diff --git a/messages/de.json b/messages/de.json
index 9a2c80392..90d17c3c7 100644
--- a/messages/de.json
+++ b/messages/de.json
@@ -10392,6 +10392,7 @@
     "vital": {
       "RESTING_HEART_RATE": "Ruheherzfrequenz",
       "HEART_RATE_VARIABILITY": "Herzfrequenzvariabilität",
+      "HRV_RMSSD": "HRV (RMSSD)",
       "RECOVERY_SCORE": "Recovery",
       "OXYGEN_SATURATION": "Sauerstoffsättigung",
       "BODY_TEMPERATURE": "Körpertemperatur",
diff --git a/messages/en.json b/messages/en.json
index 8d6ab2cd4..eba85e21c 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -10392,6 +10392,7 @@
     "vital": {
       "RESTING_HEART_RATE": "Resting heart rate",
       "HEART_RATE_VARIABILITY": "Heart-rate variability",
+      "HRV_RMSSD": "HRV (RMSSD)",
       "RECOVERY_SCORE": "Recovery",
       "OXYGEN_SATURATION": "Oxygen saturation",
       "BODY_TEMPERATURE": "Body temperature",
diff --git a/messages/es.json b/messages/es.json
index f86b42a52..f89ebd220 100644
--- a/messages/es.json
+++ b/messages/es.json
@@ -10392,6 +10392,7 @@
     "vital": {
       "RESTING_HEART_RATE": "Frecuencia cardíaca en reposo",
       "HEART_RATE_VARIABILITY": "Variabilidad de la frecuencia cardíaca",
+      "HRV_RMSSD": "VFC (RMSSD)",
       "RECOVERY_SCORE": "Recovery",
       "OXYGEN_SATURATION": "Saturación de oxígeno",
       "BODY_TEMPERATURE": "Temperatura corporal",
diff --git a/messages/fr.json b/messages/fr.json
index 3b3a92f2f..eada370a5 100644
--- a/messages/fr.json
+++ b/messages/fr.json
@@ -10392,6 +10392,7 @@
     "vital": {
       "RESTING_HEART_RATE": "Fréquence cardiaque au repos",
       "HEART_RATE_VARIABILITY": "Variabilité de la fréquence cardiaque",
+      "HRV_RMSSD": "VFC (RMSSD)",
       "RECOVERY_SCORE": "Recovery",
       "OXYGEN_SATURATION": "Saturation en oxygène",
       "BODY_TEMPERATURE": "Température corporelle",
diff --git a/messages/it.json b/messages/it.json
index 85e7d2cf8..42929075e 100644
--- a/messages/it.json
+++ b/messages/it.json
@@ -10392,6 +10392,7 @@
     "vital": {
       "RESTING_HEART_RATE": "Frequenza cardiaca a riposo",
       "HEART_RATE_VARIABILITY": "Variabilità della frequenza cardiaca",
+      "HRV_RMSSD": "HRV (RMSSD)",
       "RECOVERY_SCORE": "Recovery",
       "OXYGEN_SATURATION": "Saturazione di ossigeno",
       "BODY_TEMPERATURE": "Temperatura corporea",
diff --git a/messages/ko.json b/messages/ko.json
index cc848901c..ffd1b4c6f 100644
--- a/messages/ko.json
+++ b/messages/ko.json
@@ -10392,6 +10392,7 @@
     "vital": {
       "RESTING_HEART_RATE": "안정 시 심박수",
       "HEART_RATE_VARIABILITY": "심박변이도",
+      "HRV_RMSSD": "심박변이도 (RMSSD)",
       "RECOVERY_SCORE": "회복",
       "OXYGEN_SATURATION": "산소포화도",
       "BODY_TEMPERATURE": "체온",
diff --git a/messages/nl.json b/messages/nl.json
index 745094620..d42032ded 100644
--- a/messages/nl.json
+++ b/messages/nl.json
@@ -10392,6 +10392,7 @@
     "vital": {
       "RESTING_HEART_RATE": "Rusthartslag",
       "HEART_RATE_VARIABILITY": "Hartslagvariabiliteit",
