COME 7 a.m. the hard-core types have already been and gone, the ultramarathoners in training, the laser-focused triathlete working toward another Ironman, the body-image compulsives and exercise-bulimics with their sinewy limbs and the telltale coating of downy forearm hair. Let us not omit mention of the onetime male porn star, the guy with the bloated Popeye muscles and nipples so distended they resemble elevator buttons: Floor please!

Most every weekday the Lafayette Street outpost of Crunch Fitness is that place of New York fable, an establishment that remains open 24 hours. Contrary to all the Sinatra hype about the city that never sleeps, sidewalks tend to get rolled up by midnight in most parts of town. Yet while ordinary folks might have long since nodded off watching Letterman or washed down an Ambien and yanked the covers over their heads, gym people can still be found pumping and cycling and sweating at every hour. This I discovered after deciding recently to stage an ultramarathon of my own: that is, to see what it was like to spend 24 hours at a gym. (In two shifts, to be truthful; I tried it in one stint and could not go the distance.)

My reasons were various, random in some sense and also derived from a decades-long curiosity about a locale social scientists refer to as "the third place," a fixed place in a psychic triangle whose other points are work and home. Gyms figure in the lives of at least the 41.3 million Americans who currently belong to some kind of fitness club. They are places of refuge, escape, self-improvement or abasement and, for those who never get past the sign-in phase, rebuke. Yet, although membership numbers have not stopped climbing in the 19 years since the fitness industry began counting, Americans are slouching ever couch-ward toward epidemics of disease linked to sins the Puritans would recognize, like indolence and sloth.

"The sedentary culture that has emerged in this country in the last 30 years has had a devastating effect on our nation's health," said John McCarthy, the executive director of the International Health, Racquet & Sportsclub Association, alluding in part to alarming current rates of childhood obesity and also to a creeping rise in the adult-onset diabetes that medical science links to being overweight.

Last week, for its third consecutive year, the group sponsored an initiative (www.getactiveamerica.com) in which 1,200 participating health clubs opened their doors free to the public for a day. Simultaneously, association members went to Capitol Hill to lobby for the Workforce Health Improvement Program Act, a proposed extension of tax benefits provided by the government to employers maintaining fitness programs.

"The current tax laws allow for on-site fitness facilities to be tax deductible, but not offsite," said Brooke Correia, a spokeswoman for the organization.

This perhaps explains the Pilates class held every Wednesday in certain corporate auditoriums. The proposed law aims to make it easier for employers to subsidize health club memberships through tax cuts, thus making a dent in the roughly $77 billion spent yearly on medical problems related to inactivity.

As recently as 1987, a mere 17.3 million Americans were members of a health or fitness club. As it happens, that was about when I myself joined a gym in the hope of effecting a personal physical transformation from weedy scribbler to supertoned manly man.

Much has changed in the years since then, both culturally and about my suit size. In the beginning the goal was to bench some fairly grotesque poundage. But these days, except among hypertrophied knucklehead rappers like LL Cool J, who recently boasted to the morning radio personality Miss Jones that he routinely benches "three and change," that style of cave-man lifting has dwindled in appeal. Not the least surprising of the evolutions I have witnessed in the gym over time is an almost total alteration in tastes for certain body styles.

"The hulking, monolithic look that was in fashion for my generation seems old now," said Michael Manchino, a 49-year-old personal trainer whose clients include the designer Narciso Rodriguez.

With a low-double-digit body-fat ratio and a pectoral shelf that brings to mind the busts on Mount Rushmore, Mr. Manchino said he gets called Rambo a lot when he travels abroad.

"I dated a guy once who said, 'I don't want to lift weights. I don't want a big chest like yours because it doesn't go with the clothes,' " Mr. Manchino explained. And it's true that the pumped Armani-clad hustler Richard Gere played two decades ago in "American Gigolo" would look more than passé in a skinny suit by Hedi Slimane for Dior. He would look like he'd borrowed Pee-wee Herman's clothes. "Everybody now wants the retro 70's rock star body, the yoga body, the Jesus look," Mr. Manchino explained.

Well, perhaps not everyone does. The value of spending a full day inside a gym is that it gives one the opportunity to survey a rich gallery of human types: the male gymbots with their proud bosoms and stick legs, the flesh mountains, the solitary ponytailed hippie who passes hours leisurely pedaling a recumbent bike while meandering through "Within a Budding Grove," the aging rockers with taut bodies and faces like Salvador Dalí clocks, the young men and women — New York University students, at a guess — in the first flush of adulthood, their flesh firm, their carriages still limber because the ravages of serial hangover, student loan terror and mortgage payments for closet size co-ops have not yet made inroads on their faces and physiques.

There are the ramen-thin fashion models, perennially in the company of a personal trainer, a kind of duenna always struggling to spark tiny model brains long enough for them to hoist toy weights and pump out a few effortful curls. There are the Hells Angels with necks as thick as oak stumps, useful for accommodating Gothic-lettered Harley tattoos.

