First off, don't forget that there's a Sleep spell in the main book that uses Thaumaturgy (YS 293), which would make a good baseline.

Apart from that, it looks about right. I'm not sure you'd want to have the Aspect be "Terribly Sleepy", since invoking that wouldn't actually put them to sleep. Invoking an Aspect that implies something will happen when invoked (such as say, "Off Balance") doesn't make that happen; it just makes it easier for someone to make it happen. Invoking "Off-Balance" won't automatically knock someone over, you'd still have to roll Fists or Athletics or whatever to actually knock them over; invoking the Aspect would get you the +2/reroll making it much more likely.

It might be better to have the Aspect be "Light Sleeper" or "Catnapping", since that puts them out straight out (which is what you you want), but still lets people invoke to get a bonus to sneaking or whatnot, but at the same time implies that the target can be woken up easily.

Sleep ignores undead creatures as well as creatures immune to being charmed

Quotes from Sleep, PHB p.276:

Creatures within 20 feet of a point you choose within range are affected in ascending order of their current hit points (ignoring unconscious creatures).

[...]

Undead and creatures immune to being charmed aren't affected by this spell.

Sleep states that creatures within the range are affected, but also states undead aren't affected. Specific beats general so you ignore the undead.

Mike Mearls supports this interpretation:

Radim Havlíček @Tavicz
@mikemearls Spell sleep: If the creature with the lowest current HPs has imunity to sleep, are its HPs subtracted from the total?
_{12:30 AM - 24 Mar 2015}

Mike Mearls @mikemearls
@Tavicz no, just skip it and move to the next
_{5:20 PM - 25 Mar 2015}