Yeah, Sleep was weird. It had a pretty modest effect initially, but then if you happened to get several creatures in it and they weren't lucky on their saves it could easily end the whole encounter in a single action, effectively. I always thought it was a much better candidate to be a ritual...
Oh, and handled as rituals are in HoML, that means it is a PLOT DEVICE, so you basically cast it and say something like "Oh, now I use my Arcana skill to get past the guards, they fall asleep!" hehe.
Even better, "I expend a Healing Surge and it is an auto-success (in the ongoing SC, which is the only time you actually roll dice in HoML, that and combat).