Having a black market dealer come out at night would be great for shopping and atmosphere, but in terms of NPC schedules not getting in the way of what you want to do, there could also be some way to quickly pass the time (magic, not walking to the nearest inn and renting a room to sleep for a certain number of hours). Of course for some quests it could makes sense an NPC would be fine if you tracked them down at home, but if you wake someone up in the middle of the night to finish off something relatively minor, any rewards you receive (gold, reputation, etc) should realistically be reduced.
