I spent a few weeks sleeping in a giant plastic bubble as part of an unscientific experiment on athletic performance. The air in the tent contained less oxygen than the air in Los Angeles. By using the tent, I hoped to improve my body's ability to supply oxygen to my running muscles — and shorten my marathon time.

Other hard-core runners, cyclists and cross-country skiers — especially aging ones — understand this desperate attempt to get faster. At 36, I've already started to slow down — at least 1 1/2 minutes over a 6.2-mile race compared with my college days.

I'd move to the mountains if I could. Above 7,000 feet, where significantly less oxygen is available than at sea level, the body increases its production of oxygen-toting red blood cells within a few weeks.

In races at sea level, athletes with extra red cells can supply their muscles more efficiently with oxygen than their competitors from the lowlands.

The promise of such a boost draws top endurance athletes to live in oxygen-thin places, such as Boulder, Colo., or Mammoth, Calif. But for the rest of us, trapped in desk jobs at sea level, a tent filled with oxygen-thin air is a tempting alternative.

In theory, spending a large portion of each day in a low-oxygen environment can trick the body into producing extra red cells. Research suggests this might even be better than living and training in the mountains.

"Live high, train low" goes the current mantra on altitude for athletes. Living high builds up the red cell count. Training at lower elevations allows the body to exploit those cells; it is simply not possible to push as hard or run as fast at high altitude.

Inside the tent, I cannot get back to sleep. And because I'm not yet acclimated, it feels hard to breathe.

SLEEPING HIGH

Over the past decade or so, tents and other types of altitude chambers have become standard equipment for many elite runners, cyclists and cross-country skiers. Lance Armstrong was one of the early believers. Now it is hard to find a Tour de France rider not sleeping "at altitude."

And Nike has turned an entire house into a high-altitude dormitory for running stars.

The technology has become so popular that last year the World Anti-Doping Agency considered adding altitude tents to its list of "banned methods."

The case against tents came down to the question of whether they "violated the spirit of sport." Ultimately, tents were judged to be just another training aid.

Not only elites are using them. Amateurs account for about half of the several thousand people who have bought altitude tents, according to the two companies that control the U.S. market, Colorado Altitude Training and Hypoxico Inc.

The most affordable way to try a tent is renting, for about $600 a month. Buying a tent requires more commitment: A basic model sells for $6,000.

Since I already was training hard, it seemed unlikely that sleeping at simulated altitude could help much. But Hypoxico agreed to lend me a tent.

The tent arrived in two boxes weighing a total of 85 pounds.

The smaller box contained the tent, the "king-size." I unrolled it in my bedroom and set it up in about 30 minutes.

The arching roof was nylon and the sides were clear plastic, with zippers.

It looked like a giant version of the packages that pillows and blankets come in, the kind that say, "Keep away from children: could cause suffocation."

The second box, the heavy one, held the generator. It takes in normal air — which at sea level contains 20.9 percent oxygen — and pumps out a new mix.

That night, I set the dial at 4,500 feet, an oxygen content of about 17.7 percent. The company had advised me to start low and build up to 9,000 feet or 15 percent, over the course of a week. Going up too quickly could cause headaches and fatigue.

I climbed inside and zippered the door shut.

The end of the hose was breathing. Heavily, like Darth Vader or someone with a respiratory disease.

I would get used to it.

RESTLESS NIGHTS

It seems unlikely that tents or altitude rooms will become standard equipment for athletes.

Sleeping in the tent is not fun even for a few weeks, much less month after month.

A key issue is how many hours a day an athlete spends in an oxygen-thin environment. A normal night's sleep may not be enough.

In the seminal study on altitude training, well-trained runners that spent 20 to 22 hours a day at 8,200 feet and trained at 4,100 feet showed an average improvement of 1 percent to 2 percent in a 3.1-mile run at sea level. In contrast, runners who lived and trained at 8,200 feet — or those who lived and trained at sea level — did not improve.

As for me, in the few weeks before the marathon, I ran 5 seconds per mile faster than I ever had for my standard 4.4-mile loop in Griffith Park.

Was it the tent? Maybe. But I'd also been training harder than I had in years.