Here is what happens... Say I plug it in so no sleep occurs (energy saver settings to no sleep when on power cord), and leave the computer on overnight (screen saver on or just sleeping screen, doesn't matter) perhaps to run a task or not. What happens in the morning is that the computer tried to shut down, reboot, log out, or whatever because I have all kinds of warnings and stuff indicating that possible the computer wanted to shut down, but couldn't because of some open program. If no program was open when I left it overnight, then I would get the login screen as though I restarted the computer. Well, this of course causes major problems if you are trying to run something like a DOD swipe of a hard drive that takes 24 hours or anything that takes more than a couple hours. Technical support (I have only had this 2009 model MB Pro for a month) was helpful and checked permissions upon booting (not after booted and this makes a difference although it didn't fix the problem). I ended up having to backup everything (which I already had from Time Machine) and reinstall Mac OS X as an erase and install. This fixed it. I had all kinds of permission problems that were not being fixed by Disk Utility and that was probably caused by some cleaner programs like MacCleanse, etc... that really do a good job swiping, but perhaps too good of a job. This is just a guess. I fortunately was able to skip installing a bunch of programs I don't use (e.g. iPhoto, etc...) this time and have a bit leaner machine. We are good now, but wanted to share this that if your machine is arbitrarily trying to restart after sitting for a spell (assuming you are on the cord power), then you likely will have to erase and install.