diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index bf628854e..e1fd8c8cc 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -8982,6 +8982,11 @@ components:
                   type: integer
                   minimum: 0
                   maximum: 9007199254740991
+                reconstructed:
+                  type: boolean
+                  description: True when the source has no per-stage onset timestamps and the server synthesised a contiguous timeline in
+                    a fixed physiological order (WHOOP). The client renders the hypnogram but labels it an approximate
+                    layout and never recomputes. False for real-series sources (Apple Health / Withings / Fitbit).
                 stages:
                   type: object
                   propertyNames:
@@ -9047,6 +9052,7 @@ components:
                 - inBedMinutes
                 - awakeMinutes
                 - awakenings
+                - reconstructed
                 - stages
                 - segments
               additionalProperties: false
@@ -9102,6 +9108,11 @@ components:
                 type: integer
                 minimum: 0
                 maximum: 9007199254740991
+              reconstructed:
+                type: boolean
+                description: True when the source has no per-stage onset timestamps and the server synthesised a contiguous timeline in
+                  a fixed physiological order (WHOOP). The client renders the hypnogram but labels it an approximate
+                  layout and never recomputes. False for real-series sources (Apple Health / Withings / Fitbit).
               stages:
                 type: object
                 propertyNames:
@@ -9167,6 +9178,7 @@ components:
               - inBedMinutes
               - awakeMinutes
               - awakenings
+              - reconstructed
               - stages
               - segments
             additionalProperties: false
diff --git a/prisma/migrations/0172_v1171_sleep_timeline_backfill/migration.sql b/prisma/migrations/0172_v1171_sleep_timeline_backfill/migration.sql
new file mode 100644
index 000000000..b5ea8e75b
--- /dev/null
+++ b/prisma/migrations/0172_v1171_sleep_timeline_backfill/migration.sql
@@ -0,0 +1,20 @@
+-- v1.17.1 — one-shot sleep-timeline backfill markers.
+--
+-- WHOOP's v2 API returns only per-stage sleep DURATION totals (no onset
+-- timestamps), and the pre-fix mapper stamped all five stage totals on the one
+-- sleep-END instant. The hypnogram then reconstructed every stage as a span
+-- touching the night's right edge — the stacked-bar artefact with no clock
+-- times. The fix reconstructs an ordered, contiguous per-segment timeline.
+--
+-- Withings stamped each sleep segment with its START while every reader treats
+-- `measured_at` as the segment END, shifting each night one segment-length
+-- earlier. The fix stamps the END.
+--
+-- Both fixes change the stored row shape / instant, so existing rows must be
+-- re-synced. The boot-time backfill (per connection) deletes the affected
+-- SLEEP_DURATION rows, re-syncs with the corrected mapper, re-folds the rollup
+-- tier, and stamps `sleep_timeline_backfill_at` so the discovery query drops
+-- the connection. Null = backfill not yet run; the marker makes the one-shot
+-- idempotent across reboots.
+ALTER TABLE "whoop_connections" ADD COLUMN "sleep_timeline_backfill_at" TIMESTAMP(3);
+ALTER TABLE "withings_connections" ADD COLUMN "sleep_timeline_backfill_at" TIMESTAMP(3);
diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index 715611d14..669592fd5 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -2173,6 +2173,13 @@ model WithingsConnection {
   /// has never re-authed since v1.4.25 — the UI surfaces a banner
   /// inviting them to reconnect.
   scope          String?   @map("scope")
+  /// v1.17.1 — one-shot marker for the sleep-stamp backfill. Pre-fix Withings
+  /// sleep rows were stamped with the segment START; the reader treats
+  /// `measuredAt` as the END, shifting every night one segment-length earlier.
+  /// The boot-time backfill deletes the affected WITHINGS / SLEEP_DURATION rows
+  /// and re-syncs with the corrected END stamp, then sets this so the discovery
+  /// query drops the connection. Null = backfill not yet run.
+  sleepTimelineBackfillAt DateTime? @map("sleep_timeline_backfill_at")
   createdAt      DateTime  @default(now()) @map("created_at")
   updatedAt      DateTime  @updatedAt @map("updated_at")
 
@@ -2249,6 +2256,13 @@ model WhoopConnection {
   /// Set once the self-converging history backfill walks every collection to
   /// completion. Null = backfill still in flight (or never started).
   backfillCompletedAt DateTime? @map("backfill_completed_at")
+  /// v1.17.1 — one-shot marker for the sleep-timeline backfill. Pre-fix WHOOP
+  /// sleep rows stamped all five stage totals on the one sleep-END instant, so
+  /// the hypnogram stacked them on the night's right edge. The boot-time
+  /// backfill deletes the legacy summary rows and re-syncs the reconstructed
+  /// per-segment timeline, then sets this so the discovery query drops the
+  /// connection. Null = backfill not yet run.
+  sleepTimelineBackfillAt DateTime? @map("sleep_timeline_backfill_at")
   /// Max heart rate from the WHOOP body-measurement profile — a profile
   /// constant, not a time series, so it lives here rather than as a
   /// Measurement row.
diff --git a/src/app/api/sleep/night/route.ts b/src/app/api/sleep/night/route.ts
index a930d3a3d..29d8874f0 100644
--- a/src/app/api/sleep/night/route.ts
+++ b/src/app/api/sleep/night/route.ts
@@ -12,6 +12,11 @@
  * priority ladder — so two sources' timelines never overlay. A view over the
  * existing per-stage `SLEEP_DURATION` rows; no schema, no new type.
  *
+ * `reconstructed` (per session) is true when the winning source has no
+ * per-stage onset timestamps and the server synthesised a contiguous timeline
+ * (WHOOP) — the client renders the hypnogram but labels it an approximate
+ * layout. A real-series source (Apple Health / Withings / Fitbit) is false.
+ *
  * `date` omitted → the most recent night in the trailing-year window.
  *
  * Auth: cookie session OR Bearer token (`requireAuth`). Soft-delete-filtered.
@@ -64,6 +69,10 @@ function serializeSession(s: SleepSession) {
     inBedMinutes: s.inBedMinutes === null ? null : Math.round(s.inBedMinutes),
     awakeMinutes: s.awakeMinutes === null ? null : Math.round(s.awakeMinutes),
     awakenings: s.awakenings,
+    // iOS #18 — `reconstructed` is true when the source has no per-stage onset
+    // timestamps and the server synthesised a contiguous timeline (WHOOP). iOS
+    // labels the hypnogram as an approximate layout; it never recomputes.
+    reconstructed: s.reconstructed,
     // iOS #18 — round the per-stage map too; the underlying totals sum
     // second-precision segments and otherwise serialise as e.g. 88.4999.
     stages: Object.fromEntries(
diff --git a/src/lib/analytics/__tests__/sleep-night.test.ts b/src/lib/analytics/__tests__/sleep-night.test.ts
index 17b32973c..00221cea0 100644
--- a/src/lib/analytics/__tests__/sleep-night.test.ts
+++ b/src/lib/analytics/__tests__/sleep-night.test.ts
@@ -380,6 +380,48 @@ describe("reconstructSleepSessions", () => {
     expect(sessions[0].asleepMinutes).toBe(420);
   });
 
