DULUTH, Ga. -- At 11 p.m., or 1 a.m. on the many nights she must work overtime, Allison McClure stops at her mother's house to retrieve her 3-year-old daughter, Mia, take her home and put her back to bed. Then she packs Mia's bag for nursery school, does the ironing, cleans house and takes a shower.
"I'm in bed by 3 or 4," she said during her 9-to-9:30 p.m. "lunch" break at a mail processing center.
"Then I get up at 6:45," she said. "I get her up. I fix her breakfast, I feed her and I take her to school. I'll lie down for about an hour and a half, and then I'm up. Any business I have to do, I try to fit in between then and the time I have to be at work."
Ms. McClure, 30, is a shift worker. She is paid around $30,000 a year and is hard at work for the U.S. Postal Service when most people take it easy or sleep.
Because of her personal obligations, she said: "I never sleep eight hours. Sometimes you get tired, so you have to push yourself."
She has jet lag without the jet, and she has it all the time.
Ms. McClure and millions of others who work unconventional hours have become objects of concern to psychologists and occupational-health specialists.
While some people, like police officers, nurses and factory workers, have often worked unconventional hours, more and more do now. And among them new strains have developed as two-earner families and single working mothers have displaced one-earner, married-couple families.
Generally, statisticians define shift workers as people who spend less than half of the daylight hours on the job or who work some hours in the morning and some at night.
Others put in long, dawn-to-dawn shifts or rotating schedules -- days one week, nights the next.
Growth in shift work, like the growth in part-time and temporary jobs, is a part of a sea change in the labor force, which now has to accommodate a global economy that runs increasingly around the clock.
Shift work accounts for a quarter to nearly half the jobs in the fastest-growing occupations. It is built into eight of the 10 occupations that the Bureau of Labor Statistics predicts will add the most workers in the next decade: sales clerk and cashier in stores, nurse, nursing aide, truck driver, janitor, waiter and fast-food worker.
Shift workers also breed shift work. They work in convenience stores, supermarkets and gas stations that stay open long after dark to serve others people who work long after dark.
No one knows just how many shift workers there are. The last time it looked, in 1985, the Census Bureau found 20 million, or a fifth of the work force.
It counted them again last year, but the results will not be reported until later this year.
Donald I. Tepas, an industrial psychologist at the University of Connecticut, said, "Our society is demanding more and more round-the-clock operation at all levels, from the working-class blue-collar guy to doctors in hospitals and financiers."
Occupational-health experts find that shift work exacts a toll beyond just losing sleep.
Years of research, assembled in a recent study of shift work and health by the Congressional Office of Technology Assessment, show that shift workers typically suffer from insomnia and chronic fatigue.
They have more trouble with their health and dispositions than other workers, more marital problems, more strains raising children.
Night workers, the study says, complain inordinately of gastrointestinal disorders, including peptic ulcers.
It also says that pregnant shift workers, according to some limited research, have more miscarriages, premature births and lower birth weights than women who work regular hours.
The congressional study says the health effects of shift work become more acute with age, raising concern about the baby-boom generation, which is now the largest part of the work force and whose oldest members are approaching 50.
Timothy H. Monk, director of the human chronology program at the University of Pittsburgh, said he wonders how employers will manage with older, less malleable workers, "when they don't have enough young people to shuffle."
Many foreign governments regulate shift work, but in the United States, neither the government nor the institutions that support workplace regulation, like labor unions, have tackled the issue.