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 Torrance, "my body is tired and my mind is exhausted."
In a 24/7 world, such fatigue passes for normal. Twenty percent of American workers are night-shift workers, and the number is growing by about 3% 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 patients alive, streets safe and packages moving. 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 may also 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: As much as 15% 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 schedule of scores of biological functions, from metabolism to 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."
Evolution supports that theory. Life on Earth began with single-cell organisms that depended on sunlight for converting energy to food. "Life has been adapting to a light-dark cycle since the beginning of the planet," says Paolo Sassone-Corsi, chairman of the department of pharmacology at UC Irvine.
But modern humans wrongly think they can override their natural sleep patterns with impunity, says Dr. Charles Czeisler, director of the division of sleep medicine at Harvard Medical School. "It's a myth that we alone, among all animals, have the power to sleep when we want," he says.
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 L.A. 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 now 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. But in night workers, melatonin continues to peak at night -- even though they're awake -- and cortisol levels continue to peak in the early morning hours, even when night-shift workers 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.