Warning LONG STORY!
I used to do tech support for University of Michigan and we sent out emails telling everyone to leave their computers on at all times except to reboot occasionally. We do updates overnight so this is very important.
Well, some total envirnment guy freaked out and basically told us that we are destroying the world by doing this. So someone did a study....
In the college we had several thousand computers. The two options were:
1. powersave the monitors at night and spin down the hard drives or
2. powering off the computers
Option 1 only cost the University an extra few thousand dollars A YEAR.
A computer running overnight on minimal power (ie monitor powersaved and disks powersaved) runs on very minimal electricity.
So, go to your control panel and set your powersave options and don't worry about turning it off at night
