I am almost positive I saw that spell somewhere. It's not in the SRD, though. I seem to recall it being a low level spell and I seem to recall thinking how horrible broken it was:
Functionally, Sleep is actually worse than Stun. You can't coup de grace someone who is stunned; you
can c.d.g. someone who is asleep.
So if you can put them to sleep, you might as well kill them. (To paraphrase Monte's thoughts on the subject from one of his design diaries a while back.)
So, if
Power Word: Blind is 7th level (blinded creatures can't defend but can act, and aren't helpless);
Power Word: Stun is 8th level (stunned creatures can't defend and can't act, but aren't helpless);
Power Word: Kill is 9th level, then...
Power Word: Sleep is probably also 9th level.
If you want someone dead, Power Word: Sleep is
almost as good as Power Word: Kill. I'd say that the utility you get from the option to take someone alive, or kill them, with a single spell, is offset by the fact that some creatures are immune to sleep, and that a single coup de grace isn't necessarily fatal (although it almost always is).
I'd have no problem substituting Power Word Sleep for Power Word Kill on a 1-to-1 basis.
EDIT: Found it. Power Word Sleep was a 2nd level arcane spell in Baldur's Gate 2. No idea where they pulled it from, though. It has the hallmarks of a poorly-thought-out WOTC supplement spell, though. (Sorry to say...)
And there's this:
http://www.enworld.org/showpost.php?p=168924&postcount=20