I've read different opinions (and personal stories) about whether the battery drains in sleep mode. All I know is that mine does not seem to drain. I was just noticing this last night. Before I went to bed my battery meter read 97%. I simply closed the smart cover to put it in sleep mode (I heard the click). When I opened the cover this morning it was still at 97%. I used it for about 15 minutes and only then did it drop to 96%. I did not turn off any wifi or location settings.
I should disclose that my battery meter was acting weird the other day. After a full charge, it was stuck on 100% during a few hours of light use (internet, email, kindle) before dropping again. Maybe my meter is just screwed up, but it seemed to be back to normal again and draining at a normal rate prior to my observation about it not draining at all during sleep mode.