In June, Lauren Warren bought a sauna after monitoring a sale on Wayfair.com and installed it in her living room. “With the winter coming — I really loathe winter — especially with the pandemic, I knew I wasn’t going to be able to get away to get warm,” she said.

Ms. Warren, 54, an exercise therapist who lives in Tillson, N.Y. has competed in Ironman triathlons. She eats organic raw food, intermittently fasts and leads a “holistic lifestyle all the way — that’s the sauna!” she said.

Spending a lot of money in an uncertain economy concerned her, but the price kept dropping, from $5,100 to $1,500. When only two of her preferred four-person, infrared models remained, she pounced.

“Next thing I know, I’m typing in my info and here goes! I hit the button. I was jumping all over, excited,” she said.

Infrared units bear little similarity to the traditional Finnish-style sauna. They heat up to around 150 degrees Fahrenheit, instead of over 200. Light wavelengths warm the body. There is no blast of hot air, no pouring water on heated rocks for steam, and users can sit safely for extended periods of time.

The myriad benefits of saunas have been documented in numerous studies. Sitting in one is said to, among other things, improve insomnia, inflammation, pain, blood pressure and brain function. Near infrared has been touted to help heal skin ailments from acne to psoriasis to surgical scars, and, particularly in Japan, as therapy for heart patients.

Heat increases blood circulation. “I’m a pasty white Dutch guy,” said Rick Mouw, 59, the president of Almost Heaven in Holland, Mich., which mainly sells traditional saunas — including trendy barrel-shaped ones — electric, wood-burning and steam. “People ask if I’m tanning! You get a healthy glow.”

Some liken sweating in a sauna to passive cardiovascular conditioning, which may interest anyone who has packed on pounds in quarantine.

But maybe the best way to approach the overwhelming barrage of health claims lobbed at sauna shoppers trying to justify a four-figure purchase to get through winter is a study out of the University of Eastern Finland, which suggests frequent sauna use reduced risks of “all-cause mortality.”

There is also the simple sensual aspect. “As an athlete, I saw in quite a few journals that it can build blood volume, almost like training at altitude,” said Ms. Warren, who said she uses her sauna almost daily. “I do a lot of research, then I use myself as a guinea pig. You do feel it the next day — wow, that was good!”

And therein lies the sauna’s current appeal, up there with Peloton bikes, fire pits and outdoor heat lamps. Here we all are, facing climbing coronavirus rates and, in many parts of the country, declining temperatures. All we want is to feel good.

“More people are at home, they aren’t spending money like they used to — going to shows or on vacation or at movies,” said Corey Smee, 30, manager at Health Mate Sauna. “They are cooking more home meals. They can’t go to the gym and sweat. So people are investing in health more than ever.”

Amortizing the cost helped Ms. Warren make her investment. She used to go frequently to a sauna spa, which charge about $40 for 30 minutes. Like gyms, such studios — Perspire, HigherDose, SaunaBar, Glow — have had to restrict customers, or close altogether, during the pandemic.

In August, Ksenia Avdulova, 32, did similar math. The ClassPass points she had been using for sauna time pre-Covid were “pretty expensive.” With no other social activities, and after months of being tempted by photos of infrared saunas on Instagram, she installed one at her tiny cabin near Callicoon, N.Y. Yes, #saunaselfie is now a hashtag. Instagrammers including Gwyneth Paltrow and Lady Gaga, who uses hers for chronic pain relief, have posed in the structures.

For those with less space, Sunlighten sells a portable “Solo” sauna, which looks a little like a fancy human pet bed. Sales of this unit are currently up 140 percent, according to Connie Zack, a founder of the company, adding that overall sales rose significantly in July compared with last year, mostly thanks to buyers in California, New York, Pennsylvania, Texas and Florida. Ms. Zack calls infrared sauna “part of my lifestyle, it’s what I do to keep myself healthy.”

Sunlighten has been able to meet increased demand at its factory in Vietnam. So has Health Mate at its Canadian facility. But other companies have sold out of certain models.

“We are back ordered two to four months depending on the sauna right now,” said Andy Kaps, 58, the president of Clearlight Infrared. (Ms. Paltrow has posted its “Sanctuary” unit, with a half glass roof and a glass front, on social media.) Clearlight’s Chinese manufacturing facility closed for six weeks early on. “People are looking for things to enhance house and health — so wellness exploded,” Mr. Kaps said.

Almost Heaven is also experiencing “extended lead times in many cases,” said Mr. Mouw, who said the company has expanded production at its factory in West Virginia. “With Covid, it’s a busy market on steroids. No one can say sauna helps with Covid, but it does provide people who want to live a healthier lifestyle overall with another activity you can experience at home at the moment.”

(Almost Heaven was acquired in 2018 by the Finnish company Harvia, a maker of sauna stoves, among other related items. Mr. Mouw said affably that he was hazed by his new colleagues in Lapland, 50 miles north of the Arctic Circle.)

Can’t afford a sauna of your own? Rental is an option. Henning Grentz, 47, runs Spa Fleet, a mobile sauna rental based in High Falls, N.Y. Over the summer, he had several long-term bookings, a warm-weather rarity. “The first, for six weeks, was a family that fled New York City,” he said. “They had a beautiful piece of property in Saugerties with a creek.”

Mr. Grentz built his dry Finnish sauna, the type he grew up using in the north of Germany, from scratch. “It was a happy time — a hobby with the intention of bringing the goodness out there,” he said. He even named it after Scandinavian fruit: Cloudberry. Renting Cloudberry, a large barrel sauna designed for off-grid use, with a changing room, a wood stove and sauna rocks, costs $650 for one or two nights, and there are weekly prices, too.

A group of young Finns booked it for their yearly crayfish event and then Thanksgiving; there are winter dates available. Mr. Grentz delivers Cloudberry and enough firewood for at least 15 hours burning time on a 14-foot trailer anywhere within a two-hour radius.

Those outside this range can seek rentals on SaunaShare.com, or rationalize the splurge as one might a car. Mr. Kaps of Clearlight, whose saunas cost $5,500 on average, swears his product is worth it: “It’s not like that piece of equipment you buy and don’t use; if you sit in it, it’s going to make you feel great.”

Maybe too great. Ms. Avdulova has a 30-foot walk from her sauna to her tiny cabin, which she considers her “cold therapy” post-heat. She stargazes, recently saw a 22 degree halo on an almost full moon, has heard coyotes and once even found the remains of their prey, a baby deer.

“Sometimes I think, What if a wolf approaches me on my way back from the sauna and kills me?” she said. “It wouldn’t be a bad way to die — I feel so amazing.”