THE CHIEF EXECUTIVE of Tyco International, L. Dennis Kozlowski, calls his job "a game of endurance."

He starts work each day at dawn and goes nonstop for 16 hours or more. Last Wednesday, for example, he had a 6 a.m. meeting with investment bankers in Boston and then flew to Livingston, N.J., to meet for several hours with employees of CIT Group, the financial-services business that Tyco just announced plans to acquire.

He then traveled to New York City to have dinner with the chief executive of a company Tyco does business with, and didn't check into his hotel there until after 11 p.m.

He slept for just five hours -- his usual regimen -- and was on the phone the next morning at 5:30 a.m. with staffers in Europe. Then, he held a series of back-to-back meetings all day with employees and investors before heading to a dinner at a Chinese restaurant "to talk about synergies with CIT," he says.

"This is what it takes to be a CEO, and if you're not willing to do it you should step out of the way so someone else can take your place," says Mr. Kozlowski, who is 54 years old and believes he was born with the ability to keep going when everyone around him wants to stop and rest. "As a kid, I drove my parents crazy."

THIS BOUNDLESS ENERGY is a common trait in most CEOs, and a critical one. As business becomes more global and technology calls for 24/7 performance, those who lead companies must be able to move through several time zones several times a week, and communicate with staff and customers around the clock.

"Travel time used to be downtime, but now I'm on the phone or e-mail while on airplanes," says Mr. Kozlowski, who worries that if he stays out of touch for even a few hours he'll miss something important. "While you sleep, things are going on in Asia."

So how do CEOs sustain and replenish their high energy levels? Mr. Kozlowski says he refuels on weekends -- but that doesn't mean rest. He rides his motorcycles and races his sailboat.

Apple Computer's Steve Jobs, 46, stays energized during the week by avoiding caffeine and drinking lots of water throughout his long workdays. T.J. Rodgers, president and CEO of Cypress Semiconductor, rarely breaks for lunch. But five times a week, the 55-year-old Mr. Rodgers leaves his office midday for a five- or six-mile run. When he wants to do an extra mile or two, he'll give up changing time and sit in his wet running clothes for the rest of the afternoon. "The jogging keeps him focused and strong," says a spokesperson.

Unlike scientists, scholars or even surgeons, who must concentrate for long periods of time on one task or goal, business leaders must move quickly from situation to situation, handling scores of disconnected events each day.

CEOs tend to thrive on the pace and diversity. "Sometimes you have fires blazing in one place that you need to put out, and then you must quickly shift gears and go calmly to another location where you have to do something ceremonious," says Kay Koplovitz, the 55-year-old CEO of Working Women Network.

SHE TRIES TO RESERVE some energy for reflection and some strategic thinking. "If you are always reacting to people and things, you are not moving your business forward," says Ms. Koplovitz.

Each day, she ensures she takes some time to think without interruptions. She typically does this while working out at the gym -- or when traveling. She also boosts her energy by participating in adventure sports.

"When you are whitewater rafting in class-five waters or altitude hiking, you have to pay rapt attention and can't let thoughts about business intrude," she says. That focus "takes my energy but also frees me [from everyday concerns] and so restores my energy."

Even the most vigorous CEOs, however, must learn to let go of some workplace tasks, or risk burnout. Kimberly-Clark Chairman and CEO Wayne R. Sanders, 53, realized while on a 15-day swing through Asia two years ago that he needed help overseeing his company's expanded global businesses.

He had been logging thousands of miles flying overseas each month and then putting in marathon hours between trips to catch up with work at Dallas headquarters. Unable to sleep one night in a foreign hotel, reading a book and eating a candy bar, he realized he needed to delegate some of his workload. Soon after, he named a No. 2, President Thomas J. Falk.

Mr. Sanders now focuses on investor relations and other CEO matters at home, while Mr. Falk, 42, travels extensively, managing Kimberly-Clark's world-wide operations.

Michael Fisch, the 38-year-old president of American Securities Capital Partners, a New York leveraged buyout firm, admits, "I find work more exciting when I'm running very hard and approaching things with a sense of urgency."

As a child, he says, he thought about "whether it's more efficient to button your shirt top to bottom or bottom to top." But he knows the danger of never resting. "I know I'm in trouble," he says, "when I start thinking that sleep is inefficient."