Sleep powers would be very viable if they did something like force enemies powers to set to a 'used state'. So like you sleep a crowd and it forces all their abilities into recharge, meaning they only have a delay of a couple seconds or so... in most cases. Where it would be super handy is combined with -recharge abilities, the pairing able to shut an enemy down quite fast, or sleeps used in succession to hold enemies in a state of inability to take action. My favorite use for this would be prevention of powers like moment of glory. If you opened with a sleep attack those annoying paragon protectors would have their moment of glory power placed on recharge, Illusionists decoys and copies on recharge when the fight starts, meaning it takes the ability out of the fight entirely. Like any change this could be insanely game breaking if many things were not taken into account.
The only other idea I personally have is if disorient and sleep were merged into a 'dazed' effect and become one in the same.