diff --git a/src/lib/oura/__tests__/client.test.ts b/src/lib/oura/__tests__/client.test.ts
index 6a44695ae..1d51c364d 100644
--- a/src/lib/oura/__tests__/client.test.ts
+++ b/src/lib/oura/__tests__/client.test.ts
@@ -3,8 +3,10 @@ import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import {
   OURA_OAUTH_SCOPE,
   RESILIENCE_LEVELS,
+  derivePeriodDaysFromCyclePhases,
   exchangeCode,
   fetchCardiovascularAge,
+  fetchDailyCyclePhases,
   fetchReadiness,
   fetchResilience,
   fetchVo2Max,
@@ -524,3 +526,101 @@ describe("OURA_OAUTH_SCOPE", () => {
     expect(OURA_OAUTH_SCOPE).toBe("daily personal");
   });
 });
+
+describe("fetchDailyCyclePhases", () => {
+  it("reads the daily_cycle_phases collection path", async () => {
+    const fetchMock = installFetchMock([
+      {
+        status: 200,
+        body: {
+          data: [{ id: "1", day: "2026-06-10", phase: "follicular" }],
+          next_token: null,
+        },
+      },
+    ]);
+    const r = await fetchDailyCyclePhases("tok", {
+      startDate: "2026-06-01",
+      endDate: "2026-06-10",
+    });
+    expect(r).toHaveLength(1);
+    const [url] = fetchMock.mock.calls[0]!;
+    expect(url).toContain("/v2/usercollection/daily_cycle_phases");
+  });
+
+  it("propagates a 403 (the undocumented / scope-gated case) as an OuraApiError", async () => {
+    installFetchMock([{ status: 403, body: { detail: "forbidden" } }]);
+    await expect(
+      fetchDailyCyclePhases("tok", {
+        startDate: "2026-06-01",
+        endDate: "2026-06-10",
+      }),
+    ).rejects.toBeInstanceOf(OuraApiError);
+  });
+});
+
+describe("derivePeriodDaysFromCyclePhases", () => {
+  it("marks a literal 'menstrual' phase day directly", () => {
+    const days = derivePeriodDaysFromCyclePhases([
+      { day: "2026-06-05", phase: "menstrual" },
+      { day: "2026-06-10", phase: "follicular" },
+    ]);
+    expect(days).toEqual(["2026-06-05"]);
+  });
+
+  it("marks the single day a luteal phase transitions into a follicular phase", () => {
+    const days = derivePeriodDaysFromCyclePhases([
+      { day: "2026-06-08", phase: "luteal" },
+      { day: "2026-06-09", phase: "luteal" },
+      { day: "2026-06-10", phase: "follicular" },
+      { day: "2026-06-11", phase: "follicular" },
+    ]);
+    expect(days).toEqual(["2026-06-10"]);
+  });
+
+  it("does NOT mark a luteal→follicular transition across a gap in the series", () => {
+    const days = derivePeriodDaysFromCyclePhases([
+      { day: "2026-06-08", phase: "luteal" },
+      // 2026-06-09 missing — not calendar-adjacent.
+      { day: "2026-06-10", phase: "follicular" },
+    ]);
+    expect(days).toEqual([]);
+  });
+
+  it("is order-independent — sorts records by day before scanning", () => {
+    const days = derivePeriodDaysFromCyclePhases([
+      { day: "2026-06-10", phase: "follicular" },
+      { day: "2026-06-09", phase: "luteal" },
+    ]);
+    expect(days).toEqual(["2026-06-10"]);
+  });
+
+  it("ignores an unrecognised / missing phase string (never guesses)", () => {
+    const days = derivePeriodDaysFromCyclePhases([
+      { day: "2026-06-08", phase: "luteal" },
+      { day: "2026-06-09", phase: "unknown_future_phase" },
+      { day: "2026-06-10", phase: "follicular" },
+    ]);
+    // The unrecognised phase still updates the "previous phase" tracker (to
+    // a value that is neither luteal nor follicular), so the 06-09→06-10
+    // step is no longer a luteal→follicular transition and nothing fires.
