NEW YORK - Quick-service chains are starting to reap the benefits of keeping the lights on later and opening for business earlier.

The Wall Street Journal reports that McDonald??s is recognizing late and early operating hours (12 am to 5 am) as the "fastest growing time segment in its U.S. business," noting that nearly 40% of its U.S. outlets are now open around the clock, up from about 30% seven years ago. Burger King now has about 300 restaurants open 24/7, and Dunkin?? Donuts has doubled its number of 24/7 locations in the past 10 years to a third of its 7,000-plus U.S. outlets.

According to industry analysts, the trend is indicative of changes in the American work force. The newspaper notes that one-fifth of the U.S. workforce is punching the clock in the evening, at night or on a regularly rotating schedule, according to Harriet Presser, a University of Maryland sociology professor and author of the book, "The Economy that Never Sleeps."

"Whether it's taking on an additional job or just working a late shift, we're seeing more people out later at night," commented Steve Levigne, McDonald's vice president of consumer and business insights. The company didn??t say what percentage of overall sales are made during the 12 am to 5 am time period.

McDonald's credits its 4Q11 growth to efforts such as offering more convenience, which includes extending operating hours.

Dunkin' Donuts, a solid morning destination, has been tapping into the evening daypart because "our research and feedback from franchisees indicates we're seeing very strong growth in the evening hours," John Costello, chief marketing and innovation officer for Dunkin Brands Group Inc., told the newspaper.

"I think we really have moved to a clockless day," Costello said, adding, "People are working longer hours, in many cases multiple jobs, and are more time-starved than ever before and they want the flexibility to have a full variety of products that aren't limited by time of day."