There should be something for regiments to retreat to at night because when the day night cycle was shown it got pitch black and Generals could place encampments behind the frontlines for the regiments to retreat to at night. But the Generals can only place them far back behind the front lines. This would be historically accurate because at night the north and south would call seize fire but that still doesn't mean that you cant go out at night but it would be a good time to resupply at a camp at night.