Late at night, the General Services Administration regional headquarters in southwest Washington is a shadow of its daytime self.

In a silent shell in a still city, the hum of computers muffles the sounds of human conversation and the night workers -- the 9 p.m.-to-5 a.m. part of the work force -- seem the anomaly they are.

A few blocks away, at the Hyatt Regency Hotel on Capitol Hill, the scene is different. Conventioneers carpet the escalators and populate the bars and restaurants. Waiters, kitchen workers, cashiers and others scurry to accomodate the guests, mimicking daytime commerce.

"I come on drinking coffee," said Mazhar Banday, an assistant manager at the Hyatt Regency who works from 11 p.m. to 7 a.m. "If you all bring me a steak now I couldn't eat it, because I just got up."

It is a daytime town, a 9 a.m.-to-5 p.m. town, where night work is exceptional. No huge shifts file into factories or daylight bright refineries.

Night work in the Washington area is at isolated islands of activity that break the nighttime stillness like clusters of lights in a dark rural landscape.

Even so, there are at least 68,000 workers out there, working shifts that mirror most workers' hours, yawning over their coffee and getting ready for work when their daytime counterparts are home opening a beer nd turning on the evening news.

In general the nightworkers work in service jobs -- hotels, restaurants, utility companies, transportation, hospitals and police and fire protection. At 24-hour stores, retail clerks also work at night and some printing and publishing activities keep presses running during the early morning hours -- putting out, among their publications, the city's daily newspaper and regional editions of The Wall Street Journal.

Moreover, as is happening across the nation, a growning number of data processors are joining the ranks of night workers, providing continuous operations for equipment too expensive to be left idle. And electricity rates are being restructured to encourage consumption at night, to help reduce daytime peak demands, which points to more night work in the coming decade.

Based on figures compiled by the Bureau of Labor Statistics, the nightworkers amount to roughly 10 percent of the work force, significantly less than the approximately 16 percent of the work force nationally that works evening, night and miscellaneous shifts.

In fact, of 10 major metropolitan areas analyzed by the BLS, Washington had the highest percentage of workers working days. "We attribute that basically to the presence of the federal government," said Edward S. Sekcenski, one of the authors of the BLS 1978 study. "There's very little shift work in the federal government."

That may be good news for area workers. Although shift work frequently carries a differential that results in slightly higher pay, it carries substantial liabilities as well, according to research by the National Institute of Occupational Safety and Health and others.

Night workers and workers whose shifts rotate can count on increased incidence of sleep disorders; more serious industrial accidents, and chronic low-level health complaints such as lingering sore throats and colds, according to Michael Smith, a psychologist who worked on the NIOSH study. The effects are most pronounced among workers whose shifts change, giving their bodies less chance to adjust to odd hours.

Families and workers' social lives also suffer from hours at odds with the norm, other research has shown. "There is a great deal of evidence that this work is not socially, or in the long term, economically desirable," said John Zalusky, an economist in the AFL-CIO Department of Research.

Where are the night workers in the Washington metropolitian area? Here and there are telephone operators, nighttime construction crews, police officers, fire fighters, hospital workers, bartenders, radio disc jockeys or talk-show hosts, reporters or editors and hotel clerks. Some lawyers have been known to work through the night on an important case headed for trial.

In addition some 1,700 Government Printing Office workers, including compositers, bindery workers and press operators, labor overnight to put out the Congressional Record and to print bills and committee reports.

Some 3,000 full time sales people at Woodward and Lothrop work two nights a week until 9:30 or 10 p.m., keeping the stores open. An estimated 1,200 part-timers also work nights.

An average of 436 persons work the night shift for Potomac Electric Power Co., and hundreds of other persons in service departments are on call. If a storm develops and wires are downed, other Pepco workers come to work immediately. Involved are such divisions as engineering, transmission, distribution, plant workers, meter people, security staff members and information specialists.

Giant Food stores employ about 500 nightworkers -- cashiers, meat cutters and clerks. In addition, about 20 persons work through the night at the firm's Landover headquarters in computer operations, 60 persons bake bread and work on maintenance at the chain's bakery, 23 persons work at the diary in Landover and some 400 drivers and warehouse people load and haul food to supermarkets in time for the morning buying crunch.

