That will be true for many more cities as the world gets hotter.

Evening hike: 94°F

Dozens of hikers set out for the summit of Piestewa Peak on a July evening, their flashlights dancing in the dark. “You feeling O.K.?” Trevor Plautz, a park ranger, asked two women, one of whom had stumbled and was breathing hard. “You have enough water?” Both soon turned back, moving slowly down unlit rocky switchbacks. An owl chittered.

“You definitely feel the heat, but the nights are better,” Mr. Plautz said. “A lot of people hike right now instead of during the day because it is a lot cooler.”

Phoenix, which had 128 days at or above 100 degrees Fahrenheit last year, is one of the hottest and fastest-warming cities in the United States. Although it is on the leading edge, it is not alone: Most American cities are expected to drastically heat up in the next decades. Many may have summers with heat waves and triple-digit days — summers that resemble Phoenix today.

Here in the Valley of the Sun, that means work and play shift into the cooler hours. Neighborhoods thrum with activity at dawn and dusk when residents hike, jog and paddleboard. In the hottest months, the zoo opens at 6 a.m., for the benefit of both animals and visitors. And across the city, certain construction work starts in the middle of the night — not only for the safety of workers, but also because even some building materials can be affected by intense heat.

For workers doing concrete pours, jobs can start close to midnight so the material doesn’t get too hot or dry too quickly and later crack. At 1:30 a.m., on a construction site near Peoria, a Haydon Building Corporation crew listened for the approach of mixer trucks along a dusty dirt road marked with green flares. Moths and grasshoppers dashed against the stadium lights.

The crew was working on a bridge, hooking future housing developments to the suburban road matrix.

Night jobs are not easy on the workers, said Katie Perry, a director at Haydon. “You are getting up and awake in the middle of the night, and you have to be highly alert.” But such shifts mean they avoid the worst of the heat on those arduous jobs.

Since last year, parking lots at Piestewa Peak and two other popular trailheads stay open two hours later during the summer months so hikers can come out after dark. Encouraging hikers to avoid the worst heat is part of a “Take a Hike. Do it Right” campaign that began in 2015 to reduce, among other incidents, heat-related rescues and deaths on the city’s 200 miles of hiking trails.

Evening can mean the start of a new day for some residents. “I switch everything around. I do everything at night,” said Tee McKee, as she folded clothes in the community laundry room at the Shady Grove Mobile Home and RV Park in Mesa. “I try to put in insulation and old curtains to keep the cool in and the heat out.” But, she said, metal RVs roast in the sun and radiate heat long into the night.

Last year, heat caused or contributed to the deaths of 182 people in Maricopa County, which includes Phoenix. Preliminary figures suggest the toll this year will be similar, if not higher, according to the health department. Thirty-six percent of those who died in 2018 were 65 or older and at least 23 percent were homeless. Recent research found that mobile home residents are also especially vulnerable. In 2012 and 2014, nearly half the indoor heat deaths occurred in mobile homes, said Patricia Solís, a geographer at Arizona State University.

Below the misting machines: 99°F

Dr. Solís is one of dozens of heat experts in the region — part of what Mark Hartman, Phoenix’s chief sustainability officer, described as “the epicenter of research related to heat.” They are trying to determine, among many things, how best to mitigate heat through urban planning and how to help vulnerable populations, including people who cannot afford air-conditioning. Ultimately, Phoenix could become a model for what it means to be “heat ready.”

Some of the research takes place after sunset, as on a July night when Ariane Middel, a professor of urban climate at Arizona State University, and two colleagues pulled Marty, an assemblage of meteorological sensors, through downtown Tempe to examine the microclimate.

Other cities with temperate climates may start to experience heat like Phoenix’s in the coming decades, said Dr. Middel. “We are almost a living laboratory. We can test strategies and see different ways to keep adapting and mitigating.”

“By the time it gets hot in other places,” she said, “they can take what we have learned here.”

Night is not a respite from heat in the way it once was. According to the Arizona Department of Water Resources, nights are roughly nine degrees hotter than in 1948. The increase is due to global climate change and to the urban heat island effect: Sunbaked structures release the day’s heat and air conditioners pump heat outside.

As the researchers walked the circuit, their measurements with an infrared thermometer revealed details of the urban heat island: concrete was 113 degrees, asphalt 112, beneath a tree 103, a patch of grass 88.

“Around here it will still be over 100 degrees at 10 at night,” said Jerod W. Teller, a superintendent at Haydon Building Corporation. “They say it is always darkest before the dawn. Here it is always coolest before dawn.”

In that cooler pre-dawn, greater Phoenix seems most vibrant as many people exercise, do errands and commute. On construction sites, on roofs, on vast swaths of desert undergoing landscaping, on some farms, many workers start before daybreak. The heat comes on fast once the sun is up.

