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I know this is an energy saving option, but I can't seem to find it.

I'm working from a PC right now, so I might not be 100% correct.

Click on the battery timer, go down to energy saver, then ajust the two sliders at the bottom to your preferences.
 
I believe it is the "automatically reduce the brightness of the display before display sleep" option under energy saver --> under the options tab.
 
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