A few OS X releases ago, there was a checkbox in the Energy Saver settings for "Automatically reduce brightness before the display goes to sleep". This doesn't appear to be there any more, and hasn't been since 10.8 afaik.
Since I updated the family laptops to Yosemite, my laptop (mid-2010 MBP) still dims a few seconds before going to sleep, but my wife's (late-2009 MacBook) no longer does, and she wants to re-enable that behaviour. But I've no idea how.
Any suggestions? I've searched Secrets for a hidden preference, but I don't see one.