To get that high, you have to keep pushing your performance. If you just level off like I pretty much do, it doesn't show up, or at least isn't as apparent. I do enjoy the feeling of being tired, from physical work. I've been wondering if that is related to endorphins, as it doesn't really feel the same. Sometimes I wonder if some of the sedentary people that I know experience that tired feeling much differently from how I experience it.