These are not words you’d expect to hear from the CEO of Australia’s fifth largest company.

But in his October interview with Qantas magazine, BHP chief executive Andrew Mackenzie has opened-up on the issue of overwork.

“A rested Andrew can do more in four hours than a tired Andrew can do in eight,” he said.

“It’s not only diminishing returns; [not being rested] is like a scorpion’s tail — it can undo things. That’s true of everyone’s productivity and particularly in an intellectual role like that of a CEO.”

“A lot of boards don’t get that. People need to be fresh.”

“You don’t have to be the smartest person in the room to be a CEO, but you do need to be sharp,” he said, adding you won’t be sharp if you are tired.