[Psyke]

it's more or less keeping the RAM active, so as not to have to load the entire OS or book you might happen to be reading back into it. When you turn the device off, it flushes the memory and records anything it needs to the flash storage unit (ie - bookmarks). I also believe that if the wireless is left on, it's also on in sleep mode, but not 100% sure on that one.

Another thing to keep in mind is if you have magazine/newspaper subscriptions; if the system is set to auto check for new content, it will wake itself up to do so, and then go back to sleep. I think it's only supposed to do it once a day though, unless it can't make a connection, then it checks once an hour (need confirmation on this...I usually keep it off since the bug was reported in october, even though it was since fixed).