+  it("flags a WHOOP-won night as reconstructed (synthetic stage order) with clock-time-distinct segments", () => {
+    // The ingest mapper reconstructs an ordered, contiguous WHOOP timeline.
+    // The reconstructed segments carry distinct measuredAt instants, so the
+    // hypnogram lays them at distinct clock times (no shared right edge).
+    const rows: SleepStageRow[] = [
+      srcRow("2026-06-04T01:00:00.000Z", "CORE", 240, "WHOOP"), // 21:00→01:00
+      srcRow("2026-06-04T03:00:00.000Z", "DEEP", 120, "WHOOP"), // 01:00→03:00
+      srcRow("2026-06-04T05:00:00.000Z", "REM", 120, "WHOOP"), //  03:00→05:00
+    ];
+    const sessions = reconstructSleepSessions(rows, "UTC");
+    expect(sessions).toHaveLength(1);
+    const s = sessions[0];
+    expect(s.source).toBe("WHOOP");
+    expect(s.reconstructed).toBe(true);
+    // No two segments end on the same instant — not stacked on the right edge.
+    const ends = s.segments.map((seg) => seg.end.getTime());
+    expect(new Set(ends).size).toBe(ends.length);
+  });
+
+  it("does NOT flag an Apple-Health night as reconstructed (measured timeline)", () => {
+    const rows: SleepStageRow[] = [
+      srcRow("2026-06-04T01:00:00.000Z", "CORE", 240, "APPLE_HEALTH"),
+      srcRow("2026-06-04T03:00:00.000Z", "DEEP", 90, "APPLE_HEALTH"),
+      srcRow("2026-06-04T04:30:00.000Z", "REM", 90, "APPLE_HEALTH"),
+    ];
+    const sessions = reconstructSleepSessions(rows, "UTC");
+    expect(sessions).toHaveLength(1);
+    expect(sessions[0].source).toBe("APPLE_HEALTH");
+    expect(sessions[0].reconstructed).toBe(false);
+  });
+
+  it("does NOT flag a Withings night as reconstructed (real per-segment series)", () => {
+    const rows: SleepStageRow[] = [
+      srcRow("2026-06-04T01:00:00.000Z", "CORE", 240, "WITHINGS"),
+      srcRow("2026-06-04T03:00:00.000Z", "DEEP", 90, "WITHINGS"),
+    ];
+    const sessions = reconstructSleepSessions(rows, "UTC");
+    expect(sessions).toHaveLength(1);
+    expect(sessions[0].source).toBe("WITHINGS");
+    expect(sessions[0].reconstructed).toBe(false);
+  });
+
   it("drops the bare ASLEEP aggregate from segments + stages when granular exists (HIGH H1, v1.11.5)", () => {
     // Apple-Health shape: granular CORE/DEEP/REM PLUS the unspecified ASLEEP
     // aggregate covering the same period, plus IN_BED. The hypnogram must not
diff --git a/src/lib/analytics/sleep-night.ts b/src/lib/analytics/sleep-night.ts
index 590361df7..88024e6c1 100644
--- a/src/lib/analytics/sleep-night.ts
+++ b/src/lib/analytics/sleep-night.ts
@@ -281,6 +281,19 @@ export interface SleepNight {
 /** Sentinel for rows that carry no source — collapses to one bucket. */
 const NO_SOURCE = "__none__";
 
+/**
+ * Sources whose per-segment stage ORDER is synthesised, not measured. WHOOP's
+ * v2 API exposes only per-stage duration totals (no onset timestamps), so the
+ * ingest mapper reconstructs a contiguous timeline in a fixed physiological
+ * order (`whoop/client.ts` `mapSleep`). A session won by such a source carries
+ * `reconstructed: true` so the UI labels it as an approximate layout. Apple
+ * Health / Withings / Fitbit all store a real per-segment series and stay off
+ * this set.
+ */
+const RECONSTRUCTED_TIMELINE_SOURCES: ReadonlySet<string> = new Set<string>([
+  "WHOOP",
+]);
+
 /**
  * Separator between the source and device-type parts of a writer key.
  * `` cannot appear in a `MeasurementSource` enum literal or a
@@ -615,6 +628,17 @@ export interface SleepSession {
   awakenings: number;
   /** The canonical source's segments, sorted by start, for the hypnogram. */
   segments: SleepSegment[];
+  /**
+   * `true` when the winning writer's stage ORDER is synthesised rather than
+   * measured. WHOOP's v2 API returns only per-stage duration totals — no onset
+   * timestamps — so the server reconstructs a contiguous timeline in a fixed
+   * physiological order (`whoop/client.ts` `mapSleep`). The segment positions
+   * are then approximate, not measured, and the UI / iOS must label the night
+   * as an approximate layout (never present it as measured stage timing).
+   * Sources that store a real per-segment series (Apple Health, Withings,
+   * Fitbit) stay `false`.
+   */
+  reconstructed: boolean;
 }
 
 /** Resolve a stage row to its absolute span (start = end − duration). */
@@ -760,6 +784,11 @@ export function reconstructSleepSessions(
       stages,
       awakenings: countAwakenings(segments),
       segments,
+      // The winning writer's stage order is synthetic when its source
+      // reconstructs the timeline (WHOOP). The UI labels such a night as an
+      // approximate layout; a real-series source (Apple/Withings/Fitbit) is
+      // measured.
+      reconstructed: RECONSTRUCTED_TIMELINE_SOURCES.has(canonicalSource),
     });
   }
   return sessions.sort((a, b) => a.start.getTime() - b.start.getTime());
diff --git a/src/lib/fitbit/__tests__/client.test.ts b/src/lib/fitbit/__tests__/client.test.ts
index 38b54aeeb..9edb6b4e8 100644
--- a/src/lib/fitbit/__tests__/client.test.ts
+++ b/src/lib/fitbit/__tests__/client.test.ts
@@ -526,7 +526,7 @@ describe("sleep-stage mapping", () => {
     expect(mapFitbitSleepStage(42)).toBeNull();
   });
 
-  it("maps a session into per-stage SLEEP_DURATION rows with measuredAt = stage END", () => {
+  it("maps a session into per-SEGMENT SLEEP_DURATION rows with measuredAt = segment END (real timeline)", () => {
     const session = {
       sleep: {
         startTime: "2026-05-10T22:00:00.000Z",
@@ -556,23 +556,31 @@ describe("sleep-stage mapping", () => {
       },
     };
     const out = mapSleepSession(session);
-    const byStage = Object.fromEntries(out.map((m) => [m.sleepStage, m]));
 
-    // CORE = the two light segments summed (30 + 45 = 75 min).
-    expect(byStage.CORE!.value).toBe(75);
-    expect(byStage.CORE!.type).toBe("SLEEP_DURATION");
-    expect(byStage.CORE!.unit).toBe("minutes");
-    // measuredAt is the LATEST end for that stage (the second light segment).
-    expect(byStage.CORE!.measuredAt.toISOString()).toBe(
+    // One row PER segment (no collapse) — 4 segments → 4 rows.
+    expect(out).toHaveLength(4);
+    expect(out.every((m) => m.type === "SLEEP_DURATION")).toBe(true);
+    expect(out.every((m) => m.unit === "minutes")).toBe(true);
+
+    // Each row carries its OWN end instant + duration — a real clock-time
+    // timeline, not a stage collapse.
+    const core = out.filter((m) => m.sleepStage === "CORE");
+    expect(core.map((m) => m.value)).toEqual([30, 45]);
+    expect(core.map((m) => m.measuredAt.toISOString())).toEqual([
+      "2026-05-10T22:30:00.000Z",
       "2026-05-11T00:15:00.000Z",
-    );
-    expect(byStage.DEEP!.value).toBe(60);
-    expect(byStage.REM!.value).toBe(45);
+    ]);
+    const deep = out.find((m) => m.sleepStage === "DEEP")!;
+    expect(deep.value).toBe(60);
+    expect(deep.measuredAt.toISOString()).toBe("2026-05-10T23:30:00.000Z");
+    const rem = out.find((m) => m.sleepStage === "REM")!;
+    expect(rem.value).toBe(45);
 
