diff --git a/src/lib/insights/derived/__tests__/coincident-deviation.test.ts b/src/lib/insights/derived/__tests__/coincident-deviation.test.ts
index 5e035a06d..6b08af942 100644
--- a/src/lib/insights/derived/__tests__/coincident-deviation.test.ts
+++ b/src/lib/insights/derived/__tests__/coincident-deviation.test.ts
@@ -33,6 +33,7 @@ import {
   computeCoincidentDeviation,
   classifyDeviation,
   COINCIDENT_FIRE_THRESHOLD,
+  sameHoursStanding,
 } from "../coincident-deviation";
 
 const PROFILE = { ageYears: 40, sex: "MALE" as const };
@@ -351,3 +352,40 @@ describe("glucose — a day in progress is compared with the same hours", () =>
     ).toBeUndefined();
   });
 });
+
+describe("sameHoursStanding", () => {
+  it("reads an hourly-mean day by its day mean, not by sum / n", () => {
+    // Twelve dense readings in one hour and one in another: the mean of the
+    // hours' means is 100, the raw mean is 140. A type on the hourly-mean
+    // reader carries `dayMean`, and that is the day's value.
+    const day = (d: string, dayMean: number) => ({
+      day: d,
+      n: 13,
+      sum: 140 * 13,
+      dayMean,
+    });
+    const days = [
+      ...Array.from({ length: 8 }, (_, i) =>
+        day(`2026-05-${String(20 + i).padStart(2, "0")}`, 100),
+      ),
+      day("2026-06-02", 101),
+    ];
+    const standing = sameHoursStanding(days, "2026-06-02", "BLOOD_GLUCOSE");
+    expect(standing?.value).toBe(101);
+    expect(standing?.center).toBe(100);
+  });
+
+  it("falls back to sum / n where the reader has no day mean", () => {
+    const days = [
+      ...Array.from({ length: 8 }, (_, i) => ({
+        day: `2026-05-${String(20 + i).padStart(2, "0")}`,
+        n: 2,
+        sum: 180,
+      })),
+      { day: "2026-06-02", n: 1, sum: 88 },
+    ];
+    const standing = sameHoursStanding(days, "2026-06-02", "BLOOD_GLUCOSE");
+    expect(standing?.value).toBe(88);
+    expect(standing?.center).toBe(90);
+  });
+});
diff --git a/src/lib/insights/derived/coincident-deviation.ts b/src/lib/insights/derived/coincident-deviation.ts
index 5588e3616..235a514fa 100644
--- a/src/lib/insights/derived/coincident-deviation.ts
+++ b/src/lib/insights/derived/coincident-deviation.ts
@@ -94,20 +94,22 @@ export const SAME_HOURS_MIN_DAYS = 7;
  * there is nothing like-for-like to compare today with, and no verdict.
  */
 export function sameHoursStanding(
-  days: readonly { day: string; n: number; sum: number }[],
+  days: readonly { day: string; n: number; sum: number; dayMean?: number }[],
   todayKey: string,
   type: MeasurementType,
 ): { value: number; low: number; high: number; center: number } | null {
+  // A type on the hourly-mean reader carries `dayMean` (the mean of its
+  // hours' means); every other type's day is the mean of its readings.
+  const value = (d: { n: number; sum: number; dayMean?: number }) =>
+    d.dayMean ?? d.sum / d.n;
   const today = days.find((d) => d.day === todayKey && d.n > 0);
   if (!today) return null;
-  const earlier = days
-    .filter((d) => d.day < todayKey && d.n > 0)
-    .map((d) => d.sum / d.n);
+  const earlier = days.filter((d) => d.day < todayKey && d.n > 0).map(value);
   if (earlier.length < SAME_HOURS_MIN_DAYS) return null;
   const band = buildBaselineBand(earlier, type);
   if (!band) return null;
   return {
-    value: today.sum / today.n,
+    value: value(today),
     low: band.low,
     high: band.high,
     center: band.center,