diff --git a/src/lib/ai/coach/snapshot-blocks/__tests__/sleep-block-night-key.test.ts b/src/lib/ai/coach/snapshot-blocks/__tests__/sleep-block-night-key.test.ts
new file mode 100644
index 000000000..6bac1aa00
--- /dev/null
+++ b/src/lib/ai/coach/snapshot-blocks/__tests__/sleep-block-night-key.test.ts
@@ -0,0 +1,44 @@
+/**
+ * v1.39.4 — the Coach names a night by the same key as the sleep table
+ * under its answer: the local day the night ends on.
+ */
+import { describe, expect, it, vi } from "vitest";
+
+vi.mock("@/lib/logging/context", () => ({ annotate: vi.fn() }));
+
+import { reconstructSleepNights } from "@/lib/analytics/sleep-night";
+import { buildSleepTimelineBlock } from "../sleep-block";
+
+const TZ = "Pacific/Auckland";
+
+describe("buildSleepTimelineBlock — night keys", () => {
+  it("keys a night east of UTC by its local wake day, as the table does", () => {
+    // Woke at 07:00 on 27 September in Auckland: still the 26th in UTC.
+    const rows = [
+      {
+        value: 450,
+        measuredAt: new Date("2026-09-26T19:00:00.000Z"),
+        sleepStage: null,
+        source: "APPLE_HEALTH" as const,
+      },
+    ];
+    const snapshot: Record<string, unknown> = {};
+    buildSleepTimelineBlock({
+      sleepRows: rows,
+      sourcePriorityJson: null,
+      userTz: TZ,
+      recentCutoff: new Date("2026-09-01T00:00:00.000Z"),
+      snapshot,
+      metrics: new Set(),
+      counts: {},
+      registerBlock: () => {},
+      groundingValues: new Map(),
+    });
+    const recent = (
+      snapshot.sleep as { timeline: { recent: Array<{ date: string }> } }
+    ).timeline.recent;
+    const tableKey = reconstructSleepNights(rows, TZ, null)[0].night;
+    expect(tableKey).toBe("2026-09-27");
+    expect(recent[0].date).toBe(tableKey);
+  });
+});
diff --git a/src/lib/ai/coach/snapshot-blocks/sleep-block.ts b/src/lib/ai/coach/snapshot-blocks/sleep-block.ts
index b67802d55..c90da3d84 100644
--- a/src/lib/ai/coach/snapshot-blocks/sleep-block.ts
+++ b/src/lib/ai/coach/snapshot-blocks/sleep-block.ts
@@ -97,7 +97,11 @@ export function buildSleepTimelineBlock(
       .filter((n) => n.measuredAt >= recentCutoff)
       .map((n) => {
         const row: Record<string, unknown> = {
-          date: n.measuredAt,
+          // The night's own key, the local day it ends on: the key the
+          // sleep table under the answer and every sleep surface use. The
+          // wake instant in UTC named the day before east of UTC, so the
+          // prose and the table disagreed about which night it was.
+          date: n.night,
           weekday: tzWeekday(n.measuredAt, userTz),
           minutes: Math.round(n.asleepMinutes),
         };