I keep my computers on 24/7 other than upgrades and patches. I just retired an 11 year old P4 2Ghz Williamette system this past week after the motherboard (Asus P4S333) finally died. The PATA hard drives (from 2000, 2002,& 2004), soundblaster sblive, PSU, and other hardware from that computer still works great, although it's hard to find a new motherboard with PCI slots and PATA. Not that it matters, but I don't plan on reusing these parts as they are old and slow compared to new hardware.
Anyhow, I don't think leaving it on overnight will hurt it.