On a test machine, if you set the sleep timeout, what happens when the machine reaches that timeout? Does it wake right back up? Does it go to the lock screen?
I've had hardware in the past that would prevent the machine from sleeping. In my case it was a batch of Logitech keyboards that had some media controls on them. When the machine would attempt to go to sleep, there would be an event log error saying "Device x prevented the machine from entering sleep".
You can set the machines to wake up at a certain time for updates if you want using the task manager. http://www.howtogeek.com/119028/how-to-make-your-pc-wake-from-sleep-automatically/ is an example.
I know this is going to sound "anti-green" but if you need to update machines in the middle of the night, then leaving them on is likely worth the cost of electricity. If you have to perform updates during the day, you lose that productivity time for your users and it likely costs you more than you save. OR schedule updates for one night a week and have your users shut down ever night before they leave except for x day. Or, setup WSUS to deploy updates and wait for the user to reboot/shutdown. Then you don't have to leave them on overnight as updates will be applied on shutdown. (Just make sure you pre-flight those updates, nothing worse than coming in Monday morning and having everything broken cause MS was super helpful and updated a network driver and broke everything).