Eh, I dunno if this applies to anyone else, but when I'm dead tired, a beating is practically the last thing I'd enjoy - I'd much prefer the cozy comfort of my sheets, and a lot of comfort food.In fact, anything that involves exertion or pain is the last thing on my list... which rather narrows the scope of BDSM.
Then again, I suppose that if I'm that tired everyday, I really should do something about my job and stuff instead. Le sigh.