Incidentally someone just resurrected one of my old threads about this same topic in the "Suggestion and Feedback" subforum and made a fairly good summary of how this could be approached to several degree of commitment.
Here's a quote of his post, not sure if he will bother coming posting here himself:
I'm not a game designer, but I believe this can be solved with degrees of complexity, implementing one step at a time.
Step 1 - Only Cosmetic Change: Place two instances, day and night, in the map, adjusting the position of the light for each one. Everything else remains the same.
Step 2 - The Basic: Set two positions for each NPC – one for the day (a) and one for the night (b). You can talk to them normally. Applies lighting-based stealth modifiers, with the ability to extinguish torches for better modifiers.
Step 3 - Extra Dialogues: NPCs approached at night, if they are sleeping, will speak a line of dialogue complaining, but after that, they speak normally with the characters. Sellers may refuse to sell items before dawn. Open doors at day can be closed at night.
Step 4 - Dangers of the Night: Some enemies and new creatures appear during the night (like worgs or skeletons).
Step 5 - Four Stages: Adds four stages of the day. Morning (a), Afternoon (b), Evening (c), and Night (d). NPCs can now have four different positions instead of just two (day and night).
Step 6 - Gradual Time Passage: Now just let time pass naturally, from one stage to another. NPCs will walk to their next location when the switch happening.
- When a player asks for time to pass (by pressing the short rest button) the other players must agree. Simple.