Help, how to make an entity that sleeps at night?

...Maybe? It depends on if you just want it to sleep like foxes, or use a bed like villagers.

Either way, you could probably use some variety of the 'make entity pathfind to x/y/z' function, and a 'search for block in 6*6*6 box' procedure template to search for a bed nearby and force the entity to pathfind to it, then transform the entity into a sleeping variant of itself by spawning in a new variant and despawning the original entity. This isn't a great solution, and will likely be a bit janky, (anything involving pathfinding will be janky), but it might be worth a try. I don't think you'd need custom code for this, though it might help.