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.