Originally Posted by pdragon

My only recommendation is to really look at what applications you've installed. If you're like most people that get a new gadget (me included) you go a little nuts trying new things, and with a new phone like this playing with new apps is one of those things. Check the settings of any apps that would do anything on any kind of schedule. Anything that checks for updates online, changes your phone settings at specific times or when things happen. Use programs like Advanced Task Killer and Taskiller to see which programs are actively running that aren't part of the core system.

What probably happened when the other person did the factory reset is it cleared all his apps, so whichever one was keeping the phone from sleeping was removed.