I don't think this is necessarily a fix to sleep, but instead changes it to something else. The power itself is not meant for damage but instead to control outright. Though I don't know what else would be need, if anything is needed to change it at all.So change Sleep to something like:
Hit: 1d8 + Int psychic and ongoing 10 psychic and slowed (save ends both)
Miss: Half damage, ongoing 5 psychic, and slowed (save ends both)
Special: Any creature reduced to 0 hp by this spell falls unconscious, instead of dying.
In a very real way, the old Wizard "big guns" were all about bypassing hit points. 4e has done a pretty good job of not letting anything bypass HP, so Sleep is a big anomaly.I dont know hitpoints simulate energy and luck... suppressing effective hitpoints might be another way to do it, and I would like it to trash somebodies temporary hitpoints.
IMHO Flaming Sphere is the correct power level for Wizard Daily powers. Wizards do need other options at 1st level, but not because Flaming Sphere is too good: rather because the other options are too weak.
Cheers, -- N

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.