Let me assure you you do not how tired you are until you retire. After about 6 months of not working, I suddenly realised that although older, I felt much better, and looking back, I think you become chronically tired, but become so used to it, you are not aware of it. I don't think there is a answer because of the sheer nature of the job, and the crazy hours. I always thought one day I would write a book about the job and call it "Too Many Sunrises" however in saying all of that, I would not change it for another career, and would certainly do it all over again, however buggered.