commented

code/modules/mob/living/carbon/human/species/shadow.dm

@@ -42,7 +42,7 @@
 		H.set_sight(new /datum/vision_override/nightvision)
 		to_chat(H, "<span class='notice'>You adjust your vision to pierce the darkness.</span>")
 	else
-		H.set_sight()
+		H.set_sight(null)
 		to_chat(H, "<span class='notice'>You adjust your vision to recognize the shadows.</span>")
 
 /datum/species/shadow/on_species_gain(mob/living/carbon/human/H)

code/modules/mob/mob.dm

@@ -1357,6 +1357,8 @@ GLOBAL_LIST_INIT(slot_equipment_priority, list( \
 	sync_lighting_plane_alpha()
 
 /mob/proc/set_sight(datum/vision_override/O)
+	if(vision_type)
+		qdel(vision_type) //qdel the old vision_type unless it is null.
 	vision_type = O
 	update_sight()