LOS ANGELES - At 6 a.m., the hospital's bright hallway lights flicker on, signaling the start of a new day. Doctors in crisp business clothes appear on their early-morning rounds, and the clang of breakfast carts will soon echo through the unit.

For registered nurse Liberty Bunag, however, it's finally time to go home and sleep. She began her shift 12 hours ago with an extra-large coffee and since has consumed a liter of caffeinated soda, more coffee and lots of rice, her personal energy food. Sometimes she and the other nurses on the orthopedic ward of White Memorial Medical Center in Los Angeles practice foreign languages to stay alert, squelching the yawns and drowsiness - the body's way of protesting this nocturnal activity.

Bunag's head throbs as she walks to her car. "When I get home," says the 26-year-old from the Los Angeles suburb of Torrance, "my body is tired and my mind is exhausted."

In a 24/7 world, such fatigue passes for normal. Twenty percent of U.S. workers are night-shift workers, and the number is growing by about 3 percent per year, according to the Bureau of Labor Statistics. While the rest of society sleeps, police officers, security guards, truck drivers, office cleaning crews, hotel desk clerks, nurses, pilots and many others keep streets safe, packages moving and patients alive. But at a price.

These workers - and people with more conventionally sleep-deprived lifestyles - are known to be at higher risk for accidents, sleep disorders and psychological stress due to daytime demands, such as family and other obligations, that interfere with sleeping. Now scientific evidence suggests their disrupted circadian rhythms also may cause a kind of biological revolt, raising their likelihood of obesity, cancer, reproductive health problems, mental illness and gastrointestinal disorders.

The evidence for an increased cancer risk is so compelling that, in December, the International Agency for Research on Cancer, a unit of the World Health Organization, declared that shift work is "probably carcinogenic to humans."

Researchers are beginning to understand why. Among the most significant - and startling - reasons: Up to 15 percent of human genes function on a schedule, with highly regulated, oscillating patterns of activity.

These clocklike genes are common features of most cells and can be found in every major organ in the body. They, in turn, affect the schedules of scores of biological functions, from metabolism and cell division to cognitive processes.

"Less than 10 years ago, it was thought that sleep was for the brain and not for the rest of the body, so lack of sleep would make you tired, moody and more likely to have accidents," says sleep researcher Eve Van Cauter, a professor of medicine at the University of Chicago. "But sleep deprivation may be bad for the body, too, representing a risk for a variety of abnormal conditions."

Dennis Corrigan sometimes questions his decision to switch to a night shift 12 years ago.

By working nights, the UPS truck driver from West Covina, age 52, avoids the physical demands of the day shift, when lifting boxes is part of the job, plus the worst of city traffic. The 10:45 p.m.-to-11 a.m. shift also allowed him to attend all of his son's high school football games.

But Corrigan sleeps only about six hours a day. He has put on weight and gets less exercise than before the switch and was diagnosed with diabetes five years ago.

"The rough part is, when I come home, I'm hungry," he says. "I eat a heavy meal before going off to bed. You're not supposed to do that. It's a worry."

His circadian rhythms may be to blame. Those rhythms determine when certain body processes take place. For example, melatonin, the hormone that aids sleep, is released at night; the hormone cortisol is low at night and pours out in the morning, jump-starting the body's daytime functions. Even in night workers, melatonin continues to peak at night and cortisol levels continue to peak in the early morning hours, when they are eager to get some sleep.

Those disrupted circadian rhythms are why night-shift workers sleep less and with poorer quality, Van Cauter says. They try to sleep when their bodies want to be awake.

Chronic sleep deprivation may carry some of the same risks as disrupted circadian rhythms, she says. Today, Americans average about one hour less of sleep per night than they did 30 years ago.

Bunag feels the effect of night-shift work on her days off. If she tries to sleep at night, she often wakes around 3 a.m. and is alert until dawn, when she falls back to sleep, often for 10 hours. On workdays, she sleeps about six hours during the day but awakens still tired.