+      "HRV_RMSSD": "HRV (RMSSD)",
       "RECOVERY_SCORE": "Herstel",
       "OXYGEN_SATURATION": "Zuurstofverzadiging",
       "BODY_TEMPERATURE": "Lichaamstemperatuur",
diff --git a/messages/pl.json b/messages/pl.json
index d87da90db..6985f25f3 100644
--- a/messages/pl.json
+++ b/messages/pl.json
@@ -10392,6 +10392,7 @@
     "vital": {
       "RESTING_HEART_RATE": "Spoczynkowe tętno",
       "HEART_RATE_VARIABILITY": "Zmienność rytmu serca",
+      "HRV_RMSSD": "HRV (RMSSD)",
       "RECOVERY_SCORE": "Recovery",
       "OXYGEN_SATURATION": "Saturacja tlenem",
       "BODY_TEMPERATURE": "Temperatura ciała",
diff --git a/src/__tests__/i18n-english-leak-guard.test.ts b/src/__tests__/i18n-english-leak-guard.test.ts
index 451ab270f..b091256da 100644
--- a/src/__tests__/i18n-english-leak-guard.test.ts
+++ b/src/__tests__/i18n-english-leak-guard.test.ts
@@ -98,6 +98,7 @@ const LEGIT_IDENTICAL = new Set<string>([
   "insights.derived.composite.READINESS.component.hrv",
   "labs.catalog.hs-crp",
   "measurements.typeHrvRmssd",
+  "illness.vital.HRV_RMSSD",
   "settings.sections.sources.metrics.hrvRmssd",
   "measurements.typeVo2Max",
   // "{hours} h {minutes} min": the SI symbols for hour and minute.
diff --git a/src/lib/illness/__tests__/correlation.test.ts b/src/lib/illness/__tests__/correlation.test.ts
index 4c7e5cbd9..754026c7a 100644
--- a/src/lib/illness/__tests__/correlation.test.ts
+++ b/src/lib/illness/__tests__/correlation.test.ts
@@ -17,6 +17,7 @@ import {
   computeIllnessCorrelation,
   dayDiff,
   FUNCTIONAL_IMPACT_RETURN_KEY,
+  ILLNESS_SCAN_TYPES,
   isAdverseDeviation,
   MIN_BASELINE_DAYS,
   MIN_EPISODE_COVERAGE_DAYS,
@@ -803,6 +804,12 @@ describe("isAdverseDeviation", () => {
     expect(isAdverseDeviation("HEART_RATE_VARIABILITY", "above")).toBe(false);
   });
 
+  it("reads a drop in the nightly RMSSD series as adverse, like SDNN", () => {
+    expect(ILLNESS_SCAN_TYPES).toContain("HRV_RMSSD");
+    expect(isAdverseDeviation("HRV_RMSSD", "below")).toBe(true);
+    expect(isAdverseDeviation("HRV_RMSSD", "above")).toBe(false);
+  });
+
   it("treats neutral-direction vitals (WEIGHT) as never adverse", () => {
     expect(isAdverseDeviation("WEIGHT", "above")).toBe(false);
     expect(isAdverseDeviation("WEIGHT", "below")).toBe(false);
diff --git a/src/lib/illness/correlation.ts b/src/lib/illness/correlation.ts
index 95f47e60b..2633ff279 100644
--- a/src/lib/illness/correlation.ts
+++ b/src/lib/illness/correlation.ts
@@ -118,6 +118,7 @@ export const ADVERSE_DIRECTION: Partial<
   BLOOD_PRESSURE_SYS: "up",
   BLOOD_PRESSURE_DIA: "up",
   HEART_RATE_VARIABILITY: "down",
+  HRV_RMSSD: "down",
   RECOVERY_SCORE: "down",
   OXYGEN_SATURATION: "down",
 };
@@ -130,6 +131,9 @@ export const ADVERSE_DIRECTION: Partial<
 export const ILLNESS_SCAN_TYPES: MeasurementType[] = [
   "RESTING_HEART_RATE",
   "HEART_RATE_VARIABILITY",
+  // RMSSD, scanned on its own baseline: a nightly strap or ring reading drops
+  // during illness the same way SDNN does.
