Changing the world takes hard work. More than 40 hours of work each week to be exact, according to a tweet last week from Elon Musk.

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The SpaceX and Tesla CEO has endured an "excruciating" year that sometimes drove him to sleep on his factory's floor and work more than 120 hours a week. The billionaire says he has since pulled back to a more "sustainable" 80 to 90 hours a week.

Those looking to emulate the charismatic founder should still be careful, say experts. While the reduced hours are a start, some warn that working long hours daily could still have a tough impact on a person's health and productivity.

At issue, in part, is sleep deprivation. Working 80 to 90 hours a week could mean working 11 to 18 hours a day depending on whether you've allowed yourself a weekend. Such a schedule doesn't leave time for much else, including sleep.

Insufficient rest can hamper a person's ability to focus and think creatively. "Sleep deprivation causes cognitive impairment that can lead to dangerous and costly mistakes and accidents on the job," says Janet Kennedy, a clinical psychologist and founder of NYC Sleep Doctor. "It also makes us more prone to illness, depression and anxiety — all of which compromise productivity."

While she commends Musk for cutting back his work hours, Kennedy points out that "it still doesn't allow enough time to recharge." Those 80 hours are likely very intense, she says, and might not include the time spent tethered to work through email, text or social media.

Quality sleep isn't just about getting seven hours of shut-eye, she warns. The brain needs time to transition, slow down and let the body repair itself. "Stress takes a huge toll on sleep, especially when it is unrelenting," says Kennedy, noting that adrenaline accompanies stress and can mask one's need for rest.

Musk has acknowledged that his team has also put in long hours this year, sometimes up to 100 hours a week. "There wasn't any other way to do this," he said in an interview with Recode this year, explaining the challenges car companies face to stay successful.

For his part, Musk has recognized the need for a change. In November, Musk admitted he'd shifted to "a hard work schedule, but not an insane work schedule" and journalist Kara Swisher said the founder seemed rested.

Still, during the toughest moments this year, he said, there were times "I would just sort of sleep for a few hours, work, sleep for a few hours, work, seven days a week."

Musk also revealed that his 120-hour schedule left him feeling exhausted, "nutty," and like he'd "burnt out a bunch of neurons."

"No one should put this many hours into your work," he told Axios last week. "This is not recommended for anyone."

He admitted that the pain level significantly increases once you exceed the 80-hour mark.

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He added, "You're gonna go a little bonkers if you work 120 hours a week."

Some Silicon Valley leaders, such as Reddit founder Alexis Ohanian, have begun to bring attention to the long hours that have become synonymous with entrepreneurship in recent years. Constant "hustling," without time for family or rest, "has deleterious effects not just on your business but on your well being," he said at a European tech conference earlier this month.

Those looking to build big things should keep in mind that even an 80- to 90-hour work week can be difficult to maintain for most people, especially at top performance levels. "It's very likely that there will come a point of reckoning," says Kennedy. "That level of stress takes a very real toll."

Poor concentration, difficulty finding your words and falling asleep unintentionally are all signs that you could be overworked and reaching your breaking point, she says.

"Productivity requires fuel and we all have to recharge to maintain it," says Kennedy. "Even a Tesla has to be plugged in once in a while."

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