diff --git a/modules/applications/swayidle.nix b/modules/applications/swayidle.nix
index 135507f..7477b3f 100644
--- a/modules/applications/swayidle.nix
+++ b/modules/applications/swayidle.nix
@@ -7,12 +7,57 @@
 }:
 let
   cfg = config.my.applications.swayidle;
-  noctaliaPkg = inputs.noctalia.packages.${pkgs.system}.default;
-  noctaliaExe = lib.getExe noctaliaPkg;
 
-  lockCmd = "${pkgs.bash}/bin/bash -lc '${noctaliaExe} ipc call lockScreen lock'";
-  lockAndSuspendCmd = "${pkgs.bash}/bin/bash -lc '${noctaliaExe} ipc call sessionMenu lockAndSuspend'";
-  dpmsOn = "${pkgs.niri}/bin/niri msg action power-on-monitors";
+  noctaliaPkg = inputs.noctalia.packages.${pkgs.system}.default;
+  noctalia = lib.getExe noctaliaPkg;
+
+  bash = lib.getExe pkgs.bash;
+  brightnessctl = lib.getExe pkgs.brightnessctl;
+  niri = lib.getExe pkgs.niri;
+  rm = "${pkgs.coreutils}/bin/rm";
+
+  brightnessState = "$XDG_RUNTIME_DIR/swayidle-brightness";
+
+  noctaliaCall = target: action: "${bash} -lc '${noctalia} ipc call ${target} ${action}'";
+
+  lockCmd = noctaliaCall "lockScreen" "lock";
+
+  # AC 接続中は何もしない。つまり battery-only の処理にする。
+  skipIfOnAC = ''
+    for supply in /sys/class/power_supply/*; do
+      if [ -f "$supply/type" ] \
+        && [ "$(< "$supply/type")" = "Mains" ] \
+        && [ -f "$supply/online" ] \
+        && [ "$(< "$supply/online")" = "1" ]; then
+        exit 0
+      fi
+    done
+  '';
+
+  dimScreen = pkgs.writeShellScript "swayidle-dim-screen" ''
+    ${skipIfOnAC}
+
+    ${brightnessctl} get > "${brightnessState}" 2>/dev/null || exit 0
+    ${brightnessctl} set 10% >/dev/null 2>&1 || true
+  '';
+
+  restoreBrightness = pkgs.writeShellScript "swayidle-restore-brightness" ''
+    if [ -f "${brightnessState}" ]; then
+      saved=$(< "${brightnessState}")
+      ${brightnessctl} set "$saved" >/dev/null 2>&1 || true
+      ${rm} -f "${brightnessState}"
+    fi
+  '';
+
+  lockAndSuspend = pkgs.writeShellScript "swayidle-lock-and-suspend" ''
+    ${skipIfOnAC}
+    ${noctaliaCall "sessionMenu" "lockAndSuspend"}
+  '';
+
+  afterResume = pkgs.writeShellScript "swayidle-after-resume" ''
+    ${niri} msg action power-on-monitors
+    ${restoreBrightness}
+  '';
 in
 {
   options.my.applications.swayidle = {
@@ -24,40 +69,32 @@ in
       {
         services.swayidle = {
           enable = true;
-          systemdTarget = "graphical-session.target";
+          systemdTargets = [ "graphical-session.target" ];
 
           timeouts = [
             {
               timeout = 300;
-              command = lockAndSuspendCmd;
+              command = "${dimScreen}";
+              resumeCommand = "${restoreBrightness}";
+            }
+            {
+              timeout = 360;
+              command = "${lockAndSuspend}";
             }
           ];
 
-          events = [
-            {
-              event = "lock";
-              command = lockCmd;
-            }
-            {
-              event = "before-sleep";
-              command = lockCmd;
-            }
-            {
-              event = "after-resume";
-              command = dpmsOn;
-            }
-          ];
-        };
-
-        systemd.user.services.swayidle = {
-          Service = {
-            PassEnvironment = [
-              "WAYLAND_DISPLAY"
-              "XDG_RUNTIME_DIR"
-              "DBUS_SESSION_BUS_ADDRESS"
-            ];
+          events = {
+            lock = lockCmd;
+            before-sleep = lockCmd;
+            after-resume = "${afterResume}";
           };
         };
+
+        systemd.user.services.swayidle.Service.PassEnvironment = [
+          "WAYLAND_DISPLAY"
+          "XDG_RUNTIME_DIR"
+          "DBUS_SESSION_BUS_ADDRESS"
+        ];
       }
     ];
   };