PHILADELPHIA — At 2 a.m. Friday, in a cavernous 24-hour fitness center here, Bobby Boyle, the owner of Boyle’s World Gym, sprinted across a basketball court and then hit the weight machines when he felt his energy sag. Lonely for companionship, Boyle, who works the graveyard shift, started up a conversation (mostly one-sided) with the gym’s mascot: his 22-year-old macaw, Terese.

What does it take to make it through the grueling wee hours of an overnight shift? “It is tough, it can be a really hard time,” said Boyle, one of a group of dusk-to-dawn workers around the country who were observed by reporters.

For Aaron Byrd, a DJ broadcasting to Southern California, it requires a heavy dose of energy drinks — what he jokingly calls his “crack juice” — and dancing along to his own “global soul” music spinning on Santa Monica’s public radio station KCRW.

Jenna Walker, a registered nurse in Atlanta, runs laps on the stairway for exercise.

The one thing they all try to avoid? Fatigue on the job.

At least 15 million Americans work full-time on irregular shifts in the late evenings or overnight, according to the Bureau of Labor Statistics. The perk of more freedom during the daytime hours is one of the biggest draws, as is the serenity that often comes with the darker, slower hours. But there is also a cost, paid by the body in the form of stress and fatigue, the perils of which have been exposed anew in air traffic control towers across the nation.

Last week, after a sixth controller was discovered napping on duty, the Federal Aviation Administration said it would change scheduling practices for air traffic controllers to combat excessive sleepiness, often the result of stacking shifts too closely. The FAA also announced an end to single-person staffing on the midnight shift at 27 towers across the country.

Most industries are not federally regulated in this way, however, and it is up to individuals to manage their own time — and fatigue — as best they can to minimize the risk for errors and accidents.

Of the graveyard-shift workers interviewed early Friday, half expressed sympathy for the stresses of the air traffic controllers while the others did not understand napping on the job. “You hate to see anybody get caught sleeping,” said Sgt. Randall Malott, a campus safety officer who was patrolling the University of Denver. “It makes the rest of us look bad.”

Not all jobs carry as great a potential for danger as patrolling the air or driving a bus but there can still be hazards.

After his midnight shift, Byrd has found himself dozing off on the snaking freeway between Santa Monica and Pasadena. “I catch myself,” he said. “I never doze for too long. Usually I roll down the window, blast the radio, and try to get something to drink.”

At 1:30 a.m. Friday, Byrd, 28, was fighting sleepiness in the studio. “I definitely sympathize” with the air traffic controllers and others on overnight shifts, he said. “A heavy dose of our faithful listeners are people like firemen, night watchmen, nurses. Those are the ones who seem to call in on a more regular basis. Sometimes they’ll drop us a line saying, ‘more up-tempo music, I got to stay up.’ ”

Every night is different, busy or bottomless in its own way.

“You have those nights where you don’t stop moving and things are crazy,” Malott said. “And then you have those nights when you get to sit here and think about everything.”

Either way, Malott, 43, is hooked on the night shift, after one stint in the military and another as a sheriff’s deputy. One minute, he’s speeding in his mini-SUV to get to a young woman who ended up on campus, partly exposed and vomiting in the aftermath of a fraternity party. But in the quiet time, enjoying the silence, he might wander the gardens or visit the clock tower. And although awake, he might dream a little.

Malott never went to college; as a single father raising five children he hopes that they wind up at DU, a university he has come to adore. “I would love that to happen,” he said, singing to himself as a student bangs out Billy Joel’s “Just the Way You Are” on a piano in a dorm lobby. It is his last hours on the clock, then home in time to see his kids off to school.

There was a Starbucks for coffee in his future, but never a nap.

Even though Walker, 25, a registered nurse at the Shepherd Center, a hospital in Atlanta, is allowed by the rules to take a quick nap during her 7 p.m. to 7 a.m. shift, she prefers not to.

“It’s nice to be awake when most people are asleep,” she mused. “You have a secret feeling that you’re the only ones here — there’s no one else in the building. You’re taking care of the world’s problems while they’re sleeping.”

At work, battling the Zzzzzzzzs

At least 15 million Americans work full-time on irregular shifts in the late evenings or overnight, according to the Bureau of Labor Statistics. Here is what four people do to battle the fatigue.

“I’ve become very intimate with five-hour Energy drinks. I call them my crack juice. I try not to take it more than once a week, just for the show. I don’t want to develop any tolerance and be desensitized.”

— Aaron Byrd, DJ in Santa Monica, Calif.

“If you find yourself getting where you’re starting to doze, you park the car, you get out and walk around, you go and talk to the RA (resident assistant), you go talk to the 7-Eleven guy. You do something to get your mind off of that, and you’re generally good.”

— Sgt. Randall Malott, campus safety officer, University of Denver

“Nurses do different things to stay alert — drink coffee, take a walk. Sometimes at night, I’ll jog up and down the stairs for exercise.”

— Jenna Walker, registered nurse in Atlanta

“Here, you really can’t sleep. The door can open at any time.”

— Bobby Boyle, owner of Boyle’s World Gym in Philadelphia

Companionship is important, even if it’s just from his pet bird, a 22-year-old macaw, Terese, who perches near the entry. They chit-chat. (Terese hardly ever answers.) Around 2 a.m., Boyle, 42, also samples diet Turbo Tea or sugar-free Red Bull. “But I try not to have energy drinks past 8 or 9 o’clock at night.”