-    // externalId field-tag is session-anchored so a re-score overwrites in place.
-    expect(byStage.DEEP!.fieldTag).toBe(
-      "2026-05-11T06:00:00.000Z:sleep_deep",
-    );
+    // Every fieldTag is distinct (session anchor + stage + segment index) so
+    // the several segments of one stage never collide under the dedup key.
+    const tags = out.map((m) => m.fieldTag);
+    expect(new Set(tags).size).toBe(tags.length);
+    expect(tags).toContain("2026-05-11T06:00:00.000Z:sleep_deep:1");
   });
 
   it("anchors the session externalId on sleep.interval.end_time for an INTERVAL-shaped point", () => {
@@ -592,7 +600,7 @@ describe("sleep-stage mapping", () => {
       },
     });
     const deep = out.find((m) => m.sleepStage === "DEEP");
-    expect(deep!.fieldTag).toBe("2026-05-11T06:00:00.000Z:sleep_deep");
+    expect(deep!.fieldTag).toBe("2026-05-11T06:00:00.000Z:sleep_deep:0");
   });
 
   it("yields nothing for a session with no parseable segments", () => {
diff --git a/src/lib/fitbit/client.ts b/src/lib/fitbit/client.ts
index 25c217140..e31ee2450 100644
--- a/src/lib/fitbit/client.ts
+++ b/src/lib/fitbit/client.ts
@@ -1244,10 +1244,18 @@ function sleepSessionAnchor(point: FitbitDataPoint): string {
 }
 
 /**
- * Map one Google sleep session into per-stage `SLEEP_DURATION` rows. Each
- * stage's segments are summed into one row carrying the stage's `SleepStage`,
- * `measuredAt = the latest segment END for that stage`, and a stage-scoped
- * fieldTag so the (up to six) stage rows for one night stay distinct under the
+ * Map one Google sleep session into per-SEGMENT `SLEEP_DURATION` rows. The
+ * Google sleep payload carries a real per-stage segment series (each with its
+ * own start/end), so one row is emitted PER SEGMENT — `measuredAt = that
+ * segment's END` — rather than collapsing a stage's segments onto a single
+ * lastEnd instant. The collapsed shape stamped every stage total on its last
+ * end, so the hypnogram reconstructed all of a stage's time as one block ending
+ * at the night's right edge (the stacked-bar artefact); per-segment rows lay
+ * each block at its true clock time. The timeline is MEASURED (real onsets), so
+ * these rows are NOT flagged reconstructed — unlike WHOOP, which has no onsets.
+ *
+ * Each segment carries a stage-scoped, INDEXED fieldTag so the several segments
+ * of one stage stay distinct under the `(userId, type, source, externalId)`
  * dedup key. Unknown stage labels are skipped; a session with no parseable
  * segment yields nothing.
  */
@@ -1257,38 +1265,24 @@ export function mapSleepSession(
   const segments = readSleepSegments(point);
   if (segments.length === 0) return [];
 
