“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 U. S. Bureau of Labor Statistics estimates are in the work force (or were in 2001).

On top of the sleep problems this kind of off-hours schedule produces, there’s 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 also has been linked to obesity and depression. In fact, “every physiological system has been noted to have increased problems with shift work in general,” says Elizabeth Klerman, a physician in the division of sleep medicine at Boston’s 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 2009 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:

Try to be consistent. People most at risk of problems are those with constantly rotating schedules — say, a repeating cycle of a morning shift one day, followed by an afternoon shift the next, then an evening shift. But even someone who steadily works the night shift is unlikely to stick to that schedule on days off, when he or she reverts to normal daytime activity, says Van Cauter. The more closely you can hew to a consistent routine, the better.

Nap before you work. Daytime workers get up and go to work, then go home and spend another several hours up and about before heading to bed. But night-shift workers may head to bed immediately after getting home from work in the early morning. Then, because it’s tough to sleep through the day, they’ll get up and be awake for several hours before heading back to work — and so they may be driving home from work in the early morning, after they’ve been up for a very long time and when the circadian clock is giving its strongest signal for sleep, says Klerman.

To ameliorate this, splitting sleep can help, she says. If you sleep five or more hours after getting home in the morning and then again for a few more hours before going back to work, you’ve gotten in something closer to eight hours and at least are less likely to be dangerously exhausted when you’re driving home from work.

Don’t use caffeine. Many of us toss back coffee to keep up energy at work. Not a good idea, says Julie Carrier, a psychology professor at the University of Montreal. She led a small lab study, the results of which were published in November, in which people were deprived of sleep, then allowed to sleep three hours after being given either a caffeine pill or a placebo. Not surprisingly, the caffeine group had poor sleep quality, and the effect was even stronger in people over 40.

For most people on a reverse schedule, drinking even three hours before hitting the sack decreases the deepest stages of sleep that are crucial for overriding the circadian signal to be awake during the day, says Carrier. It’s tempting for night workers to use caffeine at the end of their shifts because it’s when they’re most sluggish, but that’s only likely to continue to chip away at sleep quality.

Don’t take melatonin. Stevens says it’s not a great idea unless specifically recommended by a physician. While you’d think it would be helpful, it can actually throw a wrench into the smooth functioning of the circadian system and worsen the disruption to your biological clock. Klerman agrees with that advice, primarily because melatonin supplements are not regulated by the U. S. Food and Drug Administration. She’s studied a compound that mimics the effects of melatonin and could be potentially useful in quieting or shifting the circadian clock so you can sleep, so stay tuned.

Change your lights. You want the period when you’re sleeping — whether you’re a night-shift or a daytime worker—to be as dark as possible. Consider blackout curtains, a sleep mask, or anything else that reduces light. Stevens says that no matter our shift, we may want to consider installing a low-wattage red bulb in the bathroom so that when we get up in the middle of our sleep cycle, it isn’t disturbed by the shock of bright lights. There is research into how light—of different wavelengths, intensity, timing and duration — might be deployed to help shift workers but no prescription yet, says Stevens.

Eat a healthful diet. It’s possible that there’s some type of eating schedule that will minimize the negative impact on the body of eating mostly at night, says Van Cauter, but as of now she knows of no such diet that’s been demonstrated to work. No matter your work schedule, however, it certainly cannot hurt to follow a balanced, nutritious diet and to avoid loading up on processed carbs at night.