For three days a week, all Sue Reimers does is eat, sleep and work.

Since the beginning of the year, Reimers, 25, has been working 4:30 p.m. to 5 a.m., late Sunday through early Wednesday, as a production assistant at Aldus Type Studio in the mid-Wilshire district of Los Angeles - a shift that doesn't leave a lot of time for frivolity."For me, the outside world doesn't exist from Sunday afternoon till Wednesday morning," she explained late one afternoon, her hair still wet as she sipped her first cup of coffee of the workday.

But come noon Wednesday, Reimers greets the world with gusto.

With four days off, she has time to go to museums and movies and to canvass for political candidates. And Reimers - whose romantic life was wanting during the three years she worked a traditional eight-hour shift, five evenings a week, at the advertising typesetting shop - even found time to fall in love and become engaged.

"I'm no longer feeling like work is my entire life and my personality," she said. "I've been able to expand my horizons."

Reimers and her co-workers on the night shift at Aldus are on the leading edge of a gigantic social experiment. Industry is revamping the lives of thousands of the 25 percent to 30 percent of American workers - from telephone operators and yen traders on the graveyard shift to factory hands on rotating schedules - whose jobs do not conform to the customary 9-to-5 workday.

In industries as diverse as oil refining, data processing and health care, hundreds of companies are altering work schedules, redesigning jobs and re-educating employees in hopes of diminishing the miseries and dangers of shift work.

Drawing on an expanding body of scientific research, managers and consultants are putting in place new schedules that take into account not only the human body's natural rhythms, but also the desire of shift workers to approximate a normal family and social life in the hours they are not working.

Workers, meantime, are learning an array of coping techniques, from dietary changes to daytime sleep strategies to ways to better plan free time.

"What it comes down to is people not realizing shift work is a lifestyle, not a work schedule," said Marty Klein, a psychologist who heads SynchroTech, a shift work consulting firm in Lincoln, Neb. "What you need is a schedule and a set of policies . . . that enable people to lead a healthy lifestyle, even though it's different from a daytime lifestyle."

The recognition that odd work hours alter almost every aspect of a worker's life has been slow to dawn on American industry.

Researchers in Western Europe, Australia and Japan, not the United States, have been the trailblazers in identifying the problems of shift workers. As a result, those nations' limits on work hours are tougher. Most American employers, experts say, have yet to question the arbitrary scheduling practices that shift workers hate.

"What companies have done over the years is say, `We'll pay you a shift differential,' " explained Richard M. Coleman, a former Stanford University researcher who now is a shift work consultant in Ross, Calif. "They throw money at the problem, and the problem never goes away."

The "problem" is considerable. Studies have found that 60 percent to 80 percent of shift workers have trouble sleeping - except at work, where an estimated 20 percent occasionally fall asleep. They are three to five times as likely to have digestive disorders as are workers on normal day shifts.

The broadest U.S. survey of shift work, based on a federal government study of 3,000 adults in 1979 and 1980, concluded that both male and female shift workers experience more job stress and emotional problems than day workers, while women working shifts were especially inclined to heavy use of drugs and alcohol.

"Whenever you go into a shift work situation and talk to shift workers, there are some people who can cope, but also some people living miserable lives because of the shift work," said Timothy H. Monk, director of human chronobiology research at the Western Psychiatric Institute in Pittsburgh, Pa.

Shift work can wreck family life and friendships, researchers add.

"Their friends are going to stop calling them, because every time they call, they're either sleeping or working," Klein said. "Their kids are disappointed all the time. Their spouses get upset because they're not helping with the kids or mowing the lawn or taking care of the house."

Employers suffer, too. Studies by the U.S. Bureau of Mines, for instance, show shift workers have more frequent and more severe industrial accidents. Most single-vehicle trucking accidents occur in the early morning hours, as well, though investigators cannot always make a decisive link between fatigue and a mishap.

"With metal fatigue, you can go out and look at the pieces of fractured metal," explained John K. Lauber, a member of the National Transportation Safety Board, which investigates air, highway and railroad accidents. "Human fatigue doesn't act that way. You have to infer it."

What is clear is that employees who work nights or rotate between day and night shifts do not, on the whole, do their jobs as well as day workers. A 20-year study of Scandinavian petroleum plant workers, for example, found that meter-reading errors peaked on the night shift. Absenteeism, turnover and health-care costs all are higher, too, for shift workers.

And with employees fleeing shift work whenever possible, businesses often must struggle to adequately staff their graveyard crews - a problem that has reached crisis dimensions in at least one field, nursing.

"There is no nursing shortage," Klein said. "What there is is a shortage of nurses willing to live a nurse's lifestyle, which depends upon them working these schedules." Hospitals have had to pay substantial premiums and offer highly flexible work hours to fill the gaps.

For the most part, though, it has taken the shock of disaster to sever employers' hidebound ties to old-fashioned shift work arrangements.

Experts say the near-catastrophe at the Three Mile Island nuclear plant in Pennsylvania - attributed largely to human errors between 4 a.m. and 6 a.m. one morning in March 1979 - began the process of focusing the business world's attention on the risks associated with shift work.

Subsequent nuclear industry crises - the April 1986 Chernobyl meltdown, which began at 1:23 a.m., and the closure of Pennsylvania's Peach Bottom nuclear plant last year after investigators found that night-shift operators were falling asleep in the control room - only reinforced the message.

"You could potentially cripple a whole industry simply by human error," Monk said. "The negative payoff can be so large that it pays to spend a few hundred thousand dollars looking at shift work's effects."

Not surprisingly, the nuclear industry itself has been among the most aggressive the last few years in exploring ways to improve shift workers' performance.

Philadelphia Electric Co., which operates Peach Bottom, paid a record $1.25 million fine and has spent millions more in a massive overhaul of plant systems designed to win Nuclear Regulatory Commission permission to restart the power station early next year.

Many of the changes aim to increase shift workers' alertness, according to Bill Jones, a spokesman for the utility. Control room operators' cozy high-back chairs have been replaced with a less comfortable model, and lighting levels have been brightened.

On the advice of top shift work researchers, control room workers' schedules were revised, Jones added. Instead of rotating backward, from night shift to evening shift to day shift, workers' schedules now rotate forward.