-  // Accumulate per-stage minutes + track the latest end instant for each stage.
-  const perStage = new Map<
-    FitbitSleepStage,
-    { minutes: number; lastEnd: Date }
-  >();
+  const anchor = sleepSessionAnchor(point);
+  const out: FitbitMappedMeasurement[] = [];
+  let segIndex = 0;
   for (const seg of segments) {
     const stage = mapFitbitSleepStage(seg.stage);
     if (!stage) continue;
     const mins = minutesBetween(seg.startTime, seg.endTime);
-    if (mins === null) continue;
+    if (mins === null || !(mins > 0)) continue;
     const end = new Date(seg.endTime as string);
-    const prev = perStage.get(stage);
-    if (prev) {
-      prev.minutes += mins;
-      if (end > prev.lastEnd) prev.lastEnd = end;
-    } else {
-      perStage.set(stage, { minutes: mins, lastEnd: end });
-    }
-  }
-
-  const anchor = sleepSessionAnchor(point);
-  const out: FitbitMappedMeasurement[] = [];
-  for (const [stage, agg] of perStage) {
-    if (!(agg.minutes > 0)) continue;
     out.push({
       type: "SLEEP_DURATION",
-      value: round2(agg.minutes),
+      value: round2(mins),
       unit: "minutes",
-      measuredAt: agg.lastEnd,
-      fieldTag: `${anchor}:sleep_${stage.toLowerCase()}`,
+      measuredAt: end,
+      fieldTag: `${anchor}:sleep_${stage.toLowerCase()}:${segIndex}`,
       sleepStage: stage,
     });
+    segIndex += 1;
   }
   return out;
 }
diff --git a/src/lib/fitbit/sync-sleep.ts b/src/lib/fitbit/sync-sleep.ts
index 953cd40f9..d7ac50f1f 100644
--- a/src/lib/fitbit/sync-sleep.ts
+++ b/src/lib/fitbit/sync-sleep.ts
@@ -2,16 +2,19 @@
  * Fitbit / Google Health sleep-bundle sync (v1.12.0, W5).
  *
  * Reads sleep sessions from the `sleep.readonly` Restricted bundle and maps
- * each into per-stage `SLEEP_DURATION` rows via `mapSleepSession` (minutes per
- * stage; `measuredAt = the stage's END instant; harmonised onto the shared
- * `SleepStage` enum IN_BED/AWAKE/ASLEEP/REM/CORE/DEEP that the night-total +
- * hypnogram readers already consume for WHOOP / Apple). Upserts as
+ * each into per-SEGMENT `SLEEP_DURATION` rows via `mapSleepSession` (one row
+ * per stage segment; `measuredAt = that segment's END instant; harmonised onto
+ * the shared `SleepStage` enum IN_BED/AWAKE/ASLEEP/REM/CORE/DEEP that the
+ * night-total + hypnogram readers already consume for WHOOP / Apple). Google
+ * carries a real per-segment series, so the rows lay each block at its true
+ * clock time (a MEASURED timeline, not reconstructed). Upserts as
  * `source = FITBIT`.
  *
- * Per-stage rows carry the `sleepStage` axis so the (up to six) stage rows for
- * one night stay distinct under the `(userId, type, source, externalId)` dedup
- * key. externalId = `<session-anchor>:sleep_<stage>` — a re-scored night
- * overwrites in place. A 24 h overlap covers Google's after-the-fact re-score.
+ * Each segment carries the `sleepStage` axis and an indexed fieldTag so the
+ * several segments of one stage stay distinct under the
+ * `(userId, type, source, externalId)` dedup key. externalId =
+ * `<session-anchor>:sleep_<stage>:<i>` — a re-scored night overwrites in place.
+ * A 24 h overlap covers Google's after-the-fact re-score.
  *
  * A per-data-class 403 soft-skips the resource — the sleep bundle is granted
  * independently of activity / metrics in the Google consent flow.
diff --git a/src/lib/jobs/__tests__/sleep-timeline-backfill.test.ts b/src/lib/jobs/__tests__/sleep-timeline-backfill.test.ts
new file mode 100644
index 000000000..710f923b9
--- /dev/null
+++ b/src/lib/jobs/__tests__/sleep-timeline-backfill.test.ts
@@ -0,0 +1,149 @@
+/**
+ * v1.17.1 — sleep-timeline backfill self-convergence tests (mocked).
+ *   - discovery only matches connections whose sleep rows predate the fix
+ *     (`sleepTimelineBackfillAt IS NULL`), for both WHOOP and Withings;
+ *   - a completed pass DELETES the source's SLEEP_DURATION rows, re-syncs, and
+ *     stamps `sleepTimelineBackfillAt` so the next discovery drops the account
+ *     (idempotent across reboots);
+ *   - the discovery enqueue is singleton-keyed per (provider, user).
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const { prismaMock, bossSend, syncUserWhoop, syncWithingsSleep } = vi.hoisted(
+  () => ({
+    prismaMock: {
+      whoopConnection: { findMany: vi.fn(), update: vi.fn() },
+      withingsConnection: { findMany: vi.fn(), update: vi.fn() },
+      measurement: { deleteMany: vi.fn() },
+    },
+    bossSend: vi.fn(),
+    syncUserWhoop: vi.fn(),
+    syncWithingsSleep: vi.fn(),
+  }),
+);
+
+vi.mock("@/lib/db", () => ({ prisma: prismaMock }));
+
+vi.mock("@/lib/jobs/boss-instance", () => ({
+  getGlobalBoss: () => ({ send: bossSend }),
+}));
+
+vi.mock("@/lib/whoop/sync", () => ({
+  syncUserWhoop: (...a: unknown[]) => syncUserWhoop(...a),
+}));
+
+vi.mock("@/lib/withings/sync-sleep", () => ({
+  syncUserSleep: (...a: unknown[]) => syncWithingsSleep(...a),
+}));
+
+vi.mock("@/lib/logging/context", () => ({
+  annotate: () => {},
+  getEvent: () => null,
+}));
+
+import {
+  enqueueBootTimeSleepTimelineBackfill,
+  runSleepTimelineBackfillForUser,
+} from "../sleep-timeline-backfill";
+
+beforeEach(() => {
+  vi.clearAllMocks();
+});
+
+describe("enqueueBootTimeSleepTimelineBackfill — discovery", () => {
+  it("queries un-backfilled WHOOP + Withings connections and enqueues one job per (provider, user)", async () => {
+    prismaMock.whoopConnection.findMany.mockResolvedValue([{ userId: "w1" }]);
+    prismaMock.withingsConnection.findMany.mockResolvedValue([
+      { userId: "v1" },
+    ]);
+    bossSend.mockResolvedValue("job-id");
+
+    const result = await enqueueBootTimeSleepTimelineBackfill();
+
+    expect(prismaMock.whoopConnection.findMany).toHaveBeenCalledWith(
+      expect.objectContaining({ where: { sleepTimelineBackfillAt: null } }),
+    );
+    expect(prismaMock.withingsConnection.findMany).toHaveBeenCalledWith(
+      expect.objectContaining({ where: { sleepTimelineBackfillAt: null } }),
+    );
+    expect(result.enqueued).toBe(2);
+    expect(bossSend).toHaveBeenCalledWith(
+      "sleep-timeline-backfill",
+      expect.objectContaining({ userId: "w1", provider: "WHOOP" }),
+      expect.objectContaining({
+        singletonKey: "sleep-timeline-backfill|WHOOP|w1",
+      }),
+    );
+    expect(bossSend).toHaveBeenCalledWith(
+      "sleep-timeline-backfill",
+      expect.objectContaining({ userId: "v1", provider: "WITHINGS" }),
+      expect.objectContaining({
+        singletonKey: "sleep-timeline-backfill|WITHINGS|v1",
+      }),
+    );
+  });
+
+  it("self-converges: no un-backfilled connections → nothing enqueued", async () => {
+    prismaMock.whoopConnection.findMany.mockResolvedValue([]);
+    prismaMock.withingsConnection.findMany.mockResolvedValue([]);
+
+    const result = await enqueueBootTimeSleepTimelineBackfill();
+
+    expect(result.enqueued).toBe(0);
+    expect(bossSend).not.toHaveBeenCalled();
+  });
+
+  it("never throws — surfaces a discovery error through the result value", async () => {
+    prismaMock.whoopConnection.findMany.mockRejectedValue(new Error("db down"));
+    prismaMock.withingsConnection.findMany.mockResolvedValue([]);
+
+    const result = await enqueueBootTimeSleepTimelineBackfill();
+
+    expect(result.error).toBe("db down");
+    expect(result.enqueued).toBe(0);
+  });
+});
+
+describe("runSleepTimelineBackfillForUser", () => {
+  it("WHOOP: deletes the source's sleep rows, full-syncs, and stamps the marker", async () => {
+    prismaMock.measurement.deleteMany.mockResolvedValue({ count: 5 });
+    syncUserWhoop.mockResolvedValue(42);
+    prismaMock.whoopConnection.update.mockResolvedValue({});
+
+    const { deleted, imported } = await runSleepTimelineBackfillForUser(
+      "w1",
+      "WHOOP",
+    );
+
+    expect(deleted).toBe(5);
+    expect(imported).toBe(42);
+    expect(prismaMock.measurement.deleteMany).toHaveBeenCalledWith({
+      where: { userId: "w1", type: "SLEEP_DURATION", source: "WHOOP" },
+    });
+    expect(syncUserWhoop).toHaveBeenCalledWith("w1", { fullSync: true });
+    const updateArg = prismaMock.whoopConnection.update.mock.calls[0]![0];
+    expect(updateArg.where).toEqual({ userId: "w1" });
+    expect(updateArg.data.sleepTimelineBackfillAt).toBeInstanceOf(Date);
+  });
+
+  it("WITHINGS: deletes the source's sleep rows, re-syncs, and stamps the marker", async () => {
+    prismaMock.measurement.deleteMany.mockResolvedValue({ count: 3 });
+    syncWithingsSleep.mockResolvedValue(7);
+    prismaMock.withingsConnection.update.mockResolvedValue({});
+
+    const { deleted, imported } = await runSleepTimelineBackfillForUser(
+      "v1",
+      "WITHINGS",
+    );
+
+    expect(deleted).toBe(3);
+    expect(imported).toBe(7);
+    expect(prismaMock.measurement.deleteMany).toHaveBeenCalledWith({
+      where: { userId: "v1", type: "SLEEP_DURATION", source: "WITHINGS" },
+    });
+    expect(syncWithingsSleep).toHaveBeenCalledWith("v1", { fullSync: true });
+    const updateArg = prismaMock.withingsConnection.update.mock.calls[0]![0];
+    expect(updateArg.where).toEqual({ userId: "v1" });
+    expect(updateArg.data.sleepTimelineBackfillAt).toBeInstanceOf(Date);
+  });
+});
diff --git a/src/lib/jobs/__tests__/sleep-timeline-queues.test.ts b/src/lib/jobs/__tests__/sleep-timeline-queues.test.ts
new file mode 100644
index 000000000..2cbccc291
--- /dev/null
+++ b/src/lib/jobs/__tests__/sleep-timeline-queues.test.ts
@@ -0,0 +1,38 @@
+/**
+ * v1.17.1 — pg-boss queue registration for the one-shot sleep-timeline backfill.
+ *
+ * The reminder-worker imports heavy infrastructure, so we read the source as
+ * text and assert the queue constant is declared, registered in `allQueues`,
+ * wired to a `boss.work` handler, and enqueued at boot — the fast guard that
+ * catches the v1.4.37 dead-queue class (a queue declared but never registered,
+ * which silently never drains).
+ */
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+import { describe, expect, it } from "vitest";
+
+const REMINDER_WORKER_PATH = join(__dirname, "..", "reminder-worker.ts");
+const source = readFileSync(REMINDER_WORKER_PATH, "utf8");
+
+describe("reminder-worker — sleep-timeline backfill queue", () => {
+  it("registers the queue in allQueues (v1.4.37 dead-queue guard)", () => {
+    const block = /const allQueues = \[([\s\S]*?)\];/.exec(source);
+    expect(block).not.toBeNull();
+    expect(block![1]!).toContain("SLEEP_TIMELINE_BACKFILL_QUEUE");
+  });
+
+  it("wires a boss.work handler to the per-user runner", () => {
+    expect(source).toMatch(
+      /boss\.work[\s\S]{0,260}SLEEP_TIMELINE_BACKFILL_QUEUE[\s\S]{0,260}runSleepTimelineBackfillForUser/,
+    );
+  });
+
+  it("wires the boot discovery", () => {
+    expect(source).toMatch(/enqueueBootTimeSleepTimelineBackfill\(\)/);
+  });
+
+  it("imports the backfill exports", () => {
+    expect(source).toMatch(/from\s*["']@\/lib\/jobs\/sleep-timeline-backfill["']/);
+  });
+});
diff --git a/src/lib/jobs/reminder-worker.ts b/src/lib/jobs/reminder-worker.ts
index 57aa681f0..056fbf574 100644
--- a/src/lib/jobs/reminder-worker.ts
+++ b/src/lib/jobs/reminder-worker.ts
@@ -21,6 +21,13 @@ import {
   enqueueBootTimeFitbitBackfill,
   type FitbitBackfillPayload,
 } from "@/lib/jobs/fitbit-backfill";
+import {
+  SLEEP_TIMELINE_BACKFILL_QUEUE,
+  SLEEP_TIMELINE_BACKFILL_CONCURRENCY,
+  runSleepTimelineBackfillForUser,
+  enqueueBootTimeSleepTimelineBackfill,
+  type SleepTimelineBackfillPayload,
+} from "@/lib/jobs/sleep-timeline-backfill";
 import { reportWorkerError } from "@/lib/jobs/report-worker-error";
 import { markWorkerStarted, recordError } from "@/lib/jobs/worker-status";
 import { setGlobalBoss } from "@/lib/jobs/boss-instance";
@@ -629,6 +636,13 @@ export async function startReminderWorker() {
     FITBIT_SYNC_QUEUE,
     FITBIT_BACKFILL_QUEUE,
     FITBIT_OAUTH_STATE_CLEANUP_QUEUE,
+    // v1.17.1 — one-shot sleep-timeline backfill for WHOOP + Withings.
+    // Discovery enqueues one job per connection whose sleep rows predate the
+    // stamp/shape fix; the pass deletes the affected SLEEP_DURATION rows and
+    // re-syncs. Idempotent across reboots. The queue MUST be registered here or
+    // pg-boss never provisions it and the boot enqueue silently never drains
+    // (the v1.4.37 dead-queue class).
+    SLEEP_TIMELINE_BACKFILL_QUEUE,
     // v1.17.0 — Nightscout CGM poll sync. Poll-only (no webhook, no OAuth, no
     // backfill queue — the hourly window walks the recent SGV set). The queue
     // MUST be registered here or pg-boss never provisions it and the schedule
@@ -1056,6 +1070,27 @@ export async function startReminderWorker() {
     { localConcurrency: 1 },
     handleFitbitOAuthStateCleanup,
   );
+  // v1.17.1 — one-shot sleep-timeline backfill. The boot enqueue below sends
+  // one job per (user, provider) whose sleep rows predate the stamp/shape fix;
+  // this handler deletes the affected rows, re-syncs, and stamps the marker so
+  // the discovery query drops the connection.
+  await boss.work<SleepTimelineBackfillPayload>(
+    SLEEP_TIMELINE_BACKFILL_QUEUE,
+    { localConcurrency: SLEEP_TIMELINE_BACKFILL_CONCURRENCY },
+    async (jobs) => {
+      for (const job of jobs) {
+        const { userId, provider } = job.data;
+        const { deleted, imported } = await runSleepTimelineBackfillForUser(
+          userId,
+          provider,
+        );
+        workerLog(
+          "info",
+          `[sleep-timeline-backfill] user=${userId} provider=${provider} deleted=${deleted} imported=${imported}`,
+        );
+      }
+    },
+  );
   // v1.17.0 — Nightscout CGM poll-cohort sync. The hourly cron tick (no
   // `userId`) walks every configured instance; one user's unreachable host is
   // warned, not fatal.
@@ -1882,6 +1917,34 @@ export async function startReminderWorker() {
     );
   }
 
