Tesla CEO Elon Musk, responding to Uber Technologies board member Arianna Huffington urging him to change the way he works, ruled out taking a different approach.

An open letter from Huffington to Musk accused him of “demonstrating a wildly outdated, anti-scientific and horribly inefficient way of using human energy”, adding in a tweet that the Tesla founder should change the way he works. It follows an interview with The New York Times last week, in which Musk said he sacrificed family milestones in the race to meet Tesla production targets.

Still, the Tesla CEO responded to the letter with a tweet in which he said that Ford Motor Co and Tesla are the only two American car companies to avoid bankruptcy.

“I just got home from the factory,” Musk wrote at 2.32am Pacific time on Sunday. “You think this is an option. It is not.”

Tesla shares plunged 8.9% on Friday after the interview with The New York Times in which he described the past 12 months as “the most difficult and painful year of my career”. References to Ambien use and driving while tweeting are fuelling calls for Tesla’s board to step up its oversight of the company’s CEO and largest shareholder.

Musk sparked controversy by tweeting on 7 August that he had “funding secured” to take Tesla private. He later said that meeting with Saudi Arabia’s sovereign wealth fund had given him confidence to announce the news. The company later received a subpoena from the Securities and Exchange Commission over the tweet. — Reported by Crystal Chui and Alex Longley, (c) 2018 Bloomberg LP