An alternative. Comments/criticism/improvements VERY welcome!
I think, in Fate, I'd be tempted to rule that a mental-attack "Sleep Spell" just needs 2 shifts' success to work. It works by applying the Aspect "Asleep" to the target. Now the target is Asleep. But they are NOT "Taken Out." As a physical attack that fatigues the target into exhausted sleep, I can see the "Taken Out" result, but I dislike the mechanical consequences of being Taken Out this way: I prefer "Aspect:Asleep" both mechanically and narratively.
They are Asleep. If they were standing up, they fall down. This certainly wakes them up! They may have a round of confusion / disorientation... after all, they remember being upright & in the middle of a conflict! Now they're on the ground, feeling sleepy... WTF??? (A mundane might take this to the doctor the next day: "Doc, I think I have narcolepsy! I fell asleep in the middle of a firefight...")
They are Asleep. If they were seated, or prone, they won't "automatically" wake up... it'd take loud noises, being physically jostled, etc. It's likely that a round of whatever Conflict is happening, would wake them... conflicts tend to be noisy, and physically rough. Obviously, anyone can just wake them up intentionally, saying their name loudly/nearby, or nudging them awake.
They are Asleep. If it's quiet and still, I'd rule they keep the Aspect until the Scene ends, or 2 hours, or sunrise (whichever comes first), unless they separately had some other Aspect that might incline them to longer/shorter sleep. Yes, that result IS much like a Taken Out, and I'm OK with that. Also, if they themselves have an aspect such as "Sleeps Like a Cat" I'd give them some extra chances to waken.
As I said up-front: I really would like some comments/criticisms. I know this isn't a "traditional" rendition of the spell.