How do I get Time Machine to work during Power Nap I tested Power Nap on my MacBook Pro Retina (Aug 2012) and my Time Machine backup hasn't made any progress.  Even the hard drive light is out while the machine sleeps.

Energy saver is at the default setting of sleep after 15 minutes and Enable Power Nap is checked.

It just doesn't seem to work.  Are my expectations wrong?  Shouldn't it just back up while in "sleep" mode?  

This is a newly re-installed OS with no special apps installed and no custom software besides MS Office.

Update:  My Mac is plugged into a Thunderbolt monitor (power and TB).  The drive is chained off the monitor.  Backups don't occur if the drive is directly connected (no monitor) nor through the monitor.  The laptop is always plugged in.
According to the Apple support doc, using Time Machine to back up in Power Nap requires the laptop to be plugged in:


  When your portable Mac is connected to a power source, it also downloads software updates, makes backups with Time Machine, performs Spotlight indexing, and can continue Mac App Store downloads.  It also updates Help Center.