The decision to turn on or off is based on the services that the server is offering. If this device is doing dhcp, vpn, or web, then turning it off during the overnight hours prevents users from interactive with it. However, please note that the mini is a real power saver:
Compare this: http://support.apple.com/kb/HT3468
to this: http://support.apple.com/kb/HT3513
The mini sips power in comparrison to other products. As strange as this may sound, that WD extrernal drive will likely draw more power and generate more heat than the mini.