Giant's major competitor, Safeway Stores, employes about 300 retail store workers to stock shelves at night as well as a few warehouse workers.

Macke Co., a vending and food services firm that also is engaged in building maintenance, employes some 1,000 nighttime janitorial workers. Virtually all of theses individuals are part-timers, working on minimum wages, after they have completed full-time day jobs.

At Geico Corp., the insurance holding company, there are 38 persons on a night data processing shift. They work three days a week with 12-hour shifts that run from 6 p.m. to 6 a.m.

Small maintenance and operations staff workers keep various International Business Machines Corp. facilities around the Washington area working at low levels during the night.

About 800 workers before midnight and about 200 from midnight to 6 a.m. keep metrorail and bus services rolling. Subway service ends minutes after midnight, but then there are cars to clean and return to their starting position.

At U.S. Postal Service facilities in the Washington area, more than 2,500 persons work through the night. Most of the postal workers are in clerical and mail classifications but there are data technicians, vehicle maintenance, mail carrying and other jobs to fill in the night hours as well.

The U.S. Department of Health, Education and Welfare employs 2,950 area residents at night, primarily medical technicians, nurses, aides and computer specialists. Most of them are located at St. Elizabeth's Hospital and the National Institues of Health.

More than a 1,000 federal General Services Asministration employes also work overnight, including members of the Federal Protective Service, computer operators, custodial workers and heating and air conditioning mechanics.

In addition there are restaurants and bars, fast food carry-outs, bowling alleys, all night groceries and the cleaning crews that work their way through high rise buildings turning off the lights.

What night workers here say about their jobs echoes the various studies on night workers.

Charles Reeves, a supervisor of computer operations at GSA, has worked night shifts off and on for about ten years. His wife works days.

"She's going out in the morning and I'm turning over trying to get that last bit of sleep," he said. "When I come in, she's sleeping. "So it's really hard on family life."

Katie Furman, a computer operator at GSA, misses spending evenings with her teenaged daughter. "She says, 'I wish you were home more.' She misses me," said Furman.

Usually shifts are assigned on the basis of seniority, with new hires making up the bulk of the workers on the night shift. Older workers have more trouble adjusting to night work, studies show.

"I still don't like working evening but I have no choice," said Kevin Lyons, another computer operator. A GSA employe for only a year, he's too junior yet to qualify for a day shift. "I know it's not forever. There aren't too many people on this shift who like being here these hours," he added.

There are exceptions -- people for whom night shifts are convenient or come to like them. Mararet Johnson, a kitchen employe at the Hyatt Regency Hotel, has worked nights for 16 years. "My husband has been employed at the Post Office and he works nights," she said. Her late hours "give us a chance to be together," said Johnson.

Joanne Lewin, a 21-year-old night baker, works 3 p.m. to midnight at the hotel, hours that generally suit her. "I like it because I can get a lot of business done during the day. I can go to the bank. I can go to school at the Corcoran School of Art," she said, as she worked on pastries for breakfast that 900 conventioneers would eat."

Another hotel worker, banquet supervisor Otho (Tiny) Bryant, said he was able to adjust his social life to his late hours. "You find ladies that work at night. That's what I did," he said.

Most night workers, however, are there out of necessity, not choice, and their bodies continue to send messages that something is awry. "You never really get used to being up all night," said Michael Brawner, who does heavy cleaning for Sullivan Services. "As long as you keep moving around, it's okay -- but when you take a break you don't want to get up."

Generally, policy makers and labor union representatives in the United States have paid less attention to the problems of nightworkers than has been paid in Europe.

"Regognition of shift work not being entirely desirable and a source of individual's problems has been reacted to the United States in one chief way -- extracting for shift workers a wage differential," said Dick Shore, a social science adviser in the Department of Labor.

Shore said he wasn't suggesting that night work should be banned because clearly some industries and services require it. "But there are other ways of dealing with the issue, including more thoughtful ways of matching employes and shifts," he said. In addition "those who are particularly debilitated by shift work in terms of health or disruptions of family or social life should not be required to work a non-traditional schedule," he said. "That's not a remedy that has been given a lot of attention in the United States."