diff --git a/include/parameter.h b/include/parameter.h
index a047652..ea98c38 100644
--- a/include/parameter.h
+++ b/include/parameter.h
@@ -109,6 +109,7 @@ unsigned long t_batteryIcon = 0;
 bool b_showBatteryIcon = true;
 // volatile: now also written from the AsyncTCP task (WS soft-sleep command).
 volatile bool b_softSleep = false;
+unsigned long t_lastSerialActivity = 0;
 #if defined(ACC_MPU6050) || defined(ACC_BMA400)
 bool b_gyroEnabled = true;
 #endif
diff --git a/platformio.ini b/platformio.ini
index 78afc7d..0f56430 100644
--- a/platformio.ini
+++ b/platformio.ini
@@ -92,3 +92,37 @@ platform = native
 test_build_src = no
 test_framework = unity
 build_flags = -Iinclude
+
+# Tier-2 automatic light-sleep build (AI FYI, see PR). Opt-in, from-source: the
+# CONFIG_PM_* / tickless / BT-controller-sleep flags below are compile-time and
+# require rebuilding the framework from source (custom_sdkconfig triggers it).
+# The BT low-power clock is RTC_SLOW because this board has no external 32kHz
+# crystal (CONFIG_RTC_CLK_SRC_INT_RC). Pair with the CONFIG_PM_ENABLE-guarded
+# esp_pm_configure() + loop yield + UART wake in src/hds.ino. Build:
+#   pio run -e esp32s3_pm -t upload
+# custom_component_remove skips unused managed IDF components that the from-source
+# build would otherwise compile (esp-modbus fails a static assert); none are
+# #included by this firmware, so removal is image-identical.
+[env:esp32s3_pm]
+extends = env:esp32s3
+custom_component_remove =
+  espressif/esp-modbus
+  espressif/esp-sr
+  espressif/esp-zboss-lib
+  espressif/esp-zigbee-lib
+  espressif/esp_insights
+  espressif/esp_diagnostics
+  espressif/esp_diag_data_store
+  espressif/esp_rainmaker
+  espressif/rmaker_common
+  espressif/esp_modem
+  espressif/esp-dsp
+  chmorgan/esp-libhelix-mp3
+  espressif/qrcode
+custom_sdkconfig =
+  CONFIG_PM_ENABLE=y
+  CONFIG_FREERTOS_USE_TICKLESS_IDLE=y
+  CONFIG_PM_DFS_INIT_AUTO=y
+  CONFIG_BT_CTRL_MODEM_SLEEP=y
+  CONFIG_BT_CTRL_MODEM_SLEEP_MODE_1=y
+  CONFIG_BT_CTRL_LPCLK_SEL_RTC_SLOW=y
diff --git a/src/hds.ino b/src/hds.ino
index e99c9bf..9831f0d 100644
--- a/src/hds.ino
+++ b/src/hds.ino
@@ -1,5 +1,8 @@
 #include <Arduino.h>
 #include <cstring>
+#include "esp_pm.h"
+#include "esp_sleep.h"
+#include "driver/uart.h"
 #include "config.h"
 
 #include "parameter.h"
@@ -882,6 +885,21 @@ void setup() {
   } else {
     hdsOtaRollbackMarkValid();
   }
+
+#if CONFIG_PM_ENABLE
+  {
+    esp_pm_config_t pmcfg = {
+      .max_freq_mhz = 240,
+      .min_freq_mhz = 80,
+      .light_sleep_enable = true,
+    };
+    esp_err_t pmrc = esp_pm_configure(&pmcfg);
+    Serial.printf("[pm] esp_pm_configure ls=on 240/80 -> %s\n", esp_err_to_name(pmrc));
+    uart_set_wakeup_threshold(UART_NUM_0, 3);
+    esp_err_t urc = esp_sleep_enable_uart_wakeup(UART_NUM_0);
+    Serial.printf("[pm] uart0 wake-on-rx -> %s\n", esp_err_to_name(urc));
+  }
+#endif
 }
 
 /**
@@ -1605,6 +1623,7 @@ void loop() {
       data[len++] = Serial.read();
     }
     usbCallbacks.onStream(data, len);  // Process serial byte stream
+    t_lastSerialActivity = millis();
   }
   usbCallbacks.poll();
 
@@ -1785,6 +1804,14 @@ void loop() {
       }
     }
   }
+
+#if CONFIG_PM_ENABLE
+  if (millis() - t_lastSerialActivity > 250) {
+    vTaskDelay(pdMS_TO_TICKS(10));
+  } else {
+    vTaskDelay(1);
+  }
+#endif
 }