Hi Stwert, Yeah, I've had this happen too. It can cause problems with major downloading from the web as well if it goes to sleep in the middle of it.

I'm using windows 7x64 as well and the way to get round the problem is before you do an overnight render go to Control Panel > Power Options and on the left hand side pick Change When the Computer Sleeps in there you can set your sleep and display settings to never and it should fix the problem.