diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index ad690c61f..046d46b73 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -41340,6 +41340,12 @@ components:
           type: integer
           minimum: 0
           maximum: 9007199254740991
+        windowFrom:
+          description: "Set on the per-night SLEEP_DURATION mode when the requested window was longer than 365 days: the instant
+            the read started from. The window is a year ending at `to` (or now); page further back by moving `to`."
+          type: string
+          format: date-time
+          pattern: ^(?:(?:\d\d[2468][048]|\d\d[13579][26]|\d\d0[48]|[02468][048]00|[13579][26]00)-02-29|\d{4}-(?:(?:0[13578]|1[02])-(?:0[1-9]|[12]\d|3[01])|(?:0[469]|11)-(?:0[1-9]|[12]\d|30)|(?:02)-(?:0[1-9]|1\d|2[0-8])))T(?:(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d(?:\.\d+)?)?(?:Z|([+-](?:[01]\d|2[0-3]):[0-5]\d)))$
       required:
         - total
         - limit
diff --git a/src/app/api/measurements/route.ts b/src/app/api/measurements/route.ts
index ffcaafece..278ddb899 100644
--- a/src/app/api/measurements/route.ts
+++ b/src/app/api/measurements/route.ts
@@ -543,9 +543,18 @@ async function sleepListResponse(
     loadUserSourcePriority(user.id),
   ]);
 
-  // Bound the read window: honour an explicit from/to, otherwise cap to
-  // the last year so the per-stage scan stays small.
-  const since = from ?? new Date(Date.now() - SLEEP_LIST_MAX_DAYS * 86_400_000);
+  // Bound the read window to a year ending at `to` (or now). An explicit
+  // `from` narrows it but never widens it: an early `from` used to walk every
+  // per-stage row back to that date, which on a multi-year, multi-source
+  // account is the unbounded read this cap exists to stop. Older nights are
+  // reached by moving `to` back, and `meta.windowFrom` says where a cut
+  // window started so a client can tell a cut from an empty history.
+  const windowEnd = to ?? new Date();
+  const floor = new Date(
+    windowEnd.getTime() - SLEEP_LIST_MAX_DAYS * 86_400_000,
+  );
+  const since = from && from > floor ? from : floor;
+  const windowCut = from !== undefined && from < floor;
   const rows = await prisma.measurement.findMany({
     where: {
       userId: user.id,
@@ -690,7 +699,13 @@ async function sleepListResponse(
   });
   return apiSuccess({
     measurements,
-    meta: { total: totalNights, limit, offset, groupBy: "night" },
+    meta: {
+      total: totalNights,
+      limit,
+      offset,
+      groupBy: "night",
+      ...(windowCut ? { windowFrom: since.toISOString() } : {}),
+    },
   });
 }
 
diff --git a/src/lib/openapi/routes/measurements.ts b/src/lib/openapi/routes/measurements.ts
index 864baeb4a..dea56ced2 100644
--- a/src/lib/openapi/routes/measurements.ts
+++ b/src/lib/openapi/routes/measurements.ts
@@ -389,6 +389,12 @@ const listMeasurementsMeta = z
       .describe(
         "Rows discarded by the cross-source canonical picker on the day-collapse mode.",
       ),
+    windowFrom: z.iso
+      .datetime({ offset: true })
+      .optional()
+      .describe(
+        "Set on the per-night SLEEP_DURATION mode when the requested window was longer than 365 days: the instant the read started from. The window is a year ending at `to` (or now); page further back by moving `to`.",
+      ),
   })
   .meta({ id: "ListMeasurementsMeta" });
 
diff --git a/tests/integration/measurements-sleep-list.test.ts b/tests/integration/measurements-sleep-list.test.ts
index 9e952c4dc..9524bd854 100644
--- a/tests/integration/measurements-sleep-list.test.ts
+++ b/tests/integration/measurements-sleep-list.test.ts
@@ -56,7 +56,7 @@ interface ListEnvelope {
       napCount?: number;
       awakenings?: number;
     }>;
-    meta: { total: number; groupBy?: string };
+    meta: { total: number; groupBy?: string; windowFrom?: string };
   };
 }
 
@@ -160,4 +160,61 @@ describe("GET /api/measurements?type=SLEEP_DURATION", () => {
     const stages = body.data.measurements.map((m) => m.sleepStage).sort();
     expect(stages).toEqual(["CORE", "DEEP", "REM"]);
   });
+
+  it("reads at most a year ending at `to`, however early `from` is", async () => {
+    // An explicit early `from` used to walk every per-stage row back to it.
+    const prisma = getPrismaClient();
+    const user = await seedSession("sleep-window-user");
+    const day = 86_400_000;
+    const morning = new Date();
+    morning.setUTCHours(4, 0, 0, 0);
+    // One night of a single CORE stage every day for two years.
+    await prisma.measurement.createMany({
+      data: Array.from({ length: 730 }, (_, i) => ({
+        userId: user.id,
+        type: "SLEEP_DURATION" as const,
+        value: 420,
+        unit: "minutes",
+        source: "APPLE_HEALTH" as const,
+        measuredAt: new Date(morning.getTime() - i * day),
+        externalId: `uuid-night-${i}`,
+        sleepStage: "CORE" as const,
+      })),
+    });
+    const spy = vi.spyOn(prisma.measurement, "findMany");
+    const from = new Date(morning.getTime() - 800 * day).toISOString();
+    const res = await callList(
+      `type=SLEEP_DURATION&from=${encodeURIComponent(from)}&limit=1000`,
+    );
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as ListEnvelope;
+    expect(body.data.meta.total).toBeLessThanOrEqual(366);
+    expect(body.data.meta.total).toBeGreaterThanOrEqual(364);
+    expect(body.data.meta.windowFrom).toBeDefined();
+    const windowFrom = Date.parse(body.data.meta.windowFrom!);
+    expect(Date.now() - windowFrom).toBeLessThan(366 * day);
+    expect(Date.now() - windowFrom).toBeGreaterThan(364 * day);
+    // The read itself was bounded, not just the answer.
+    const read = spy.mock.calls
+      .map(([args]) => args as { where?: { measuredAt?: { gte?: Date } } })
+      .find((args) => args?.where?.measuredAt?.gte);
+    expect(read?.where?.measuredAt?.gte?.getTime()).toBe(windowFrom);
+
+    // Older nights are reached by moving `to` back.
+    const to = new Date(morning.getTime() - 400 * day).toISOString();
+    const older = await callList(
+      `type=SLEEP_DURATION&from=${encodeURIComponent(from)}&to=${encodeURIComponent(to)}&limit=1000`,
+    );
+    const olderBody = (await older.json()) as ListEnvelope;
+    expect(olderBody.data.meta.total).toBeGreaterThanOrEqual(329);
+    expect(olderBody.data.meta.total).toBeLessThanOrEqual(331);
+
+    // A window inside the year is untouched and carries no marker.
+    const recent = await callList(
+      `type=SLEEP_DURATION&from=${encodeURIComponent(new Date(morning.getTime() - 10.5 * day).toISOString())}`,
+    );
+    const recentBody = (await recent.json()) as ListEnvelope;
+    expect(recentBody.data.meta.total).toBe(11);
+    expect(recentBody.data.meta.windowFrom).toBeUndefined();
+  });
 });