Many companies have desktops that are on 24/7, but are not likely to be used between, say, 6:00 at night and 7:00 in the morning. How can we schedule them to sleep, wake up before the user is likely to arrive at work, and still install those midnight updates?
Assuming you have Windows desktops, you can use GPO to set an automatic shut down after a certain time of inactivity - you don't want to automatically shut down someones computer while they are working late.
Ensure corporate software has some enforced auto-save setting so people who have had to run out without shutting down and forgetting to save don't lose work.
Make sure you know which users use non-standard software that can't have such a setting enabled, and either exclude their desktops from the rule, or clearly warn them to save before leaving work.
That takes care of shutting down and saving power.
You can then use Wake On LAN within something like SCCM to wake up computers and install updates at a scheduled time.
I'm not sure how this works with things like hibernation or standby.