Tesla shares plunged on Friday as investors reacted to an interview in which its chief executive Elon Musk admitted taking the sleeping pill Ambien, working 120 hours a week and described how exhaustion was taking its toll.

He broke down at times as he told the New York Times he was suffering the most painful year of his career, while company executives said they were worried his erratic tweeting and unpredictable behaviour could be caused by his use of sedatives to sleep.

By the end of trading, shares in his company were down almost nine percent - wiping more than $5 billion off its value

Mr Musk has long delighted in his reputation as one of Silicon Valley’s brashest entrepreneurs but admitted struggling to cope with exhausting production deadlines.

“It’s not been great, actually,” he said. “I’ve had friends come by who are really concerned.”