I always feel better after riding, so I'm going to assume it's the endorphin thing. My wife says the same thing, although she doesn't ride as often or as far as I do. I always sleep better after riding, too. One thing I noticed that you didn't mention is that riding makes my chronic low back pain less noticeable - whether that's the endorphins or just the relaxation from the ride and being outdoors where I can clear my head, I don't know and I don't care - but I do enjoy the relief!