Steve Jobs left incredibly big shoes for Cook to fill. However, the man got the top job for a reason. He's always been a workaholic, and Fortune reports that he begins sending emails at 4:30 a.m.

A profile in Gawker reveals that he's the first in the office and last to leave. He used to hold staff meetings on Sunday night in order to prepare for Monday.

Barra started at the very bottom of General Motors at age 18, when she enrolled in an engineering college sponsored by the company. There, she spent half the year inspecting parts at a Pontiac plant, according to Fortune.

She worked her way up the ladder with smart decision-making and a willingness to give the company everything she had. The Financial Times reports colleagues recall her being the first person in the office every morning and responding to emails after 11 p.m.

In 2013, her dedication was rewarded when she was named GM's first ever female CEO.

At first glance, Cuban's amazing success looks like a stroke of luck. He sold his first company at the peak of its value and got into technology stocks at exactly the right time.

Cuban writes on his blog that it took an incredible amount of work to benefit from his luck. When starting his first company, he routinely stayed up until 2 a.m. reading about new software, and went seven years without a vacation.

The current head of Brazilian Oil Giant Petrobras spent her childhood in a favela collecting cans to pay for school. She started as an intern in 1978, but quickly became the company's first female head of field engineering.

Bloomberg reports that her tireless work ethic has earned her the nickname Caveirao, for the armored vehicles police use to clean up crime-ridden Brazilian neighborhoods.

Bezos always had a relentless work ethic. A former classmate told Wired that once Bezos made it clear that he intended to be high school valedictorian, "everyone else understood they were working for second place."

The early days at Amazon were characterized by working 12-hour days, seven days a week, and being up until 3 a.m. to get books shipped.

Now that Amazon's a giant, Bezos personally emails teams about customer service issues and has them present directly to him about how they're going to solve them, according to an excerpt from Brad Stone's book, "The Everything Store."

The Williams sisters, who between them have won 25 Grand Slam singles championships, were all but raised on the court.

From an extremely young age, their lives revolved around tennis. Their sister, Isha, describes their daily routines to the New York Times as such: "Get up, 6 o’clock in the morning, go to the tennis court, before school. After school, go to tennis."

Carlos Ghosn runs two of the world's largest automakers, which should tell you something about his work ethic. A profile in Forbes describes how Ghosn works more than 65 hours a week, spends 48 hours a month in the air, and flies more than 150,000 miles a year.

His turnaround of Nissan is the subject of many case studies. Within a month of taking over in 1999, he deployed a system that completely changed ingrained practices, helping save a company many thought irredeemable.

One of the richest men in Asia and a dominant figure in Hong Kong's economy, Ka-Shing started outworking everybody as a teenager en route to building a $31 billion empire.

By age 15, Ka-Shing had left school and was working in a plastics factory. He told Forbes how he quickly became a salesman, outsold everybody else, and became the factory's general manager by 19. In 1950, he started his own plastics business and did almost everything, including the accounting, himself.

Sandberg's famous for leaving work at 5:30 to eat dinner with her husband and spend time with her children. But according to an interview with Harvard Business Review, she gets right back to work online after she puts her kids to bed.

Any one of the things Sandberg does would be impressive and take an incredible amount of work on their own. In all, she's the highly successful and influential COO of a multi-billion dollar company, a massively successful author, and one of the most recognizable advocates in the world for women in the workplace.

Jordan had prodigious physical gifts. But as his long-time coach Phil Jackson writes, it was hard work that made him a legend. When Jordan first entered the league, his jump shot wasn't good enough. He spent his off seasons taking hundreds of jumpers a day until it was perfect.

In a piece at NBA.com, Jackson writes that Jordan's defining characteristic wasn't his talent, but having the humility to know he had to work constantly to be the best.

The chief of advertising giant WPP is described by the Financial Times as a "notorious workaholic and micro manager." His typical workday begins at 6 a.m. and never seems to end.

A former client described sending Sir Martin a message while he was in a different time zone in the earliest hours of the morning. Sir Martin responded almost immediately.

Mayer is known for her incredible stamina and work schedule. She used to put in 130-hour weeks at Google, according to Entrepreneur, a schedule she managed by sleeping under her desk.

Even people critical of her management style acknowledge that she "will literally work 24 hours a day, seven days a week," Business Insider's Nich Carlson reports. That paid off with one of the biggest jobs in technology.

A 2005 Fortune article on Immelt describes him as "The Bionic Manager." The article highlights his incredible work ethic, saying he worked 100-hour weeks for 24 years. Immelt strictly divides that time, devoting a specific portion of each day to deal with every part of his business.

All of that comes after a 5:30 a.m. workout, during which he's already reading the papers and watching CNBC.

Nobody in basketball drives their body harder than Bryant. A profile in GQ describes how he has changed his shooting technique repeatedly rather than take time for dislocated and broken fingers.

When growing up outside of Philadelphia, ESPN describes how Kobe would spend his free time endlessly practicing jump shots in the park. The Lakers staff finds him doing the same thing at their practice facility at all hours of the day and night.

Now one of the most powerful and well-known women in business, Nooyi worked midnight to 5 a.m. as a receptionist to earn money while getting her masters degree at Yale.

Today, she wakes up at about 4 a.m., according to Fortune. At the Aspen Ideas Festival earlier this year, Nooyi said that at times during her career it has been normal for her to work until midnight. During that same talk, she said her intense dedication to her job may have made her a worse parent.

PayPal cofounder, SpaceX founder, and Tesla CEO Elon Musk is known for his tireless work ethic, and in an interview with Vator, he advises other entrepreneurs to work twice as hard as everyone else to achieve success.

"You just have to put in 80- to 100-hour weeks every week," he tells Vator. "If other people are putting in 40-hour work weeks and you're putting in 100-hour work weeks, then, even if you're doing the same thing, you know that you will achieve in four months what it takes them a year to achieve."

This ability to keep pushing was on display in August 2008 after SpaceX's third unsuccessful attempt to launch a rocket into space. Musk put out a statement telling the world, "For my part, I will never give up and I mean never."

Seven weeks later, SpaceX became the first private company to successfully launch a rocket that reached orbit.

McMahon's life has revolved entirely around World Wrestling Entertainment (formerly the World Wrestling Federation) for more than 40 years.

His former creative partner, Vince Russo, writes in a piece for What Culture that no matter what he did, he could never get to the office before McMahon or leave after him.

Wrestler Mark Henry says in an episode of Grantland's Cheap Heat podcast that McMahon is interested in nothing besides wrestling. This isn't entirely true — McMahon, 69, is a big bodybuilder, and he is in great shape for someone his age.

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