact on the endocrine system. One, because it's tough to sleep soundly during the day, when your body clock is screaming for you to be up and at it. Two, because much of the world—including family and friends—is not on the same schedule, and workers want to participate in those lives, too.
But there's very likely something going on besides a sheer lack of sleep: the disruption of the circadian rhythm—the internal clock that governs eating, sleeping, body temperature, and other regular biological processes. As it turns out, messing around with that clock can have consequences; increased rates of breast cancer among shift workers, for example, may be caused by exposure to light during the night, when you should be sleeping. In the case of breast cancer, melatonin may play a role, says Richard Stevens, a cancer epidemiologist at the University of Connecticut Health Center who's been studying the so-called light-at-night theory for more than 20 years. Production of the hormone, which usually occurs during the "dark" period of a person's day, is disrupted by light exposure. The resulting dearth of melatonin may allow very small colonies of existing cancerous cells to flourish. Lower amounts of melatonin exposure at key points in a woman's life may also cause the breast tissue to change its growth characteristics, says Stevens.
When it comes to obesity and other metabolic issues, the issue could be either increased hunger (from, say, hormonal changes) that leads to overeating, a difference in how calories are metabolized, or both, says Van Cauter. "The one thing that is clear is that shift workers have a misalignment of their eating schedule relative to their own biological clock," she says. The notion is that the amount of insulin that the pancreas has to produce to absorb the same amount of carbohydrates is greater in the evening than in the morning. When people are forced by their schedule to eat more in the evening, they're ingesting carbs when the body needs a higher amount of insulin—which can promote fat storage—to dispose of them, putting the pancreas into overdrive, she says. A study published in September found that mice who ate during their customary sleeping hours gained more weight than those who ate during their usual wakeful hours—even though they consumed the same amount of calories.
This entire area of research is still excitingly new, but researchers have some early ideas on how these bad effects might be eased: