How to change "Automatically reduce brightness before the display goes to sleep" in Yosemite?
That preference slider seems to be missing in Energy Saver for some Mac portables, and newer Mac portables seem to combine both settings if "Power Nap" is enabled:
http://www.extensions.in.th/amitiae/2014/prefs/energy.html
http://forums.macrumors.com/threads/no-computer-sleep-setting-in-energy-saver-for-nmp.1692340/