Personally, I don't think it's a physical issue, Jon has always kept himself physically fit and I'm sure that he still does. I think the reasons that he seems so drained by the end of the show is because he's having to put so much effort into his singing, which is also why he doesn't move around much, and by the end of the show it's taken so much out of him. But I don't get why he does that, as he seems to sound better when he is relaxed and moving around more...it's very bizarre to me....