diff --git a/mesecons/internal.lua b/mesecons/internal.lua
index 8b82fa1..1bc0666 100644
--- a/mesecons/internal.lua
+++ b/mesecons/internal.lua
@@ -368,6 +368,48 @@ function mesecon.is_power_off(pos, rulename)
 	return false
 end
 
+-- This is the callback for swap_node_force in turnon and turnoff. It determines
+-- whether a conductor node necessitates a lighting update.
+local cached_update_light = {}
+local function get_update_light_conductor(pos, name)
+	local update_light = cached_update_light[name]
+
+	if update_light == nil then
+		-- Calculate and cache whether the conductor needs light updates.
+
+		local def = minetest.registered_nodes[name]
+
+		local conductor = mesecon.get_conductor(name)
+		local other_states
+		if conductor.onstate then
+			other_states = {conductor.onstate}
+		elseif conductor.offstate then
+			other_states = {conductor.offstate}
+		else
+			other_states = conductor.states
+		end
+
+		update_light = false
+		for _, other_state in ipairs(other_states) do
+			local other_def = minetest.registered_nodes[other_state]
+			if (def.paramtype == "light") ~= (other_def.paramtype == "light")
+			or def.sunlight_propagates ~= other_def.sunlight_propagates
+			or def.light_source ~= other_def.light_source then
+				-- The light characteristics change depending on the state.
+				update_light = true
+				break
+			end
+		end
+
+		cached_update_light[name] = update_light
+		for _, other_state in ipairs(other_states) do
+			cached_update_light[other_state] = update_light
+		end
+	end
+
+	return update_light
+end
+
 -- Turn off an equipotential section starting at `pos`, which outputs in the direction of `link`.
 -- Breadth-first search. Map is abstracted away in a voxelmanip.
 -- Follow all all conductor paths replacing conductors that were already
@@ -398,7 +440,7 @@ function mesecon.turnon(pos, link)
 					end
 				end
 
-				mesecon.swap_node_force(f.pos, mesecon.get_conductor_on(node, f.link))
+				mesecon.swap_node_force(f.pos, mesecon.get_conductor_on(node, f.link), get_update_light_conductor)
 			end
 
 			-- Only conductors with flat rules can be reliably skipped later
@@ -470,7 +512,7 @@ function mesecon.turnoff(pos, link)
 					end
 				end
 
-				mesecon.swap_node_force(f.pos, mesecon.get_conductor_off(node, f.link))
+				mesecon.swap_node_force(f.pos, mesecon.get_conductor_off(node, f.link), get_update_light_conductor)
 			end
 
 			-- Only conductors with flat rules can be reliably skipped later
diff --git a/mesecons/util.lua b/mesecons/util.lua
index 234775b..7fb95cc 100644
--- a/mesecons/util.lua
+++ b/mesecons/util.lua
@@ -343,7 +343,7 @@ function mesecon.vm_commit()
 		if tbl.dirty then
 			local vm = tbl.vm
 			vm:set_data(tbl.data)
-			vm:write_to_map()
+			vm:write_to_map(tbl.update_light)
 			vm:update_map()
 		end
 	end
@@ -364,7 +364,7 @@ local function vm_get_or_create_entry(pos)
 		local vm = minetest.get_voxel_manip(pos, pos)
 		local min_pos, max_pos = vm:get_emerged_area()
 		local va = VoxelArea:new{MinEdge = min_pos, MaxEdge = max_pos}
-		tbl = {vm = vm, va = va, data = vm:get_data(), param1 = vm:get_light_data(), param2 = vm:get_param2_data(), dirty = false}
+		tbl = {vm = vm, va = va, data = vm:get_data(), param1 = vm:get_light_data(), param2 = vm:get_param2_data(), dirty = false, update_light = false}
 		vm_cache[hash] = tbl
 	end
 	return tbl
@@ -388,8 +388,11 @@ end
 -- Sets a node’s name during a VoxelManipulator-based transaction.
 --
 -- Existing param1, param2, and metadata are left alone.
-function mesecon.vm_swap_node(pos, name)
+--
+-- See mesecon.swap_node_force for documentation about get_update_light.
+function mesecon.vm_swap_node(pos, name, get_update_light)
 	local tbl = vm_get_or_create_entry(pos)
+	tbl.update_light = tbl.update_light or (get_update_light == nil or get_update_light(pos, name))
 	local index = tbl.va:indexp(pos)
 	tbl.data[index] = minetest.get_content_id(name)
 	tbl.dirty = true
@@ -423,13 +426,15 @@ end
 -- Outside a VM transaction, if the mapblock is not loaded, it is pulled into
 -- the server’s main map data cache and then accessed from there.
 --
--- Inside a VM transaction, the transaction’s VM cache is used.
+-- Inside a VM transaction, the transaction’s VM cache is used. If a third
+-- argument is supplied, it may be called. If it returns false, the swap does
+-- not necessitate a lighting update.
 --
 -- This function can only be used to change the node’s name, not its parameters
 -- or metadata.
-function mesecon.swap_node_force(pos, name)
+function mesecon.swap_node_force(pos, name, get_update_light)
 	if vm_cache then
-		return mesecon.vm_swap_node(pos, name)
+		return mesecon.vm_swap_node(pos, name, get_update_light)
 	else
 		-- This serves to both ensure the mapblock is loaded and also hand us
 		-- the old node table so we can preserve param2.