(FORTUNE Magazine) – Patrick Heaney comes wobbling up the road on a mud-caked bicycle, looking grim. His dirt-spattered body trembles with fatigue; his eyes are deep tunnels of red. In the past two days he has bushwhacked, biked, climbed, and rafted 90 miles, through rain and freezing temperatures, all on scarcely an hour's sleep. He looks like a man in need of a vacation. In fact, he's on one.

Heaney, 28, is on the cutting edge of the latest leisure trend: extreme vacations. Premised on the notion that rest is for the weak, these excursions into pain reassure type-A tourists that their schedules are being disrupted for nothing as unproductive as mere relaxation. And people are clamoring to sign up.

Heaney's ordeal has been organized by Odyssey Adventure Racing Academy of Lookout, W. Va., one of a half-dozen similar schools around the country. After four days of learning the basics of rafting, rope climbing, and compass navigation in the New River Gorge, his class of 11 launched directly into a 36-hour nonstop race up, down, and across the rugged countryside. Known as the Endorphin Fix, the contest is billed as "the toughest two-day race in the United States."

Having survived acute exhaustion and borderline hypothermia--and narrowly escaped being trampled by a herd of deer --Heaney says it "feels good" to cross the finish line. And the engineer is pleased that his first vacation in a year was so punishing. "When I'm on vacation, I've got to do something productive," he says. "I can't just vegetate on a beach somewhere."

There have always been stress addicts. In the past they had a single vacation option: Stay at work. But in the last two years or so, the travel industry has begun offering high achievers grueling packages set in virtually any corner of the planet.

Backroads, one of the country's largest such organizers, has seen an explosion in demand for physically intense, multiple-sport vacations. "Since we introduced the category in 1997 it has grown by leaps and bounds--up 44% in the last year alone," says spokeswoman Megan Gaynor. The company now offers 30 different tours, including a weeklong circuit of coastal Alaska that alternates 50-to-90-mile bike rides with five-hour glacier hikes and Class IV whitewater rafting.

Unpleasant as that may sound to the traditional recreationist, Backroad's Alaska journey pales in comparison with the 35-day Karakoram Traverse organized by KE Adventure of Aspen, Colo. The trek is an unrelenting odyssey across some of the earth's most daunting terrain--the Karakorams, just west of the Himalayas in northern Pakistan--and includes a trip to the base of K2, the second-highest peak in the world. "Back when we started offering it, seven years ago, we didn't think anyone was really going to do this," says director Mark Van Alstine. "But it's one of our most popular trips, which amazes us."

Why has leisure-time masochism become so popular? Some outfitters hypothesize that their clients are so stressed from years of multitasking that they crave the mental hose-down that comes of sheer physical depletion. "By having a physical purpose, our clients have time to relax mentally," explains Lisa Didus of Active Journeys, which offers 300-mile bike rides in the Dominican Republic and 200-mile hikes across England. "If they're sitting on a beach, all they can think about is the work they left behind. But if the challenge is to get over the next hill, there's no time to think about what's waiting back on their desk."

And so the old paradigm of vacation--work, then rest--has become inverted. The new credo: Work, then work harder. The truly motivated actually train to get ready for their time off. Odyssey recommends that its clients start getting in shape eight weeks in advance. And San Francisco's Latitudes Expeditions East, which operates multisport Asian tours, is working with the fitness chain Crunch to offer a 12-week training program before each trip. For instance, clients will practice on an indoor climbing wall to prepare for rock climbing during a 14-day tour of Thailand that also includes trekking, sea kayaking, and mountain biking.

Back in West Virginia, Patrick Heaney is lying faceup on a picnic table, blinking. Tomorrow it's back to work, putting in 60-hour weeks designing advanced jet engines for Pratt & Whitney. For now, though, he can enjoy the sweet tranquility that exhaustion brings.

The grass around him is strewn with battered bicycles and the sleeping bodies of fellow racers. "Without a doubt, this is much worse than a full triathlon," says one bleary-eyed competitor. Indeed, of the 62 men and women who started the race, only 14 have finished. Among those who bailed out were a man pulled off the race course with hypothermia and a woman who quit after suffering multiple head-over-heel bike crashes.

For his part, Heaney says, "I'm just stoked I finished the race."

Any take-home lessons from his trip? Yes, he says: "You've got to work in sleeping and eating."

Words every vacationer should live by.