Did you not use oozes/slimes, plants, undead or animated objects?
As the key encounter in a level 1-3 adventure? No, not ussually. 99% of those adventures have goblins, kobolds, orcs and other similar creatures as key encounter (the key word, is key). Few published adventures (to use as a reference) have the key encounter with plants or oozes, and those with undead, often have a necromancer of some sort leading them (which is vulnerable to sleep).
Even if it "only" dominates the non-plant, ooze and undead encounters, that's like 85% of the CR 1-3