I think that this is an interesting idea.

Three challenges come immediately to mind:

-- It's probably impractical logistically. Imagine all of the chaos in assigning seats and then processing all of the change requests at the gate.

-- It might create customer expectations that airlines will never be able to manage. (I can just see the threads on FT now: "I booked the sleeping section, but the guy next to me was talking, so I demand a refund, 100,000 miles, and 4 free upgrades for future flights as compensation.")

-- People might free-ride. Someone who wants to quietly work on his laptop might choose to sit with the sleepers -- good for him, perhaps a bother to the others.

At a minimum, I like the idea of a family section to keep all of the screaming kids together. I could see that being feasible (although various political-correctness issues might prove to be barriers).