Or, you could do it as a maneuver to apply "Asleep" as an Aspect.
This is somewhat easier to pull off
Complexity equals targets Conviction (+5 if you want to be certain he fails the resist) X the number of times you want to be able to tag(invoke it for free) the sleep aspect.
The downside here is that after you run out of tags, you will have to give the target a Fate point everytime he tries to act, in order to invoke the asleep aspect.
you can also do it as a mild consequence
Complexity equals targets Conviction (again with the optional +5), + his mental stress track +2 for the mild Consequence
In this option, you still need to keep invoking the Consequence to keep him down, but the consequence will last beyond the scene.
To full on put him to sleep, so you don't have to worry about him at all, you need to take him out.
Complexity equals targets Conviction (ditto the +5) + his mental stress track+20 to bypass all his Consequence levels (Mild, Moderate, Severe, Extreme) and thats if he doesn't have any extras from anything, +1 to take him out.
Then you've taken him out of the scene with the descriptor that hes asleep, and don't have to worry about him anymore... But a Complexity of 26+targets Conviction, is pretty steep just to put someone to sleep. I'd pay the fate points.