This is something that has bugged me since 3.0 and especially in 3.5. Sleep spells have an experation date and aren't even guarenteed to work at level 1 if your DM throws something kind of beefy. I figured this might be a way to keep useful:
Sleep- Effects 4 HD (I'd personally like 5 as thats the statistical average of the original 2d4) of creatures + 1HD/2 Levels.
Deeper Sleep- 8 HD (10 HD) +1/Level.