More gyms are remaining open round the clock, experts say, spurred by advances in surveillance and security technology, clients' ever more fluid work habits and a generation of multi-tasking consumers.

"A lot of people work untraditional times and they take advantage of clubs open at all hours," said Carl Liebert, CEO of 24 Hour Fitness, an international chain of health clubs, most of them open around the clock.

Liebert said his 30-year-old chain has seen an increase in after-hours traffic, with five to 15 percent of clients, depending on location, working out between midnight and 5:00 a.m.

"I think people have changed," he said. "They're online; they're connected day and night. When I grew up I went to bed at night and got up in the morning. Between those times I wasn't multitasking."

Liebert said common night visitors include people who work out to relieve stress, insomniacs, and hard-core fitness fanatics who wouldn't miss a workout and prefer a gym less crowded.

The number of health clubs open 24 hours has soared from just couple hundred five years ago to more than 2,000 today, according to IHRSA, the International Health, Racquet & Sportsclub Association, a trade association of fitness facilities.

Spokesperson Meredith Poppler said while most club goers still work out during regular hours, technology has enabled small, often rural, clubs to operate overnight, and sometimes unstaffed.

"Cameras on the doors, cameras in the clubs, safety buttons each member carries set them apart," said Poppler. "Many of the operators are in the clubs at certain times of days, but other times the clubs are truly unstaffed."

Access is granted via a swipe card or key fob.

Poppler said the largest franchising company of 24-hour key clubs is Anytime Fitness, based in Hastings, Minnesota.

Earlier this year the chain reported that late-night workouts from midnight to 3:00 a.m. totaled 1.2 million in 2011, up from 900,000 the previous year.

"Anytime Fitness finds that 24/7 access caters to everyone from your typical night owls, to doctors and nurses, and other public service officials who work odd hours," said spokesperson Tara Dosh.

Liz Neporent, spokesperson for the American Council on Exercise, belongs to a 24-hour gym in New York City.

She said physiologically it doesn't matter when you choose to work out, although some studies suggest that morning exercisers may be more likely to stick with their program. More important is finding a time that's convenient.

Neporent, who has experience operating health clubs, said while she prefers to see staff on duty at night, sometimes it's not economically feasible.

"A lot of residential gyms in apartment buildings are lightly manned or unmanned," she said, adding that for turnkey gyms, cameras are a must.

Neporent said when she does hit the gym at 3:00 a.m., the crowd's different.

"At 6:00 a.m. you get the professionals going to work. Late morning you see a lot of stay-at-home moms. Overnight I tend to see more creative types," she said. "More piercings, more tattoos. I have met interesting people at three or four in the morning."

(Reporting by Dorene Internicola; editing by Patricia Reaney)

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