I'd say since the Sleep spell can be broken at once--which happens on the turn of the Hero who cast the spell--then the breaking of a spell by a monster shouldn't be counted as an action (at least not in this case). Breaking a spell is instead a reaction, like defending against an attack (which also happens on a Hero's turn). Other forms of resisting spell effects, such as defending against a Genie's attack or rolling red dice to escape damage from a Fire spell, are also examples of immediate responses by a monster on a Hero's turn.
The two monster actions listed on p. 21 of the NA Instruction Booklet (ATTACK and CAST A CHAOS SPELL) only happen on a monster's turn, and so they may be combined with a reaction that occured on a Hero's turn. I'd say that much is clear.
Can a monster reaction also combine with a monster action on Zargon's turn? Yes is probably the best answer. Why? Consider a lucky Fimir that survives a Firestorm Chaos spell cast on Zargon's turn by a Chaos Sorcerer. It basically reacts as if it resisted a Hero's Fire spell. Going by the text on the spell card in either case (a Firestorm or Hero Fire spell), if the Fimir hasn't yet attacked it isn't prevented from attacking that turn. Thus, a monster may react and take an action in the same turn. I'd maintain that breaking a Sleep spell is yet another form of a reaction as demonstrated above, so a monster should by extension also be allowed to break a Sleep spell and take an action in the same turn.
