Implementing a day/night cycle is not that difficult. They just need the following things:
1. All NPCs will have two places where they can be found. One during the day and one at night. Most of them will be sleeping in their homes/beds/sleeping bags at night, but you can talk to them (maybe some may complain, but talk to you in the same way).
2. Entrances to some places/buildings will be closed at night.
3. For cosmetic matters, the days will be divided into three cycles: day (a), evening/sunset (b), and night (c). When you click on the short rest button you go from one to the other. After some time playing (something like 30 min) the time jumps from one of these cycles to the other (a to b, b to c, c to a).
4. If the characters spend a day without resting, they need to go camping to sleep, or they will win two Exhaustion levels (PHB, p. 291), per day.
5. When sleeping in the camp the time skips two cycles. Example: from day (a) to night (c).
