Arianna Huffington will step down as editor-in-chief of The Huffington Post, the media company she launched in 2005.

"I thought HuffPost would be my last act," Huffington said via Twitter on Thursday. "But I've decided to step down as HuffPost's editor-in-chief to run my new venture, Thrive Global."

Thrive Global is a platform that encourages health and wellness, and aims to reduce burnout, through science-based training courses and content. 

Huffington declined Inc.'s request for comment, but noted in a press release that she found it too difficult to run both firms simultaneously, especially as Thrive Global grew quickly "from an idea to reality."

"When I decided to create Thrive Global, I thought it would be possible to build a startup and continue as editor-in-chief of The Huffington Post. Today, it's clear that was an illusion," she said. 

Thrive Global has already closed a Series A funding round led by Lerer Hippeau Ventures, featuring billionaire investors like Sean Parker (Facebook, Spotify), and Ray Dalio (Bridgewater Associates).

In her most recent book, The Sleep Revolution, the entrepreneur emphasizes sleep as a key to long-term success. In the release, Huffington suggested the irony that leading the two companies would have caused her to burn out. "Running both companies would have involved working around the clock, which would be a betrayal of the very principles of Thrive I've been writing and speaking about," she said.

It is unclear who will succeed Huffington as editor-in-chief, which was acquired by AOL for $315 million in 2011. Inc. had previously predicted the Huffington would step down from the firm to launch a wellness brand, when it was confirmed that Verizon had purchased AOL, HuffPost's parent company, for $4.4 billion last year.