most probably when u have to leave yr comp on while u r away is when u are waiting for it to complete some task or u r running a server or downloading/ uploading stuff.
generally even if u leave an OEM computer on for hours it will not overheat unless there is some problem with yr house circuitry...fear that if not one is attending to it when it causes a fire u will not be there to react or put it off on time.
disable screen saver if u have to. go to "power options" in "control panel" and make sure u select "never" to "hdd", "power standby" etc. since u r expecting hours of inactivity. u can switch off yr monitor to save power.
no worries if you are not connected to the internet while leaving yr computer on. but if it is connected online while u are sleeping or away, weird things can happen depending on what actions yr computer shall be performing.
otherwose u just have to take precaution against other users in the family or yr pets.
happy computing
