There was no one moment when Jon Stewart knew it was time for him to leave what he describes as "the most perfect job in the world"; no epiphany, no flashpoint. "Life," he says, in the lightly self-mocking tone he uses when talking about himself, "doesn't really work that way, with a finger pointing at you out of the sky, saying, 'Leave now!' That only happens when you're fired, and trust me, I know about that."

Instead, he describes his decision to quit The Daily Show, the American satirical news programme he has hosted for 16 years, as something closer to the end of a long-term relationship. "It's not like I thought the show wasn't working any more, or that I didn't know how to do it. It was more, 'Yup, it's working. But I'm not getting the same satisfaction.'" He slaps his hands on his desk, conclusively.

"These things are cyclical. You have moments of dissatisfaction, and then you come out of it and it's OK. But the cycles become longer and maybe more entrenched, and that's when you realise, 'OK, I'm on the back side of it now.'"