waking from "sleep" problem

mid 2012 non retina MBP. over the past year i've noticed a change in my MBPs behavior when waking from long sleep. after a few hours, any activity will wake up the screen and computer as normal, but if it sits overnight or an extending period of time, when i press a button i hear the hard drive spinning (like when its initially turned on) and i get a grey screen with bars on the bottom that signify time elapsing. the login screen pops up but is frozen for about a minute, before it registers key strokes or mouse movement. I've tried changing the sleep settings, and not having "put hard disk to sleep when possible" checked, and reset the NVRAM and SMC. in terminal it showed no changes to the sleep mode for my MBP. wondering what's causing this and why/how to keep it from happening.