diff --git a/src/persona/IntentEngine.cpp b/src/persona/IntentEngine.cpp
index 17cbc772..4f0d7839 100644
--- a/src/persona/IntentEngine.cpp
+++ b/src/persona/IntentEngine.cpp
@@ -28,7 +28,7 @@ void IntentEngine::begin() {
   activeSpeechUntilMs_ = 0;
   soundOrientUntilMs_ = 0;
   lastEventAtMs_ = lastUpdateMs_;
-  demoEnabled_ = true;
+  demoEnabled_ = STACKCHAN_DEMO_ENABLED_AT_BOOT != 0;
   reducedMotion_ = false;
   soundAzimuthNorm_ = 0.0f;
   lastEventStrength_ = 0.0f;
diff --git a/src/persona/IntentEngine.hpp b/src/persona/IntentEngine.hpp
index a78429f8..bc00ea7e 100644
--- a/src/persona/IntentEngine.hpp
+++ b/src/persona/IntentEngine.hpp
@@ -9,6 +9,22 @@
 #include "persona/SpeechPlanner.hpp"
 #include "persona/StateMatrix.hpp"
 
+// Demo mode injects a random mode change and a synthetic event every 2.5-6 s.
+// That is useful on a bench when showing the face off, and actively harmful the
+// rest of the time: it makes the character look random rather than responsive,
+// and because every injected event counts as stimulus the robot can never
+// accumulate enough drowsiness to fall asleep. The character design has him
+// getting heavy-lidded at fatigue 0.45, yawning at 0.62 and asleep at 0.80 after
+// roughly 8.5 idle minutes, and none of that can happen while demo mode runs.
+//
+// Every native test covering sleep, idle life, or character behaviour calls
+// setDemoEnabled(false) first for exactly this reason, so the behaviour under
+// test was never the shipped default. Boot with it off; `demo on` over serial
+// still turns it on for a bench demonstration.
+#ifndef STACKCHAN_DEMO_ENABLED_AT_BOOT
+#define STACKCHAN_DEMO_ENABLED_AT_BOOT 0
+#endif
+
 namespace stackchan {
 
 class IntentEngine {
@@ -74,7 +90,7 @@ class IntentEngine {
   uint32_t activeSpeechUntilMs_ = 0;
   uint32_t soundOrientUntilMs_ = 0;
   uint32_t lastEventAtMs_ = 0;
-  bool demoEnabled_ = true;
+  bool demoEnabled_ = STACKCHAN_DEMO_ENABLED_AT_BOOT != 0;
   bool reducedMotion_ = false;
   float soundAzimuthNorm_ = 0.0f;
   float lastEventStrength_ = 0.0f;
diff --git a/test/test_native_logic/test_main.cpp b/test/test_native_logic/test_main.cpp
index ac2160d7..00a329d2 100644
--- a/test/test_native_logic/test_main.cpp
+++ b/test/test_native_logic/test_main.cpp
@@ -912,6 +912,7 @@ void test_intent_engine_emits_deduped_speech_cue_on_external_event() {
 void test_intent_engine_demo_can_be_disabled_and_resumed() {
   IntentEngine engine;
   engine.begin();
+  engine.setDemoEnabled(true, 0);
   TEST_ASSERT_TRUE(engine.isDemoEnabled());
 
   engine.setDemoEnabled(false, 0);
@@ -1280,6 +1281,32 @@ CharacterMode runUntilAsleep(IntentEngine& engine, uint32_t limitMs, uint32_t* a
   return mode;
 }
 
+// Demo mode injects a synthetic event every 2.5-6 s, and every injected event is
+// stimulus, so with it on the robot can never accumulate sleep pressure. Shipping
+// it enabled meant the documented drowsiness ladder could not run on a real
+// robot: one observed device stayed awake for 5.9 days.
+void test_intent_engine_boots_with_demo_off_so_the_character_can_sleep() {
+  IntentEngine engine;
+  engine.begin();
+  TEST_ASSERT_FALSE(engine.isDemoEnabled());
+
+  // Left alone, nothing injects a mode change. With demo mode on this loop sees
+  // Think/Attend/React appear within the first few seconds.
+  for (uint32_t nowMs = 1000; nowMs <= 30000; nowMs += 1000) {
+    const RobotFrame frame = engine.update(nowMs);
+    TEST_ASSERT_EQUAL(static_cast<int>(CharacterMode::Idle), static_cast<int>(frame.mode));
+  }
+
+  // And drowsiness accumulates instead of being reset by injected stimulus, so
+  // he reaches sleep the way the character design describes. Every other sleep
+  // test has to call setDemoEnabled(false) first to get here; this one must not
+  // need to, because that is now the boot state.
+  uint32_t asleepAtMs = 0;
+  runUntilAsleep(engine, 900000u, &asleepAtMs);
+  TEST_ASSERT_TRUE(engine.isAsleep());
+}
+
+
 }  // namespace
 
 void test_sleep_pressure_builds_only_when_left_alone() {
@@ -9418,6 +9445,7 @@ int main() {
   RUN_TEST(test_robot_frame_carries_character_mode_for_renderer);
   RUN_TEST(test_robot_frame_carries_speech_cue_for_output_adapters);
   RUN_TEST(test_intent_engine_emits_deduped_speech_cue_on_external_event);
+  RUN_TEST(test_intent_engine_boots_with_demo_off_so_the_character_can_sleep);
   RUN_TEST(test_intent_engine_demo_can_be_disabled_and_resumed);
   RUN_TEST(test_idle_life_breathing_moves_face_and_body_together);
   RUN_TEST(test_persona_behavior_codegen_exposes_idle_life_tuning);