This Labor Day holiday marks the 50th anniversary of the 40-hour work week, the 9-to-5 standard that sets the boundaries of America's work and leisure time. You could consider it one of the more tenacious concepts of recent social history.In 1938, the Fair Labor Standards Act established the 40-hour week by declaring that workers in interstate commerce would receive time and a half pay for working more than 40 hours. Almost as soon as the law passed, sociologists began predicting that the American work week would continue to shrink. By the late 1950s, observers, including historian Arthur Schlesinger Jr., were warning that increasing amounts of leisure time would reduce the muscular fiber of America to moral flab.

Maybe ... but not yet.

In 1988, labor economist John D. Owen says that American families are contributing more hours to the work force than they have at any time since World War II.

"Contrary to popular opinion, the number of hours supplied to the market - at least by members in the 20 to 55 age group - appears to have increased, not decreased, in recent decades," he writes in "Working Hours," his landmark study of time use.

Not only does the 40-hour work week remain standard, but Americans also work longer hours and take fewer vacations than the citizens of almost any industrialized nation except Japan. In addition, the increased number of working women - almost half of the work force - has actually shrunk the amount of family leisure time. Most working couples return home to share the full-time job of running a household.

Owen says that the leisure world forecasts of the 1940s and 1950s overlooked several things, particularly the need to work longer hours to pay for college degrees.

One of every four U.S. workers is now a college graduate, a statistic that has helped preserve not only the 40-hour work week but also the inclination to moonlight.

The biggest reason to keep long hours, however, remains America's appetite for an upper middle-class lifestyle. Labor studies show that Americans seem to prefer buying roses to smelling them.

A 1985 survey by the U.S. Department of Labor asked workers if they would rather work longer, shorter or the same number of hours for the wage they were receiving. Almost two-thirds said that they would choose to work the same number of hours and almost a third said that they would choose to work more hours.

In fact, the U.S. work culture continues to heap prestige upon those who put in long hours, at least if you take your cues from the top.

A 1985 survey of senior executives at American corporations by Korn-Ferry International, an executive recruiting company, showed that four-fifths of the executives worked between 46 and 60 hours a week. Ten percent worked more than 60 hours a week.

Still, America was the first nation to establish the idea of the five-day, 40-hour work week. Although France introduced the 40-hour week in 1937, Owen says that it never really caught on with the public; many European countries still used a 48-hour work week in the late 1940s.

Henry Ford introduced the five-day work week shortly after World War I. At that time, the average "white-collar" work week was about 51 hours - five 9-hour days with a half day on Saturday - and the average manufacturing work week was 54 hours.

With the Great Depression of the 1930s, the work week collapsed. Seeking to spread available jobs throughout the population, the 1933 National Recovery Act promoted a five-day work week, which was later made official by the Fair Labor Standards Act.

Since then, U.S. Department of Labor statistics show that the work week has ranged from a national average of 42.8 hours in 1948 to an average of 38 hours in 1982. Last year, Americans worked an average of 39 hours a week. However, as Owen points out, that figure tends to be misleading because it reflects an increasing number of part-time workers.

Last year, full-time workers put in an average of 43 hours a week.

A former professor at Johns Hopkins University, Owen has taught the economics of human resources at Wayne State University in Detroit since 1975. The Johns Hopkins University Press will publish his newest book, "Reduced Working Hours: Cure for Unemployment or Economic Burden?" next year.

"There is definitely a time shortage in the mainstream of American life," he says. "I see a trend toward an increasing bureaucratization of life, a world in which husbands and wives are both working, children are in day care and everyone is eating fast food.

"Maintaining the so-called time savers that we have bought take up a lot of our time, and so do our possessions. The reason we are better off than we were 50 years ago is that we sell our labor for a higher wage. But when you turn around and buy services, that (higher income) doesn't apply."

According to a recent Lou Harris poll, Americans' leisure time - that period defined by relaxing and entertaining activities - dropped from 26.2 hours a week in 1973 to 16.6 in 1987. During the same period, Harris found that most full-time employees' work weeks increased from 40.6 to 46.8 hours.

"It has become much more difficult to characterize certain activities than it was in the days when a field worker worked 12 hours a day," Owen says. "People's attitudes toward their activities vary so much. We all know people who enjoy their work so much that we would call them `workaholics,' but for the man with an unhappy marriage, `leisure time' is the most unhappy time of the day."