Sleep AI processing when nobody on their Zlevel

code/modules/ai/ai_holder.dm

@@ -318,9 +318,11 @@
 		if(STANCE_MOVE)
 			if(hostile && find_target()) // This will switch its stance.
 				ai_log("handle_stance_strategical() : STANCE_MOVE, found target and was inturrupted.", AI_LOG_TRACE)
+				return
 		if(STANCE_FOLLOW)
 			if(hostile && find_target()) // This will switch its stance.
 				ai_log("handle_stance_strategical() : STANCE_FOLLOW, found target and was inturrupted.", AI_LOG_TRACE)
+				return
 			else if(leader)
 				ai_log("handle_stance_strategical() : STANCE_FOLLOW, going to calculate_path([leader]).", AI_LOG_TRACE)
 				calculate_path(leader)
@@ -359,5 +361,6 @@
 	if(ai_holder)
 		ai_holder.receive_taunt(taunter, force_target_switch)
 
+#undef AI_NO_PROCESS
 #undef AI_PROCESSING
 #undef AI_FASTPROCESSING

code/modules/ai/ai_holder_disabled.dm

@@ -6,7 +6,7 @@
 // If our holder is able to do anything.
 /datum/ai_holder/proc/can_act()
 	if(!holder) // Holder missing.
-		manage_processing(AI_NO_PROCESS)
+		manage_processing(0)
 		return FALSE
 	if(holder.stat) // Dead or unconscious.
 		ai_log("can_act() : Stat was non-zero ([holder.stat]).", AI_LOG_TRACE)

code/modules/mob/living/simple_mob/simple_mob.dm

@@ -156,6 +156,9 @@
 	// contained in a cage
 	var/in_stasis = 0
 
+	// don't process me if there's nobody around to see it
+	low_priority = TRUE
+
 /mob/living/simple_mob/Initialize()
 	verbs -= /mob/verb/observe
 	health = maxHealth

code/modules/mob/mob.dm

@@ -1200,6 +1200,16 @@ mob/proc/yank_out_object()
 		else
 			registered_z = null
 
+GLOBAL_LIST_EMPTY(living_players_by_zlevel)
+/mob/living/update_client_z(new_z)
+	var/precall_reg_z = registered_z
+	. = ..() // will update registered_z if necessary
+	if(precall_reg_z != registered_z) // parent did work, let's do work too
+		if(precall_reg_z)
+			GLOB.living_players_by_zlevel[precall_reg_z] -= src
+		if(registered_z)
+			GLOB.living_players_by_zlevel[registered_z] += src
+
 /mob/onTransitZ(old_z, new_z)
 	..()
 	update_client_z(new_z)