one pc stays on 24/7 because its hosting gameservers.
my gameing pc is on pretty much 24/7 for downloading.
other pc in the house usually gets turned off at night.
and, a computer wont overheat if being left on overnight. 'overnight' is generally only 5-8 hours. so if you think 5-8 hours of being on will make it overheat, then you have some cooling problems. theres billions of computers all over the world that running 24/7 without problems. if you have the pc on all day and theres no problems then you could easily have it on all night without problems. day is generally around 16 hours compaired to 8 hours of night.