Think of it this way, you are "on" so much more than you are "off" - you are on "full alert" at a cellular and neurobiochemical level. Any system that remains in the "on" position eventually burns out. Unlike a lightbulb, at least living organisms have the ability to regenerate to an extent (death, of course, is the outer limit of that ability). If you can "get away from it" in your mind and relax, you may find some relief for your weary state. But, obviously, that ability to relax will be hard to find when you're in the thick of it.