I tend to agree with fred_yu_job. What purpose does the 'Sleep' mode really serve if the Kobo will turn off in 15 minutes.
If I know I'm coming back to the Kobo within 15 minutes I'd just leave it on. If I'm not back in 15 minutes go to sleep Kobo - that would make sense. The 'Sleep' mode seems completely pointless to me. Turning off should be a user option not a default, preferably with a user set time-out.
I miss the cover picture - it was what made the Kobo a Kobo - please bring it back.