I like this and this is why: the person is asleep. Loud noise would wake them (block vs perception), any stress would wake them etc...
I'm not sure a sleep spell is an automatic take-out. I wouldn't think it would be ultra-useful in combat. It'd be great for sneaking past guards or other non-combat situations. It would have to be a VERY powerful sleep spell to make sure the person doesn't wake in the middle of everything going on in combat.
EDIT: YPU makes a good point regarding consequences as well
Part of the whole issue I've been running into with modelling the spell is that a player wants to have a sleep-type spell which they can cast in combat, specifically to put a target to sleep during combat.
This came up during an actual game, and as a bit of handwaving, I let the player do this, albeit it took the player several castings before they could put the target to sleep so that they player could capture the target. The other part of this is that I didn't have the target (who is a significant, recurring NPC) take any consequences to avoid falling asleep and then being captured.
Went I sat down afterwards to determine how effective sleep spells would be in the future, that's when it appeared that sleep-type spells wouldn't be particularly effective, especially in combat. A sleep spell cast as a Maneuver on a target, so that a night watchman can get a Temporary Aspect of Sleepy, or as a sleep-based Block on actions seems workable.
But future attempts to use Evocation to put a target to sleep in a fight doesn't seem workable, except possibly against mooks. I just wanted to double check to make sure I wasn't missing anything.
-Cheers