It messes with your schedule and you don't get your needed recharge time. Makes complete sense.
I have one full-time job and one part-time job. (Unfortunately the pay at the FT job wasn't enough.) I have found that I have a very distinct thresh hold and that if I work any more than that that I will be exhausted and a stressed out mess because I need to be able to recharge before going back and trying to be interactive again. In a society where people tell me on a regular basis, "But I work 50+ hours all of the time..." and can't see why I can't handle that, it is hard to draw lines and set limits for myself for my own benefit. It's necessary though.