hmmm my first thought is, I would be more concerned of hazards etc whilst im tucked up in bed rather than whilst I am out
If its on all night then the cost of running it durring the day is going to be negligable.
Alternativly if hibernate works well on the system, then you can get software that will bring it out of hibernate at a set time.
Wake-on-LAN only works when a specific command is sent to the network card so this could still be an option, althouugh something i have never had to deal with.
interesting question mind..