That's a surprising problem. I can't think of what might be causing it - have you checked that nobody physically turns it off every night?
As a workaround, until someone can come up with a fix, you could try setting it up with 'wakeonlan', so that at least you don't need to go out to it every morning. To do this, set up the BIOS to 'wake on LAN', according to the instructions, make a note of the computer's MAC address, then on your own computer, assuming that the wakeonlan app is installed, type:
and it should wake up.
Sorry I couldn't be any more help than this, I know it's not a fix, but at least it should make the problem easier to deal with.