+  "HRV_RMSSD",
   "RECOVERY_SCORE",
   "OXYGEN_SATURATION",
   "BODY_TEMPERATURE",
diff --git a/src/lib/insights/__tests__/discovery-matrix-modules.test.ts b/src/lib/insights/__tests__/discovery-matrix-modules.test.ts
index 92d0d96b4..8f53be251 100644
--- a/src/lib/insights/__tests__/discovery-matrix-modules.test.ts
+++ b/src/lib/insights/__tests__/discovery-matrix-modules.test.ts
@@ -127,6 +127,21 @@ describe("assembleDiscoveryMatrix — switched-off modules", () => {
     expect(keys).toContain("CUSTOM_METRIC:abc");
   });
 
+  it("scans the nightly RMSSD series as an outcome owned by recovery", async () => {
+    const on = await assembleDiscoveryMatrix("u1", OPTS);
+    expect(
+      on.series.find((s) => s.key === "HRV_RMSSD" && s.role === "outcome"),
+    ).toBeDefined();
+    const off = await assembleDiscoveryMatrix("u1", {
+      ...OPTS,
+      modules: { recovery: false },
+    });
+    const keys = keysOf(off.series);
+    expect(keys).not.toContain("HRV_RMSSD");
+    // SDNN is a plain vital and stays.
+    expect(keys).toContain("HEART_RATE_VARIABILITY");
+  });
+
   it("does not read the weather at all with the environment module off", async () => {
     const read = vi.mocked(fetchEnvironmentSeries);
     read.mockClear();
diff --git a/src/lib/insights/correlation-discovery.ts b/src/lib/insights/correlation-discovery.ts
index db16e66c0..c08f20f70 100644
--- a/src/lib/insights/correlation-discovery.ts
+++ b/src/lib/insights/correlation-discovery.ts
@@ -662,6 +662,12 @@ export const DISCOVERY_BEHAVIOURS = [
 export const DISCOVERY_OUTCOMES = [
   "SLEEP_DURATION",
   "HEART_RATE_VARIABILITY",
+  // The nightly RMSSD series (WHOOP, Oura, Polar, Fitbit, Google Health) as
+  // its own outcome. It is not merged into the SDNN channel: the estimators
+  // differ in scale, so one series of both would correlate the switch between
+  // devices rather than the body. Neither HRV channel is a behaviour, so the
+  // two never pair with each other.
+  "HRV_RMSSD",
   "RESTING_HEART_RATE",
   "WEIGHT",
   // v1.22 — blood pressure as an OUTCOME (it was behaviour-only). This makes
diff --git a/src/lib/modules/surface.ts b/src/lib/modules/surface.ts
index 21f670926..a11e848a2 100644
--- a/src/lib/modules/surface.ts
+++ b/src/lib/modules/surface.ts
@@ -212,6 +212,8 @@ const STATIC_SURFACE_MODULE = {
   "correlation:SLEEP_DURATION": "sleep",
   "correlation:BLOOD_GLUCOSE": "glucose",
   "correlation:RESTING_HEART_RATE": "recovery",
+  // The RMSSD series is the recovery module's (see `summary:HRV_RMSSD`).
+  "correlation:HRV_RMSSD": "recovery",
   "correlation:MEDICATION_COMPLIANCE": "medications",
   // Symptom burden is read from the illness journal.
   "correlation:SYMPTOM_SEVERITY": "illness",