Early morning construction: 86°F

“We talk about hydration all the time,” said Chuck Hughes, a forklift operator at Interstate Mechanical, as he stood in the parking lot of a McCarthy Building Companies construction site in Tempe one morning. Just across the lot was a cooling station — a blue tent — and two huge fans blowing cool air through a covered lunch area. On floors where the outer walls and windows were up and the heat was intense, even in the early morning, there were fans and water jugs everywhere.

“The heat illnesses, we really have to train our guys on. They don’t recognize it right away,” said Kevin Maitland, vice president for safety at McCarthy. “We want to make sure that our workers, that the whole team, understands what those symptoms are.”

Mr. Maitland noted that heat impacts are cumulative, long-term, and of growing concern to many people in the construction industry. “The next time you have a heat stress, it is amplified,” he said. “Heat illness follows you for your career.”

In nearby Tempe Beach Park, runners, bikers, walkers and a paddleboarder exercised in the 94 degrees of first light. “I can quantify it,” said Glenn A. Dotson, who arrived at 5 a.m. to do a solo run before joining the local chapter of Black Men Run for a longer one. “It is a three-hour shift earlier.”

By 6 a.m., the group had stretched and started along the Salt River. “We don’t go anywhere without our water,” said Jarred Ervin. There are water fountains all along the route, he said. “We have them pretty much memorized and all mapped out.”

As the sky turned pink in Surprise, about 45 minutes north of downtown Phoenix, a team of landscapers raked gravel around newly planted vegetation. “This is certainly the busiest time of the year,” said Ed Macias, a division president at Service Direct Landscape. “People think that as the heat goes up, production goes down. But it is the opposite.” The company starts jobs at 4 a.m. and tries to wrap up by 1 or 2 p.m.

But even with the altered schedule, some workers — often those new to the region or to the intense labor — experience heat exhaustion every summer and need to sit in air conditioning and rehydrate, Mr. Macias said.

From June through August, the Phoenix Zoo opens two hours earlier so visitors and the animals avoid the most sweltering stretch of the day. Some staff arrive well before 5 a.m. to be ready for members, who can arrive at 6. The zoo closes at 2 p.m., three hours earlier than it does most of the year.

On the Desert Lives trail, three horticulturalists trimmed a palo verde tree. “There is no shade here. We start in the worst place and work our way down to where the shade is,” said John Sills. “We do sort of think it through — where we work each day — because it is so hot. It is kind of a survival mechanism.”

Bighorn sheep came up with the sun over a nearby butte.

The zoo at dawn: 77°F

Alicia Marcell, one of the zookeepers, wakes up at 3:15 a.m. to get to work on time. “In the summer, I don’t have a social life,” said Ms. Marcell, as she tended the African savanna habitat, picking up mesquite pods and preparing hay for eight giraffes that had been in a climate-controlled barn since 2 the previous afternoon. “I just sleep and adapt.” A hyena emerged from its night quarters as a buff-cheeked gibbon hooted nearby.

George Henry, who grew up in Phoenix and has volunteered at the zoo for nine years, said he enjoys being out all day, taking care of the grounds. “The heat has never bothered me,” he said. “The desert was my playground.”

Megan Helstab recently took advantage of the zoo’s summer hours. “Basically, anything you can get done early in the morning and after the sun goes down is a bonus,” Miss Helstab said, as she and her son watched the giraffes amble into their enclosure. “We definitely try to get all of our stuff done early in the day.”

The playground: 103°F

In Tempe’s Kiwanis Park, Patrick Smith and Stephanie Provencio watched their children in a playground and on a splash pad where loudspeakers looped a thunderstorm soundtrack. Their kids stay indoors most of the day during summer. “It’s not good,” Mr. Smith said. “They play lots of video games.” Except when it rains. “Everywhere else, people go inside when it rains,” he said. “Here, they go outside.”

Mr. Smith said they will drive 30 minutes or more to find a covered playground or water park. Unshaded playground rubber and metal can reach upward of 170 degrees.

Adaptation is a point of pride to many locals — to those who can take the summer shift in stride. “How to live in the heat is just part of the genetic code of people who live in the city,” said Mr. Hartman, the city’s sustainability officer.

Some residents even relish peak heat. Midafternoon on the second day of an excessive heat warning in July, the air at Piestewa Peak trailhead was about 112 degrees.

Lee H. Thomason had a water bottle in hand and electrolyte packets in his pocket. He makes a point of coming out on the hottest days to help hikers when he can.

Peak heat: 112°F

“You find people on the trail that shouldn’t be there. They are overheated and exhausted and unprepared,” Mr. Thomason said, adding that many don’t know about the physiological dangers of heat. “With the temperatures changing, there are new problems.”

Mr. Thomason and a few fellow heat-adapted “heat addicts” hike the peak during the day, no matter the temperature.

“We are all half lizard up here,” he said. “In fact, we are out when the lizards are not.”