It looks like what you are experiencing is the "sleep of death". It happens when the Nook Color attempts to go into deep sleep while wifi is running. My understanding of the problem is that it involves some sort of timing conflict. It is a known CM7 issue. The best workaround that I have seen is to simply turn wifi off if you are going to let it go to sleep. Wifi does use some additional battery life anyway so I only have it on when I actually need it. Most of my Nook use is reading and playing some games while commuting. An additional benefit is the adds in many free games don't function with wifi off.