As The Tonight Show host prepares to move into primetime, Leno, 58, tells PEOPLE exclusively the cause of his two days of hospitalization that forced him to take his first sick days in 17 years, canceling two shows.
"Exhaustion," he said Friday, shaking his head. He added jokingly, "That's like a rich person's condition. Poor people that work – they don't get exhausted. Only rich people get exhausted. It's an embarrassing thing."
He blames not only his talk show and grueling standup schedule, but also his lesser-known writing jobs.
"I think I just probably got worn out," he says. "I do three columns. I write for the London Sunday Times, I write for Popular Mechanics every month and I write for a British magazine called Octane. I do all automotive stuff. And then I'm on the road [doing standup comedy] 160 days. I think it's one of those deals where you just kinda just got worn out."
But the funnyman says he's ready for his next challenge: a primetime show that begins in the fall.
"No, I didn't have swine flu or anything like that at all," he says. "And really, now, I'm fine, I'm great."