Once they go to bed, and you exit the game, the game "takes over" and they will rest

If you check on them, they "think" that they need to be out of bed so that you can play with them. I always stock up on food for them before I go to bed, then I simply exit the game. When I check on them in the morning (usually 8 hours or so later - depending on how much sleep *I* get ) they are well rested and ready for a new day