I think Npc.isSleeping() is made for the Sleep of the animals in the game.
In the Java Version, there was something like "Range", thus, the vision should be determined width, at least for 3D objects.
Probably it will repel better the kidney with the sight to be Combined.
But a Stale isAktive True if the Npc is in the point of view is the area of a player.
But currently, you can maybe have the World.getAllNpcsInRange(...) the number of limit something, maybe a Player could filter out the direction of view to the rear.
Code
Man could also display a Timer in the Npc.Attributes Store the Checks Slowly as far as the players Wake up and turn Off when the Player is far enough away. With the Lambda loop, it is pretty good.