If you have any scheduled task that ran that may have kick the system back on. Hibernation is really for Lap tops to save on the battery. If you are running a desktop and do not like the appearance of the system being on all of the time you can just shut down the monitor after a time period that you select. With todays monitors a screen saver is really over kill. I like some of the screen savers so that is what I use. I had mine set up for monitor shut down after an hour which was kind of neat but served no real purpose that I could tell.