With all of the Windows versions I've had its never rebooted even after hours in sleep mode.
I had a hard drive crash a few months ago after using Windows 7 for 6 months.
Its probably since I reinstalled Windows on the new hard drive that the trouble started.
So its been since then.
I've changed the power settings to display after 30 minutes and to sleep after 1 hour
Its not a problem to wait if it reboots, its just the fact WHY doe it happen?
Is your battery low when this happens? I know that when the battery hits a criticaly low level, Windows can shut itself down to prevent data corruption.