+  // v1.17.1 — fire-and-forget boot discovery for the one-shot sleep-timeline
+  // backfill. Finds every WHOOP + Withings connection whose sleep rows predate
+  // the stamp/shape fix and enqueues one job per (user, provider). Idempotent
+  // across reboots: a completed pass stamps `sleepTimelineBackfillAt`, dropping
+  // the connection from the discovery set. Errors come back through the
+  // helper's result value — the worker boot never fails because of a miss.
+  try {
+    const { enqueued, skipped, error } =
+      await enqueueBootTimeSleepTimelineBackfill();
+    if (error) {
+      workerLog(
+        "error",
+        `[sleep-timeline-backfill] boot discovery failed: ${error}`,
+      );
+    } else {
+      workerLog(
+        "info",
+        `[sleep-timeline-backfill] boot discovery: enqueued=${enqueued} skipped=${skipped}`,
+      );
+    }
+  } catch (err) {
+    workerLog(
+      "error",
+      "[sleep-timeline-backfill] boot discovery threw an unexpected error",
+      err,
+    );
+  }
+
   // v1.7.0 — fire-and-forget boot discovery for the daily-mean
   // consolidation pass. Finds every user holding live per-sample
   // high-frequency mean-type rows and enqueues one job per account.
diff --git a/src/lib/jobs/sleep-timeline-backfill.ts b/src/lib/jobs/sleep-timeline-backfill.ts
new file mode 100644
index 000000000..d3f49f154
--- /dev/null
+++ b/src/lib/jobs/sleep-timeline-backfill.ts
@@ -0,0 +1,169 @@
+/**
+ * v1.17.1 — one-shot sleep-timeline backfill for WHOOP + Withings.
+ *
+ * Two ingest-side stamp/shape fixes changed the stored sleep rows:
+ *
+ *  - WHOOP: the old mapper stamped all five per-stage DURATION totals on the
+ *    one sleep-END instant (WHOOP v2 exposes no onset timestamps), so the
+ *    hypnogram reconstructed every stage as a span touching the night's right
+ *    edge — stacked, no clock times. The fix reconstructs an ordered,
+ *    contiguous per-segment timeline with distinct `measuredAt` per segment and
+ *    NEW indexed externalIds (`<sleep_id>:seg:<tag>:<i>`).
+ *  - Withings: each segment was stamped with its START while every reader
+ *    treats `measuredAt` as the END, shifting each night one segment-length
+ *    earlier. The fix stamps the END.
+ *
+ * Both change the row's `measuredAt` (and, for WHOOP, the externalId), and the
+ * unique index `(userId, type, measuredAt, source, sleepStage)` makes an
+ * in-place UPDATE collision-prone. So the per-connection pass DELETES the
+ * affected `SLEEP_DURATION` rows for that source, RE-SYNCS with the corrected
+ * mapper (which re-folds the rollup tier in its tail), and stamps
+ * `sleepTimelineBackfillAt` so the discovery query drops the connection.
+ *
+ * Idempotent across reboots: the discovery predicate is
+ * `sleep_timeline_backfill_at IS NULL`, and a completed pass stamps the marker.
+ * A reboot mid-pass re-runs delete + re-sync from scratch — the re-sync upserts
+ * are key-stable, so the result converges.
+ *
+ * Modelled on `whoop-backfill.ts` / `fitbit-backfill.ts`. The queue name MUST
+ * be registered in `allQueues` in `src/lib/jobs/reminder-worker.ts` or pg-boss
+ * never provisions it and the boot enqueue silently never drains (the v1.4.37
+ * dead-queue class).
+ */
+import { prisma } from "@/lib/db";
+import { annotate } from "@/lib/logging/context";
+import { getGlobalBoss } from "@/lib/jobs/boss-instance";
+import { syncUserWhoop } from "@/lib/whoop/sync";
+import { syncUserSleep as syncWithingsSleep } from "@/lib/withings/sync-sleep";
+
+export const SLEEP_TIMELINE_BACKFILL_QUEUE = "sleep-timeline-backfill";
+
+/**
+ * Serial concurrency — a pass deletes a source's sleep rows then walks a
+ * re-sync that is rate-bounded by the upstream API cap. Concurrency-1 keeps it
+ * from crowding the request pool, matching the other backfill queues.
+ */
+export const SLEEP_TIMELINE_BACKFILL_CONCURRENCY = 1;
+
+export type SleepTimelineProvider = "WHOOP" | "WITHINGS";
+
+export interface SleepTimelineBackfillPayload {
+  userId: string;
+  provider: SleepTimelineProvider;
+  enqueuedAt: string;
+}
+
+/**
+ * Delete the legacy `SLEEP_DURATION` rows for one source, then re-sync with the
+ * corrected mapper. WHOOP runs a full-history sync (the segment externalIds are
+ * new, so the deleted summary rows are not re-created); Withings re-syncs its
+ * default trailing window with the corrected END stamp. The re-sync re-folds
+ * the rollup tier in its own tail.
+ */
+export async function runSleepTimelineBackfillForUser(
+  userId: string,
+  provider: SleepTimelineProvider,
+): Promise<{ deleted: number; imported: number }> {
+  const { count: deleted } = await prisma.measurement.deleteMany({
+    where: { userId, type: "SLEEP_DURATION", source: provider },
+  });
+
+  let imported = 0;
+  if (provider === "WHOOP") {
+    imported = await syncUserWhoop(userId, { fullSync: true });
+    await prisma.whoopConnection.update({
+      where: { userId },
+      data: { sleepTimelineBackfillAt: new Date() },
+    });
+  } else {
+    imported = await syncWithingsSleep(userId, { fullSync: true });
+    await prisma.withingsConnection.update({
+      where: { userId },
+      data: { sleepTimelineBackfillAt: new Date() },
+    });
+  }
+
+  annotate({
+    action: {
+      name: "sleep.timeline.backfill.complete",
+      details: { provider, deleted, imported },
+    },
+  });
+  return { deleted, imported };
+}
+
+/**
+ * Boot-time discovery. Finds every WHOOP + Withings connection not yet
+ * backfilled (`sleep_timeline_backfill_at IS NULL`) and enqueues one job per
+ * (user, provider). Idempotent across reboots: a completed pass stamps the
+ * marker, dropping the connection from the discovery set. pg-boss
+ * `singletonKey` coalesces duplicate sends so a fast restart while a job is
+ * queued doesn't double up.
+ *
+ * Best-effort: errors are returned through the result value so worker boot
+ * never fails because of a backfill miss.
+ */
+export async function enqueueBootTimeSleepTimelineBackfill(): Promise<{
+  enqueued: number;
+  skipped: number;
+  error: string | null;
+}> {
+  const boss = getGlobalBoss();
+  if (!boss) {
+    return { enqueued: 0, skipped: 0, error: null };
+  }
+
+  try {
+    const [whoop, withings] = await Promise.all([
+      prisma.whoopConnection.findMany({
+        where: { sleepTimelineBackfillAt: null },
+        select: { userId: true },
+      }),
+      prisma.withingsConnection.findMany({
+        where: { sleepTimelineBackfillAt: null },
+        select: { userId: true },
+      }),
+    ]);
+
+    const targets: Array<{ userId: string; provider: SleepTimelineProvider }> =
+      [
+        ...whoop.map((c) => ({ userId: c.userId, provider: "WHOOP" as const })),
+        ...withings.map((c) => ({
+          userId: c.userId,
+          provider: "WITHINGS" as const,
+        })),
+      ];
+
+    if (targets.length === 0) {
+      return { enqueued: 0, skipped: 0, error: null };
+    }
+
+    let enqueued = 0;
+    let skipped = 0;
+    for (const { userId, provider } of targets) {
+      const payload: SleepTimelineBackfillPayload = {
+        userId,
+        provider,
+        enqueuedAt: new Date().toISOString(),
+      };
+      const jobId = await boss.send(SLEEP_TIMELINE_BACKFILL_QUEUE, payload, {
+        retryLimit: 3,
+        retryDelay: 60,
+        retryBackoff: true,
+        singletonKey: `sleep-timeline-backfill|${provider}|${userId}`,
+      });
+      if (jobId) {
+        enqueued += 1;
+      } else {
+        skipped += 1;
+      }
+    }
+    return { enqueued, skipped, error: null };
+  } catch (err) {
+    return {
+      enqueued: 0,
+      skipped: 0,
+      error: err instanceof Error ? err.message : String(err),
+    };
+  }
+}
diff --git a/src/lib/openapi/routes/measurements.ts b/src/lib/openapi/routes/measurements.ts
index 15de359a7..9dbb92663 100644
--- a/src/lib/openapi/routes/measurements.ts
+++ b/src/lib/openapi/routes/measurements.ts
@@ -200,6 +200,11 @@ const sleepSessionResource = z.object({
   inBedMinutes: z.number().int().nonnegative().nullable(),
   awakeMinutes: z.number().int().nonnegative().nullable(),
   awakenings: z.number().int().nonnegative(),
+  reconstructed: z
+    .boolean()
+    .describe(
+      "True when the source has no per-stage onset timestamps and the server synthesised a contiguous timeline in a fixed physiological order (WHOOP). The client renders the hypnogram but labels it an approximate layout and never recomputes. False for real-series sources (Apple Health / Withings / Fitbit).",
+    ),
   stages: z.record(sleepStageEnum, z.number().int().nonnegative()),
   segments: z.array(sleepSegmentResource),
 });
diff --git a/src/lib/whoop/__tests__/client.test.ts b/src/lib/whoop/__tests__/client.test.ts
index 74d3f12bb..385ce9008 100644
--- a/src/lib/whoop/__tests__/client.test.ts
+++ b/src/lib/whoop/__tests__/client.test.ts
@@ -320,15 +320,64 @@ describe("mapSleep", () => {
   it("maps per-stage SLEEP_DURATION rows (ms→min) with sleepStage", () => {
     const rows = mapSleep(base);
     const dur = rows.filter((r) => r.type === "SLEEP_DURATION");
-    const byStage = Object.fromEntries(dur.map((r) => [r.sleepStage, r.value]));
-    expect(byStage.CORE).toBe(240); // light → CORE
-    expect(byStage.DEEP).toBe(90); // slow-wave → DEEP
-    expect(byStage.REM).toBe(120);
-    expect(byStage.AWAKE).toBe(30);
-    expect(byStage.IN_BED).toBe(480);
+    // One asleep/awake row per reconstructed segment + one IN_BED envelope. The
+    // per-stage TOTALS are unchanged (the reconstruction only reorders timing).
+    const minutesByStage: Record<string, number> = {};
+    for (const r of dur) {
+      minutesByStage[r.sleepStage!] =
+        (minutesByStage[r.sleepStage!] ?? 0) + r.value;
+    }
+    expect(minutesByStage.CORE).toBe(240); // light → CORE
+    expect(minutesByStage.DEEP).toBe(90); // slow-wave → DEEP
+    expect(minutesByStage.REM).toBe(120);
+    expect(minutesByStage.AWAKE).toBe(30);
+    expect(minutesByStage.IN_BED).toBe(480);
     expect(dur.every((r) => r.unit === "minutes")).toBe(true);
   });
 
