Seeking serenity? Flotation therapy offers an inexpensive way to achieve it.

This summer, I spent an hour floating in a 4-by-8 isolation tank filled with tepid salt water in a basement in Manassas.

No, this wasn’t a “Silence of the Lambs” scenario. It was an exercise meant to bolster my sanity, not endanger it.

Floating in a tank of skin-temperature water saltier than the Dead Sea is said to induce an ethereal, meditative state — one that doesn’t necessitate furtive handoffs on street corners or haggling with sketchy dudes who listen to Grateful Dead bootlegs.

Flotation therapy businesses have been popping up around the country, including Bethesda, where Kimberly Boone and Tomas Hyrman opened the Hope Floats spa in April, and Manassas, where Stillness Flotation, now called Om Float Spa, debuted last year.

“The tank teaches you things about . . . the noise in your head — stuff that’s toxic and self-deprecating” says Brooks Brinson, who runs Om Float from his Manassas townhouse.

Researchers aren’t sure precisely why floating is restorative. Time in the tank may induce a neurological state similar to sleep, dialing down the sympathetic nervous system — the part of our brains associated with fight or flight — while turning up the parasympathetic nervous system, associated with rest.

Research also demonstrates the many benefits of the “restricted environmental stimulation technique,” or REST, a fancy name for the therapy. Studies show that REST may help people manage pain, battle anxiety and depression, quit smoking and lower blood pressure.

“You lose contact with where your body stops and the water starts,” Brinson says. The company motto: “Relax . . . Rejuvenate . . . Reconnect.”

Some say its curative powers go further.

Anette Kjellgren, a psychology professor who has studied flotation at Karlstad University in Sweden since the 1990s, says floating can help with a laundry list of conditions, including stress, muscular pain, addiction, fibromyalgia and disorders associated with whiplash. The technique can also facilitate traditional psychotherapy and even help address ADHD and autism spectrum disorders, she said in an e-mail interview.

These apparent benefits have yet to be tested rigorously enough to prove flotation’s effectiveness. But some researchers are convinced enough that they have become floaters themselves.

“The first 10 minutes are very long,” says Paul Morgan, a research associate at the University of Pittsburgh and an avid floater. Morgan co-authored a paper showing that, after exercising, floating decreases lactate in muscles faster than just, say, lying down. “Then you have the anxiety of ‘Are my legs still there? Where am I in the tank?’ The next thing I know, I’m waking up,” he said.

“The first time, they forgot and left me in the tank,” Kjellgren said. Several hours in the tank offered “a very intense and interesting experience,” she said, and she has since floated hundreds of times.

“If they had taken me out after 45 minutes, as planned, I would probably never have become a flotation tank researcher,” she said.

For me, the experience was like Zen meditation: boring at first, then over in what seemed like seconds. Suspended in about two feet of 94-degree water filled with Epsom salt, I didn’t just feel relaxed, I felt like the giant baby at the end of Stanley Kubrick’s “2001: A Space Odyssey.”

But the positive effects of floating — a sense of peace, clarity and general good humor — didn’t stay in the basement. After I got out of Brinson’s tank and showered, I figured that was it.

“Your float is not done yet,” he warned.

My hour-long drive back to Washington proved him right. Though I found my return to everyday existence slightly irritating, colors — of cars, of buildings, of the sky — were more lush. The world seemed enhanced, like watching HDTV. Driving I-66, I felt like Neo fighting Agent Smith at the conclusion of “The Matrix,” navigating one of America’s most congested regions but feeling like I was lane-shifting in slow motion.

When I stopped to get gas and a snack, plantain chips never tasted as plantainy. I’m no banterer, but when I called my stockbroker from the road, I asked her whether she was having a wonderful day. And when I left a voice mail for myself at The Post so I wouldn’t forget a story idea, I cracked up. It’s me, I thought, leaving a message — for me!

Why not just say it? I was high.

“You become a consciousness floating in a void,” Brinson says.

If that sounds kooky, it’s partly because of flotation’s checkered history. The practice was pioneered in the 1950s by physician turned psychedelic adventurer John C. Lilly, perhaps best known for his work on communication between humans and dolphins. In 1980, William Hurt turned into an amoeba in the flotation-themed horror film “Altered States.” And flotation’s main celebrity endorser is Joe Rogan — a bawdy comedian, “Fear Factor” host and Ultimate Fighting Championship announcer who is also really into marijuana.

“Everybody should do the tank,” Rogan, who declined to be interviewed for this story, said in 2010. “You will learn more about yourself than in any other way.”

Brinson isn’t the only floater who references Rogan’s proselytizing. Quinn Zeteda, an owner of Portland, Ore.’s Float On — billed as “the largest flotation center on the West Coast,” with four tanks offering bliss to 40 floaters per day — was so inspired by a Rogan podcast that he went into the business.

“I couldn’t believe how good and clear and calm I felt after my float,” said Zeteda, who opened Float On in 2010. “It seems to just resonate with a certain level of consciousness.”

Not every flotation advocate, however, comes from reality TV or a city known for its vegan doughnuts. The Nobel Prize-winning physicist Richard P. Feynman, who knew Lilly, wrote about his attempts to hallucinate while floating (after smoking marijuana and using the drug ketamine). The Dallas Cowboys and the Philadelphia Eagles used flotation in the 1980s to help players focus. The Cowboys’ units were equipped with closed-circuit televisions that allowed floating players to watch games. (Linebacker D.D. Lewis called it “the think tank.”)

Brinson also found flotation helped him compete — as a shooter.

“It’s really a very mental game, the most mental in the Olympics,” says Brinson, the Texas air pistol champion in 2007 and 2008.

A float can cost around $60 an hour. But if you don’t want to rent time in a tank, you can buy your own. Tanks cost a lot, but less than, say, a Lexus. According to the California company Samadhi, the first manufacturer of flotation tanks, its units are self-contained, can easily fit in a bedroom or basement, require no plumbing and are kept clean with a hydrogen peroxide solution. Chlorine, which can be toxic in enclosed places, isn’t required.

The cost? Just $8,900 plus shipping — or floaters can try their luck on eBay.

“This is small business still,” says Lee Perry, whose husband, Glenn, an acolyte of Lilly’s, founded Samadhi in 1972. “We’re a small company, and if it keeps going like it’s going now, it could get to be big business in not too long.”

Brinson, who has three more tanks in storage and is looking for a storefront in Tysons Corner, is confident about floating’s business potential, saying he has had as many as 20 floaters per week. Hope Floats’ Boone, a former client of Brinson’s, started her business out of her home with just one tank. After building up a roster of about 40 regular clients, she will have three tanks when she moves to a location near the Bethesda Metro next month.

The appeal of plugging into universal consciousness is, after all, universal.

“It’s about learning to focus your mind,” Brinson says. “The tank is a perfect tool for that.”