The corporate koan at Crunch Fitness has long been "No Judgments," a philosophy that in one sense is accurate (all these disparate types actually do seem to get along) and in another is a quaint delusion. The reality is that no one, Hells Angel included, who crosses the threshold of a gym has not at some time been tried and probably convicted in the bathroom mirror by his own private kangaroo court.

Clearly it is no longer just women who are plagued with body-image paranoia. If the widely trumpeted feminization of men has demonstrated anything, it is that the world is now a place where all are free to obsess about belly bumps, crepey knees or the cruel Newtonian joke that gravity eventually makes of everyone's aging rump.

"If I could change one thing about myself, it would be the self-consciousness about my body," said the quite acceptably fit "Lost" actor Josh Holloway recently in Star, which (ahem) I picked up from a treadmill lectern. "I feel like I'm out of shape at all times," Mr. Holloway added. "Eventually I'm going to have to quit trying to be Brad Pitt in 'Fight Club.' I just ain't that, you know?"

Who is? This is the question I find myself asking as I wander the ground floor at Crunch, where it strikes me that most of the people one finds in a gym can stand to be there. And there is something bracing in the realization that the gym is not axiomatically a temple of narcissism, a default for the solipsistic, the lonely and the bored.

To call the place communal would be overreaching. Yet in the cycles and the confines of a gym are found predictable and reassuring rhythms, familiar faces and also recognizable habits of being. One can set clocks by some of the Type A performers of early morning, the driven, the compulsive, the competitive bodybuilders, 10-egg-white-omelets-for-breakfast types for whom this slot is an opportunity to hoist massive hunks of iron without causing bystanders to huddle in corners, whimpering in fear. One can recognize, too, in the late-night warriors, the hospital workers, taxi drivers or junior lawyers who show up past midnight, elements of one's own need to carve out room in which to pace the miles of a psychic marathon.

During my hours at Crunch Fitness — a place I chose because its demographic profile is more racially and economically various than some fancier spots in town and also, full disclosure, because it is my home gym and thus the place I know best — I did as one might in assembling a jigsaw puzzle. I put down the edge pieces first and then worked to fill in the center of the image.

The thicket of chrome and steel, the infinitely mirroring perimeter walls, the ceiling-hung photomural of disembodied animals and people, the purple and yellow and pink and silver color scheme suggestive of a dungeon decorated by the Easter bunny came gradually into focus.

Only then could I begin to identify the particulars within the overall scheme: here was the editor of a major city magazine, there the architect who designed 200 Gucci stores. Over on the Roman chair was the sexily scrawny Smashing Pumpkins guitarist James Iha cranking out leg lifts. Skulking around amiably in sweats was the "Grey's Anatomy" actress Sandra Oh.

I failed to score a sighting of Matt Damon, although I often have in the past, Mr. Damon being an easy puzzle piece to find because his protective incognito is so studiedly anonymous it becomes a form of ostentation.

"He comes in with the knit cap low, and surrounded by buddies, like he doesn't want anyone to notice him," said Morena Saenz, an artist whose day job involves pulping bananas, bone meal and other even less appetizing substances in the weary blenders at the gym's juice bar.

"I don't know why he bothers since no one's paying attention anyway," Ms. Saenz added. "When Calvin Klein comes in he just works out like anyone else. He's just low-key and famous."

Her observation helps tease out the reasons gyms in New York occupy the status they do, as social free zones, leveling places rare in a city where most of us squander countless Gawker hours diagramming the minutiae of hierarchy and caste.

At any given moment at Crunch, as I found, one is likely to stumble across a Pulitzer Prize-winning novelist drenched in sweat during Matt Pestorius's 6:45 Spinning class or a hedge fund Master of the Universe prancing happily to Marc Santa Maria's hip-hop mixed tapes in a Crumpin' & Clownin' class or a prime-time television hunk, walking half-naked to the showers, a sight that must be related to a recent strict ban on cellphones (with their pesky cameras) in the locker room.

The sense that, in the gym at least, all people are the same person owes something to the anonymous universality of droopy shorts and sweat-stained T-shirts. And that, too, is another great thing about gym culture: despite the best efforts of Polo or Dolce & Gabbana, fashion has never yet found a way in.

"What I love about coming here is that it's like my living room and the people who come here are like my little crowd whether I know them or not," said Joshua Suzanne, a downtown store owner who spends two hours, five days a week at Crunch before heading to her vintage clothing shop Rags-a-GoGo. With her sailor tattoos, lip piercing, bleached sideburns and lovingly coiffed Mohawk, Ms. Suzanne may not be the most obvious exponent of the gym as a laboratory of Platonic ideals.

But there she was last week, exercising her philosophy between sets of pull-ups and arm curls.

"All of us are in here going through our changes, getting fat or skinny, getting nose jobs or boob jobs, getting muscles or not, changing our bodies or not, and it's all, like, kind of abstract," Ms. Suzanne said. "But what's cool is that there's this larger thing going on, where everybody is working toward a better self."

Whether the ideal is remotely attainable matters less, Ms. Suzanne said, than the pursuit. "The dream is what counts," she said. "If you pay attention, you can watch how the physical changes translate in people's psyches. We're all in here. We keep coming back. For some reason or other, we believe."
Link to the New York Times article.