+  it("reconstructs an ordered, contiguous, non-overlapping per-segment timeline from sleep ONSET", () => {
+    const segs = mapSleep(base)
+      .filter((r) => r.type === "SLEEP_DURATION" && r.sleepStage !== "IN_BED")
+      .map((r) => ({
+        stage: r.sleepStage,
+        end: r.measuredAt.getTime(),
+        start: r.measuredAt.getTime() - r.value * 60_000,
+        reconstructed: r.reconstructed,
+      }))
+      .sort((a, b) => a.start - b.start);
+
+    // AWAKE (lead) → CORE → DEEP → REM, laid back-to-back.
+    expect(segs.map((s) => s.stage)).toEqual(["AWAKE", "CORE", "DEEP", "REM"]);
+    // First segment starts at sleep ONSET (s.start), not the END edge.
+    expect(segs[0].start).toBe(new Date(base.start).getTime());
+    // Contiguous + non-overlapping: each segment's start == previous end.
+    for (let i = 1; i < segs.length; i++) {
+      expect(segs[i].start).toBe(segs[i - 1].end);
+    }
+    // Distinct END instants — no stacking on the night's right edge.
+    expect(new Set(segs.map((s) => s.end)).size).toBe(segs.length);
+    // Every reconstructed row is flagged so the UI labels the night approximate.
+    expect(segs.every((s) => s.reconstructed === true)).toBe(true);
+  });
+
+  it("emits IN_BED as a single un-reconstructed envelope row over [start, end]", () => {
+    const inBed = mapSleep(base).filter((r) => r.sleepStage === "IN_BED");
+    expect(inBed).toHaveLength(1);
+    const r = inBed[0]!;
+    expect(r.value).toBe(480);
+    // measuredAt = sleep END → segmentOf resolves the span back to the window.
+    expect(r.measuredAt.toISOString()).toBe(base.end);
+    expect(r.reconstructed).toBeUndefined();
+  });
+
+  it("gives each reconstructed segment a distinct indexed externalId", () => {
+    const ids = mapSleep(base)
+      .filter((r) => r.reconstructed)
+      .map((r) => r.externalId);
+    expect(ids.every((id) => id?.startsWith("sleep-uuid:seg:"))).toBe(true);
+    expect(new Set(ids).size).toBe(ids.length);
+  });
+
   it("sums SLEEP_NEED components ms→min", () => {
     const need = mapSleep(base).find((r) => r.type === "SLEEP_NEED");
     expect(need?.value).toBe(490); // 450 + 30 + 10 + 0
@@ -344,9 +393,9 @@ describe("mapSleep", () => {
     expect(byType.RESPIRATORY_RATE?.unit).toBe("breaths/min");
   });
 
