i havent seen this yet, but i think its a fun idea. if the npcs sleep then it will provide the player with a challenge, to hold the gralats until the npc is awake. or maybe make a way, that, if you do something to the npc long enough, s/he will wake up. if the npc wakes up the prices would be higher because the npc is angry.
just some ideas i came up with as i was writing this reply