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