Default What is Android doing while the Nook is sleeping?

My HD is running 10.1.3 stable. Yesterday I charged it fully, used it very little and put it to sleep with a battery charge well over 90%. This morning the charge was at 68% and when I looked at what had used the battery during the 12 hours since it was charged, Android OS used 53%. The next 3 users were tablet idle (12%), wifi and screen at 11% each.

So what was Android doing during all that idle time? FWIW, no updates occurred during that idle time and I got 4 notifications.