When billionaire adventurer Steve Fossett broke the record for around-the-world solo jet flight last March, he slept just 60 minutes in 67 hours of flight time -- 60 minutes broken into two- and three-minute naps. "I slept when I needed it and awoke refreshed," he says. Fossett, who holds world records in ballooning, sailing, and flying, adds that none of his feats could have been done without these micro-variety "power naps."

So what makes a power nap effective? Think of it as an investment with the greatest return in the least amount of time, a kind of super-efficient sleep that fits nicely in a high-pressure schedule: say, between business meetings or in the minutes before a game.

Napping in general benefits heart functioning, hormonal maintenance, and cell repair, says Dr. Sara Mednick, a scientist at the Salk Institute for Biological Studies who is at the forefront of napping research. A power nap, says Mednick, simply maximizes these benefits by getting the sleeper into and out of rejuvenative sleep as fast as possible. No surprise that Lance Armstrong's coach, Chris Carmichael, says that "naps were critical in his overall training plan." In Manhattan, napping has become a lucrative business: MetroNaps in the Empire State Building provides darkened cot-like redoubts that attract Broadway actors between shows as well as investment bankers who otherwise would fall asleep at their desks. And in Iraq, U.S. Marine commanders have mandated a power nap before patrols.

Here's how the power nap works: Sleep comes in five stages that recur cyclically throughout a typical night, and a power nap seeks to include just the first two of them. The initial stage features the sinking into sleep as electrical brain activity, eye and jaw-muscle movement, and respiration slow. The second is a light but restful sleep in which the body gets ready -- lowering temperature, relaxing muscles further -- for the entry into the deep and dreamless "slow-wave sleep," or SWS, that occurs in stages three and four. Stage five, of course, is REM, when the eyes twitch and dreaming becomes intense.

The five stages repeat every 90 to 120 minutes. Stage one can last up to 10 minutes, stage two until the 20th minute. Extenuating circumstances, like manning the controls of a jet, aside, experts believe that the optimal power nap should roughly coincide with the first 20 minutes in order to give you full access to stage two's restorative benefits. In addition to generally improving alertness and stamina, stage two is marked by a certain electrical signals in the nervous system that seem to solidify the connection between neurons involved in muscle memory. "It's like a welding machine," says Mednick. "When you wake up, your neurons perform the same function as before, but now faster and with more accuracy," making the 20-minute nap indispensible to the hard-working athlete looking to straighten out his putter or baseline shot.

Mednick's most recent research also shows that power naps can lift productivity and mood, lower stress, and improve memory and learning. In fact, Mednick has found through MRIs of nappers that brain activity stays high throughout the day with a nap; without one, it declines as the day wears on. Tell that to the boss next time he finds you passed out at your desk.

There is, however, a pitfall in all this sleeping around. You have to carefully time the duration of your nap in order to avoid waking in slow-wave sleep. This can produce what's known as sleep inertia. That's when the limbs feel like concrete, the eyes can't focus, the speech is slurred, the mind is sluggish. Sleep inertia can ruin your day. You must keep the nap to 20 minutes or slightly less, and if you need the extra sleep, wait until the 50-minute mark. This will safely keep you on the power side of your nap.