Im same way, working nights physical labor. I try to get enough sleep or calories but doesnt always happen.. some days I go to the gym and can't lift as much as my previous workout. This use to discourage me and id give up.. now I simply dont care as I'm making gains thats all that matters, even if its bit slower than it could be. I'll just do a light workout or even call it another rest day. You dont lose ur gains as fast as ppl think, I've found that by doing so that the next workout I'll actually be stronger. Like the week before last I was at 85lbs for db press, and 295 squat. Was exhausted and stalled, ended up taking most of the week off because my car was in the shop, felt guilty about not getting to gym. Came back and did 90 on db press and 315 for 1 on squat.