"My problem is not while working but on my days off," she says. "I feel unproductive, because all I do is sleep all day and I'm up the whole night, when nothing much can get done."

She finds herself less willing to socialize these days and worries that her irritability may border on depression. She also wonders about the long-term health consequences of her schedule. "I want to be able to sleep normally at night when the body does all of its detoxifying, cleansing, repairing and recharging. But I haven't figured out what's going to work for me."

Her concerns are well-founded.

n Night-shift workers have a 40 percent to 50 percent increased risk of heart disease compared with day workers, various studies have found.

n People who regularly get five hours of sleep, common among night-shift workers, are 50 percent more likely to be obese than normal sleepers, Columbia University researchers have found. Several dozen other studies have tied sleep loss to weight gain as well.

n Women night-shift workers have higher rates of miscarriage, preterm birth and low birth-weight babies.

n Night-shift workers show increased rates of breast (by 50 percent) and colon (by 35 percent) cancer in numerous independent studies. Animal studies have shown that exposure to dim light during the night can substantially increase tumor development.

"It's been known for years that there is an increased risk of a variety of medical conditions in the population of shift workers," says Dr. Diane Boivin, associate professor of psychiatry and director of the Centre for Study and Treatment of Circadian Rhythms at Douglas Mental Health University Institute in Montreal. "What is difficult to parcel out is the exact contribution of this circadian misalignment and sleep deprivation. We think it's major, but it's very difficult to be sure."

Science is inching closer to understanding how a lack of sleep - or sleep at the wrong time - can wreak biological havoc. In the past few years, researchers have made surprising discoveries about the body's sophisticated timekeeping.

Scientists once assumed the body's sole "clock" was nestled in a place in the brain called the suprachiasmatic nucleus, or SCN. Light - particularly sunlight - is the primary synchronizer for circadian rhythms.

When we open our eyes each morning, light reaches photoreceptors in the retina and creates signals that travel to the SCN to jump-start the body's hormones, neurotransmitters, temperature and metabolism for the new day.

But that's not the body's only timepiece. Circadian timekeeping genes can be found in organs all over the body.

These peripheral clocks control the activity of many cellular processes and biological functions, and their presence may explain why sleep dysfunction seems to have such a broad effect on overall health. Light sets the circadian clocks in the SCN, but scientists still aren't sure what compels the body's peripheral clocks to work in unison. After all, the liver and the digestive tract don't have direct access to sunlight.

"The SCN is like a musical conductor, and the peripheral clocks are the instruments that need to play their music with peak activity at certain times of the day to get good harmony across the body," Boivin says. "They must be in sync."

Research continues.

For now, scientists and sleep doctors implore people to show a little respect for slumber.

"People think of sleep as a waste of time," says Paolo Sassone-Corsi, chairman of the department of pharmacology at the University of California, Irvine. "But it's essential. A correct sleep-wake cycle is as important to health as any other thing in our lives."

How to cope with shift work

Sleep experts suggest various ways to cope with unusual work and sleep schedules.

* Regular hours: Keep the same sleep schedule on days off from work.

* Strategic napping: Studies show naps of 20 minutes to two hours - either just before a shift or during a shift - can improve alertness and performance at work.

* Bright light: Exposure to bright lights during the first half of a shift may help fight fatigue later in the shift.

* Light restriction: Avoiding bright lights in the morning (such as by wearing sunglasses on the way home and sleeping in a very dark room) can help night-shift workers sleep better during the day.

* Melatonin: Supplements can promote daytime sleep when taken an hour or two before bedtime.

* Modafinil: This medication, brand name Provigil, was approved in 2004 for the treatment of

shift-related sleep disorder. Taken just before a shift, it can enhance alertness at night, studies have shown.

* Caffeine: Caffeine has been shown to counteract drops in performance levels at night.