-  it("uses sleep.end as measuredAt", () => {
-    const row = mapSleep(base)[0]!;
-    expect(row.measuredAt.toISOString()).toBe("2026-06-01T07:00:00.000Z");
+  it("stamps the non-segment scores on sleep.end", () => {
+    const need = mapSleep(base).find((r) => r.type === "SLEEP_NEED")!;
+    expect(need.measuredAt.toISOString()).toBe("2026-06-01T07:00:00.000Z");
   });
 
   it("emits nothing for an unscored sleep", () => {
diff --git a/src/lib/whoop/client.ts b/src/lib/whoop/client.ts
index a75ad1e06..19c30ddc7 100644
--- a/src/lib/whoop/client.ts
+++ b/src/lib/whoop/client.ts
@@ -505,20 +505,24 @@ export interface MappedMeasurement {
   fieldTag: string;
   /** Per-stage sleep rows carry the SleepStage; everything else omits it. */
   sleepStage?: "CORE" | "DEEP" | "REM" | "AWAKE" | "IN_BED";
+  /**
+   * Full externalId for rows the mapper must key itself rather than letting
+   * the sync layer build `<resource-uuid>:<fieldTag>`. The reconstructed sleep
+   * segments (one row per synthetic span) need an index in the key so the
+   * several rows of one stage stay distinct under
+   * `userId_type_source_externalId`. When set the sync layer uses it verbatim.
+   */
+  externalId?: string;
+  /**
+   * `true` on the synthesised per-segment sleep rows whose ORDER is invented:
+   * WHOOP's v2 API returns only per-stage duration totals (no onset
+   * timestamps), so the timeline is reconstructed in a fixed physiological
+   * order. The night is flagged so the UI labels it as an approximate layout
+   * and never presents it as measured stage timing.
+   */
+  reconstructed?: boolean;
 }
 
-/** WHOOP sleep stage → HealthLog SleepStage (light→CORE, slow-wave→DEEP). */
-const SLEEP_STAGE_MAP: Record<
-  string,
-  { stage: MappedMeasurement["sleepStage"]; fieldTag: string }
-> = {
-  total_light_sleep_time_milli: { stage: "CORE", fieldTag: "sleep_core" },
-  total_slow_wave_sleep_time_milli: { stage: "DEEP", fieldTag: "sleep_deep" },
-  total_rem_sleep_time_milli: { stage: "REM", fieldTag: "sleep_rem" },
-  total_awake_time_milli: { stage: "AWAKE", fieldTag: "sleep_awake" },
-  total_in_bed_time_milli: { stage: "IN_BED", fieldTag: "sleep_in_bed" },
-};
-
 function round2(n: number): number {
   return parseFloat(n.toFixed(2));
 }
@@ -589,16 +593,71 @@ export function mapSleep(s: WhoopSleep): MappedMeasurement[] {
   const out: MappedMeasurement[] = [];
 
   const stages = s.score.stage_summary;
-  for (const [key, mapping] of Object.entries(SLEEP_STAGE_MAP)) {
-    const ms = stages[key as keyof typeof stages];
-    if (typeof ms !== "number") continue;
+
+  // WHOOP v2 returns only per-stage DURATION totals (no per-stage onset
+  // timestamps, no hypnogram endpoint). Stamping every stage total on the one
+  // sleep-END instant makes the hypnogram reconstruct five spans that all touch
+  // the night's right edge (the stacked-bar artefact). Since the API never
+  // carries an order, we RECONSTRUCT an ordered, contiguous timeline: lay the
+  // asleep stages back-to-back from sleep ONSET in a fixed physiological order,
+  // emitting one timed row per segment with `measuredAt = that segment's END`.
+  // The ORDER is synthetic, so every reconstructed row is flagged
+  // `reconstructed: true` and the night DTO advertises it — the UI labels it as
+  // an approximate layout and never presents it as measured stage timing.
+  //
+  // IN_BED stays a single envelope row spanning the whole [start, end] window:
+  // the in-bed reader wants the union envelope, not a placed segment. AWAKE is
+  // laid as a leading segment from onset so the asleep stages still partition
+  // the remaining window.
+  const onset = new Date(s.start).getTime();
+  // Order the laid-out stages: a brief AWAKE settling-in block, then the asleep
+  // stages CORE → DEEP → REM. Each carries its own duration; the cursor walks
+  // forward so the spans are contiguous and non-overlapping.
+  const TIMELINE_ORDER: ReadonlyArray<{
+    key: keyof typeof stages;
+    stage: NonNullable<MappedMeasurement["sleepStage"]>;
+    tag: string;
+  }> = [
+    { key: "total_awake_time_milli", stage: "AWAKE", tag: "sleep_awake" },
+    { key: "total_light_sleep_time_milli", stage: "CORE", tag: "sleep_core" },
+    { key: "total_slow_wave_sleep_time_milli", stage: "DEEP", tag: "sleep_deep" },
+    { key: "total_rem_sleep_time_milli", stage: "REM", tag: "sleep_rem" },
+  ];
+  let cursor = onset;
+  let segIndex = 0;
+  for (const { key, stage, tag } of TIMELINE_ORDER) {
+    const ms = stages[key];
+    if (typeof ms !== "number" || ms <= 0) continue;
+    const segEnd = cursor + ms;
     out.push({
       type: "SLEEP_DURATION",
       value: round2(ms * MS_TO_MIN),
       unit: "minutes",
+      measuredAt: new Date(segEnd),
+      // The sync layer keys reconstructed rows by this externalId verbatim; the
+      // index keeps the several rows of one night distinct under
+      // userId_type_source_externalId.
+      fieldTag: tag,
+      externalId: `${s.id}:seg:${tag}:${segIndex}`,
+      sleepStage: stage,
+      reconstructed: true,
+    });
+    cursor = segEnd;
+    segIndex += 1;
+  }
+
+  // IN_BED — single envelope row over the whole sleep window. `measuredAt` is
+  // the sleep END so `segmentOf` resolves the span back to [start, end]; the
+  // in-bed union-envelope reader consumes the full window.
+  const inBedMs = stages.total_in_bed_time_milli;
+  if (typeof inBedMs === "number" && inBedMs > 0) {
+    out.push({
+      type: "SLEEP_DURATION",
+      value: round2(inBedMs * MS_TO_MIN),
+      unit: "minutes",
       measuredAt,
-      fieldTag: mapping.fieldTag,
-      sleepStage: mapping.stage,
+      fieldTag: "sleep_in_bed",
+      sleepStage: "IN_BED",
     });
   }
 
