Looking at the sleep spells in YS again, I can't help but notice, that those are not actually sleep spells in the sense you want it to be for your evocations, Todjaeger. They are, in fact, blocks against nightmares, the sleeping part is only secondary to the whole thing and voluntary.
As many have said, it will probably come down to a taken out result. You'll want to use sleep spells on mooks with 2 stress boxes and no consequences, to take them out without killing. Everyone else doesn't even have to take a sleep themed consequence. You could take the typical "twisted ankle" and say you got dizzy for a moment when the spell hit and you tripped. Since metagaming is a regular part of the DFRPG, I don't think there is a problem for the players knowing if their characters are facing a mook or something tougher. They will probably know anyway once the first mook is taken out by looking at him too hard.