From my understanding, Undead are _magicaly_ brought back to life to varieing degrees, from instinctual awareness of the mindless zombie to the rebinding of a soul in a lich's phylactery. At the baseline of them all is a current of magic on which they survive, much as living creatures on oxygen. Supress that magical awareness, that compulsion imposed apon an inert corpse from some magical source, and it makes sense that the corpse will cease to be ambilitory, even if only short duration. Sleep might be better worded as Supress Awareness but then it loses its fantastic feel and starts drifting to the realms of sci-fi to me.
Much the same for the general "mind-effecting" field of magic. if you think about it, the necromancer who raises a skeleton is exerting his will through magic to compel the inanimate to move and act in accordance to his designs. even if left to its own actions after animation, its is the raiser's will, though magic, that allows the corpse to function untill destroyed or the magic ceases to effect it. Thus, it seems to follow suit that another could use magic in an attempt to overwrite or supress the magic compelling the corpse to function. Since it is a different type of magical compusion, should it be more difficult to overwrite the animating effect? That makes sense, but in the example of a 1st level necromancer's skeleton being happened apon by a 15th level enchanter, the latter's magic should unquestionaly by strong enough to superceed the formers.