Many a debate on forum and blog has weighed the imponderables of the situation.
Above is my current nerfing of the classic Sleep. I've waffled back and forth, at times going for the "up to 4 HD but save" approach, which makes it less all-powerful at low levels but preserves its utility well into the mid-levels. Perhaps I did the wrong thing - my 4th level party, finding it useless, traded it away at the Faerie Market for that divinational spell of most uncertain application, the Appeal to the Nine Worthy Elders (more on this later).
Probably the biggest balancing factor I applied was to make it only affect creatures that, you know, biologically sleep. This takes away its applicability to a lot of low-level pests, leaving only soldier-types, rats, and the miscellaneous animal.
The voluntary application is my attempt to make it useful for even high level parties. Restful sleep implies a higher healing rate, but also a deeper sleep from which to be awoken in case things happen in the night. You'll sleep as deeply as if you were in a safe inn, even camping in the dungeon.
Anyway, in spite of my doubts about Sleep, I'm on firmer ground with the blue spell, combining Shield, Floating Disc, and Hold Portal capabilities. Loading all three limited-use spells into one slot seems like a winning move to me.