diff --git a/src/lib/whoop/sync-sleep.ts b/src/lib/whoop/sync-sleep.ts
index 234052ad0..a97173178 100644
--- a/src/lib/whoop/sync-sleep.ts
+++ b/src/lib/whoop/sync-sleep.ts
@@ -4,8 +4,14 @@
  * `mapSleep` (per-stage SLEEP_DURATION rows, SLEEP_NEED, the SLEEP_*
  * percentages, RESPIRATORY_RATE), and upserts as `source = WHOOP`.
  *
- * Per-stage rows carry the `sleepStage` axis so the five stage rows for one
- * night stay distinct under the dedup key. externalId = `<sleep_id>:<fieldTag>`.
+ * Sleep rows carry the `sleepStage` axis so the night's rows stay distinct
+ * under the dedup key. WHOOP exposes only per-stage DURATION totals (no onset
+ * timestamps), so `mapSleep` RECONSTRUCTS an ordered, contiguous per-segment
+ * timeline (CORE/DEEP/REM/AWAKE laid back-to-back from sleep onset) and flags
+ * those rows `reconstructed`; each reconstructed segment carries its own
+ * indexed externalId `<sleep_id>:seg:<tag>:<i>`. IN_BED stays a single
+ * envelope row keyed `<sleep_id>:sleep_in_bed`. The non-segment scores keep
+ * the `<sleep_id>:<fieldTag>` shape.
  */
 import { fetchSleeps, fetchSleepById, mapSleep } from "./client";
 import {
@@ -53,7 +59,10 @@ export async function syncUserSleep(
         value: m.value,
         unit: m.unit,
         measuredAt: m.measuredAt,
-        externalId: `${s.id}:${m.fieldTag}`,
+        // Reconstructed sleep segments carry their own indexed externalId so
+        // the several rows of one stage stay distinct; everything else keeps
+        // the `<sleep_id>:<fieldTag>` shape.
+        externalId: m.externalId ?? `${s.id}:${m.fieldTag}`,
         sleepStage: m.sleepStage ?? null,
       });
     }
@@ -92,7 +101,9 @@ export async function syncWhoopSleepById(
       value: m.value,
       unit: m.unit,
       measuredAt: m.measuredAt,
-      externalId: `${record.id}:${m.fieldTag}`,
+      // Reconstructed sleep segments carry their own indexed externalId
+      // (see syncUserSleep).
+      externalId: m.externalId ?? `${record.id}:${m.fieldTag}`,
       sleepStage: m.sleepStage ?? null,
     });
   }
diff --git a/src/lib/withings/__tests__/sync-sleep.test.ts b/src/lib/withings/__tests__/sync-sleep.test.ts
index 28757ebd0..6ae04f3a1 100644
--- a/src/lib/withings/__tests__/sync-sleep.test.ts
+++ b/src/lib/withings/__tests__/sync-sleep.test.ts
@@ -194,7 +194,7 @@ describe("syncUserSleep — segment writes + idempotency", () => {
     expect(firstDuration.data.unit).toBe("minutes");
   });
 
-  it("converts startdate (unix seconds) to a Date in measuredAt", async () => {
+  it("stamps measuredAt at the segment END (enddate, unix seconds → Date)", async () => {
     installFetchMock([
       { startdate: 1715000000, enddate: 1715003600, state: 2, id: 1 },
     ]);
@@ -205,7 +205,10 @@ describe("syncUserSleep — segment writes + idempotency", () => {
     const arg = vi.mocked(prisma.measurement.create).mock.calls[0][0] as {
       data: { measuredAt: Date };
     };
-    expect(arg.data.measuredAt.getTime()).toBe(1715000000 * 1000);
+    // measuredAt is the segment END — every reader treats it as the END and
+    // resolves the span as start = end − duration. Stamping the START shifted
+    // the night one segment-length earlier.
+    expect(arg.data.measuredAt.getTime()).toBe(1715003600 * 1000);
   });
 
   it("skips state 4 (synthetic marker) without throwing", async () => {
diff --git a/src/lib/withings/sync-sleep.ts b/src/lib/withings/sync-sleep.ts
index 231d94f6b..4b92080d2 100644
--- a/src/lib/withings/sync-sleep.ts
+++ b/src/lib/withings/sync-sleep.ts
@@ -26,10 +26,13 @@
  * when Withings re-emits the night with adjusted segment boundaries.
  *
  * Date semantics: each segment's `measuredAt` is the segment's
- * `startdate` (unix seconds → `Date`). The duration in minutes lands
- * in `value`. The HealthLog analytics aggregator already groups
- * stage rows under their parent night via the per-night `dayKey`
- * helper, so the segment-level timestamp is the canonical sort key.
+ * `enddate` (unix seconds → `Date`). Every reader treats `measuredAt`
+ * as the segment END and resolves the span as `start = end − duration`
+ * (`sleep-night.ts` `segmentOf`); stamping the START shifted the whole
+ * night one segment-length earlier. The duration in minutes lands in
+ * `value`. The HealthLog analytics aggregator groups stage rows under
+ * their parent night via the per-night `dayKey` helper, so the
+ * segment-level END timestamp is the canonical sort key.
  */
 import type { MeasurementType, SleepStage } from "@/generated/prisma/client";
 
@@ -258,7 +261,11 @@ export async function syncUserSleep(
       segmentIndex++;
       continue;
     }
-    const measuredAt = new Date(segment.startdate * 1000);
+    // `measuredAt` is the segment END (`enddate`). Every reader treats
+    // `measuredAt` as the END instant — `segmentOf` resolves the span as
+    // `start = measuredAt − value·60_000` (sleep-night.ts). Stamping the START
+    // here shifted the whole night one segment-length earlier; END is correct.
+    const measuredAt = new Date(segment.enddate * 1000);
     const durationSec = Math.max(0, segment.enddate - segment.startdate);
     // Canonical SLEEP_DURATION unit is minutes (Apple Health alignment,
     // v1.4.23 schema note). Round so re-sync doesn't flip values on