I understand that Riot hasn't put in a sleep status effect yet due to its "Burden of Knowledge" mechanic on how it can be broken if your target takes damage. If it were designed in the same way it's made in Warcraft 3, whereas it breaks if your target takes ANY damage, then sure, it wouldn't fit into LoL.
But I have a possible solution.
What if a sleeping target had to take a certain amount of damage before sleep were to break?
For example, my concept for Rydia, the Dream Sylph, has a sleep spell, Fairy Lullaby, which just puts your target to sleep for 1/1.5/2/2.5/3 seconds (it's a hard disable like Fiddlesticks' Terrify). If her target takes 100/150/200/250/300 (+0.5AP) (these are just example numbers) damage while sleeping, the sleep effect breaks.
So you see, it wouldn't be just one damage tick to break the effect; it'd be a certain amount of accumulated damage, which could also be raised by AP if needed. This would counterbalance its vulnerability.
Anything to comment on this, Morello?