I've had my Nook STR since Tuesday and notice that after I charge up that 24 hours later it has gone from 100% charged to 81% in sleep mode without reading on it at all.
When I had a Nook Classic, I found that the battery lasted much longer if I turned it off between readings. Hate to think they didn't do something to improve battery life in sleep mode when they came out with the Nook STR.
I will follow some of the advice here and let the charge go down much further before re-charging. I normally hate to have a charge on my Nook or Kindle go below half.