Elon Musk's office sleeping habits have existed since the early 1990s, according to a book about his first startup

• Elon Musk founded his first startup, Zip2, with his brother in 1995.
• Jimmy Soni's "The Founders" details how he slept at the office and openly berated colleagues.
• Nearly three decades later, the billionaire is managing Twitter with the same intensity.

Elon Musk's management of Twitter has become notorious for long hours and strenuous demands on staff, but before he was a billionaire, Musk behaved similarly at his first startup, according to "The Founders" by Jimmy Soni.

Soni's book was published last February – two months before Musk first offered to buy Twitter – and covers the rise of Musk and his PayPal cofounders.

In 1995, Musk founded Zip2 with his brother Kimbal, an online business directory with maps – before Google or Apple's navigation platforms became the go-to. Per "The Founders," Musk's company rented a "spartan Palo Alto office space," and he would sleep at the office and shower at the nearby YMCA.

Soon after taking over Twitter, Musk similarly said he would be sleeping at the office "until the org is fixed." At its San Francisco headquarters, Musk has turned some of the office space into bedrooms for employees.

One person familiar with the situation told Insider's Kali Hays that Musk was also having a bedroom made for himself. The company has since been ordered by officials to properly label them as sleeping areas, or convert them back to offices.

That followed Musk's ultimatum telling Twitter staff to commit to working "extremely hardcore" or receive three months' severance – delivered via a midnight email.

Back at Zip2, Soni describes a "perpetually sleep-deprived" Musk openly berating other executives and colleagues in 1996. As Musk recapped in an interview with biographer Ashlee Vance: "I'd never been a sports captain or a captain of anything or managed a single person."

But 27 years later, these habits are still on display at Twitter. Insider reported in February that Musk walks around the office and asks people at random what they're working on, and "you have to figure out how to answer the question in a way that will not get you fired," one of the people familiar said.

Platformer reported that Musk fired a Twitter engineer who explained why his popularity was declining. "There's times he's just awake late at night and says all sorts of things that don't make sense," one employee told the tech newsletter.

Zip2 was sold in 1999 for $307 million, before Musk founded the online payment company X.com — which would later merge with Thiel's company to form PayPal.

Musk's sleeping habits continued at his second company, too, as the company's cofounder Harris Fricker told Soni. Whereas Musk would stop working at 4 a.m. "with a catnap on his office floor," his cofounder would arrive for work before the financial markets opened at 6:30 a.m, per the book.

"To Fricker, this was a sign of Musk's disconnection from the company, but to Musk, late nights were just how startups operated," Soni writes. While the $44 billion Musk bought Twitter for is over 100-times what Zip2 sold for, the world's second richest person is investing the same energy into the social-media company.

In February, Musk said he has worked an average of more than 120 hours a week after taking over Twitter. "I go to sleep, I wake up, I work, go to sleep, wake up, work—do that seven days a week," he said last November, per Observer.

Twitter did not respond to Insider's request for comment.