She likes to walk away from her computer for as long as 24 hours (sometimes more), and the additional energy cost, wear and tear on the hard drive as well as other issues make this bug unacceptable for long.
On my system I can deal with the frustration, but on my wife's system - let's just say she is not a happy camper. I boot the system up, head for a cup of coffee and when I return the system has already gone into sleep mode. I do not consider this a fix, and it is a pain to deal with if I happen to need to step away for more than 3 minutes.
If I do anything that keeps the system awake for longer than 5 minutes there will be no automatic sleep mode. This is just for testing purposes, meaning that the system is restarted, allowed to boot up and no other actions are taken (hands off the keyboard, no software or apps opened up, etc.). At five minutes or above the system stays 'awake' indefinitely unless put to sleep manually. If I set the computer to go to sleep in less than 5 minutes it will do so. I'm having the same issue but may have found a temporary workaround or at least a bit of information that can be used for further troubleshooting.