nook's battery sucks

nook's battery is advertised to be able to run for around 10 days if users trun off the wifi. But my nook can only run for 4 days at maximum, even if I always turn air mode on. On average, I used it for two or three hours per day. That's to say, its battery can only support 12 hours normal usage! It seems the culprit is even in sleep mode, the nook still consumes a lot of battery. In this review for nook, it's said nook consumes battery while turned off !! Ridiculous!