+    expect(days).toEqual([]);
+  });
+
+  it("returns [] for an empty or all-ovulatory/all-luteal series (no transition, no direct hit)", () => {
+    expect(derivePeriodDaysFromCyclePhases([])).toEqual([]);
+    expect(
+      derivePeriodDaysFromCyclePhases([
+        { day: "2026-06-08", phase: "luteal" },
+        { day: "2026-06-09", phase: "luteal" },
+      ]),
+    ).toEqual([]);
+  });
+
+  it("skips records with a missing/empty day string", () => {
+    const days = derivePeriodDaysFromCyclePhases([
+      { day: "", phase: "menstrual" },
+      { day: "2026-06-10", phase: "menstrual" },
+    ] as never);
+    expect(days).toEqual(["2026-06-10"]);
+  });
+});
diff --git a/src/lib/oura/__tests__/cycle-sync.test.ts b/src/lib/oura/__tests__/cycle-sync.test.ts
new file mode 100644
index 000000000..ef732bbf4
--- /dev/null
+++ b/src/lib/oura/__tests__/cycle-sync.test.ts
@@ -0,0 +1,166 @@
+/**
+ * `syncUserOuraCyclePhases` — Oura Cycle Insights → CycleDayLog, subordinate
+ * to any existing row. The core invariant under test: a day that already
+ * carries ANY CycleDayLog row (regardless of source) is never touched —
+ * Oura only ever fills a genuinely empty day.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const { fetchCyclePhasesMock, isCycleAvailableMock, findUserMock, createMock } =
+  vi.hoisted(() => ({
+    fetchCyclePhasesMock: vi.fn(),
+    isCycleAvailableMock: vi.fn(),
+    findUserMock: vi.fn(),
+    createMock: vi.fn(),
+  }));
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    user: { findUnique: findUserMock },
+    cycleDayLog: { create: createMock },
+  },
+}));
+
+vi.mock("@/lib/cycle/gate", () => ({
+  isCycleAvailableForUser: isCycleAvailableMock,
+}));
+
+vi.mock("../client", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../client")>();
+  return {
+    ...actual,
+    fetchDailyCyclePhases: fetchCyclePhasesMock,
+  };
+});
+
+import { syncUserOuraCyclePhases } from "../cycle-sync";
+
+function uniqueViolation() {
+  const err = new Error("Unique constraint failed") as Error & {
+    code: string;
+  };
+  err.code = "P2002";
+  return err;
+}
+
+beforeEach(() => {
+  fetchCyclePhasesMock.mockReset().mockResolvedValue([]);
+  isCycleAvailableMock.mockReset().mockResolvedValue(true);
+  findUserMock.mockReset().mockResolvedValue({ timezone: "Europe/Berlin" });
+  createMock.mockReset().mockResolvedValue({ id: "cdl-1" });
+});
+
+describe("syncUserOuraCyclePhases", () => {
+  it("no-ops without touching the DB when the collection is empty", async () => {
+    const created = await syncUserOuraCyclePhases("u1", "tok", 7);
+    expect(created).toBe(0);
+    expect(isCycleAvailableMock).not.toHaveBeenCalled();
+    expect(createMock).not.toHaveBeenCalled();
+  });
+
+  it("no-ops when the series carries no derivable period day", async () => {
+    fetchCyclePhasesMock.mockResolvedValue([
+      { day: "2026-06-08", phase: "luteal" },
+      { day: "2026-06-09", phase: "luteal" },
+    ]);
+    const created = await syncUserOuraCyclePhases("u1", "tok", 7);
+    expect(created).toBe(0);
+    expect(isCycleAvailableMock).not.toHaveBeenCalled();
+  });
+
+  it("respects the two-layer cycle gate — no write when disabled", async () => {
+    fetchCyclePhasesMock.mockResolvedValue([
+      { day: "2026-06-05", phase: "menstrual" },
+    ]);
+    isCycleAvailableMock.mockResolvedValue(false);
+    const created = await syncUserOuraCyclePhases("u1", "tok", 7);
+    expect(created).toBe(0);
+    expect(createMock).not.toHaveBeenCalled();
+  });
+
+  it("creates a source=OURA flow=LIGHT row for a detected period day", async () => {
+    fetchCyclePhasesMock.mockResolvedValue([
+      { day: "2026-06-05", phase: "menstrual" },
+    ]);
+    const created = await syncUserOuraCyclePhases("u1", "tok", 7);
+    expect(created).toBe(1);
+    expect(createMock).toHaveBeenCalledWith({
+      data: {
+        userId: "u1",
+        date: "2026-06-05",
+        tz: "Europe/Berlin",
+        source: "OURA",
+        flow: "LIGHT",
+      },
+    });
+  });
+
+  it("skips a day that already has ANY existing row (manual wins) — unique-violation swallowed silently", async () => {
+    fetchCyclePhasesMock.mockResolvedValue([
+      { day: "2026-06-05", phase: "menstrual" },
+    ]);
+    createMock.mockRejectedValue(uniqueViolation());
+    const created = await syncUserOuraCyclePhases("u1", "tok", 7);
+    expect(created).toBe(0);
+  });
+
+  it("never calls .update — a pre-existing row (its own or another source's) is left exactly as-is", async () => {
+    fetchCyclePhasesMock.mockResolvedValue([
+      { day: "2026-06-05", phase: "menstrual" },
+      { day: "2026-06-06", phase: "menstrual" },
+    ]);
+    createMock
+      .mockRejectedValueOnce(uniqueViolation()) // 06-05 already logged (manual)
+      .mockResolvedValueOnce({ id: "cdl-2" }); // 06-06 genuinely empty
+    const created = await syncUserOuraCyclePhases("u1", "tok", 7);
+    expect(created).toBe(1);
+    expect(createMock).toHaveBeenCalledTimes(2);
+  });
+
+  it("propagates a non-unique-violation write error without crashing the whole batch", async () => {
+    fetchCyclePhasesMock.mockResolvedValue([
+      { day: "2026-06-05", phase: "menstrual" },
+      { day: "2026-06-06", phase: "menstrual" },
+    ]);
+    createMock
+      .mockRejectedValueOnce(new Error("connection reset"))
+      .mockResolvedValueOnce({ id: "cdl-2" });
+    const created = await syncUserOuraCyclePhases("u1", "tok", 7);
+    // The failing day is skipped (warned, not thrown); the healthy sibling
+    // still writes.
+    expect(created).toBe(1);
+  });
+
+  it("propagates the Oura fetch error to the caller (the sync layer decides how to swallow it)", async () => {
+    fetchCyclePhasesMock.mockRejectedValue(new Error("403 forbidden"));
+    await expect(syncUserOuraCyclePhases("u1", "tok", 7)).rejects.toThrow(
+      "403 forbidden",
+    );
+    expect(createMock).not.toHaveBeenCalled();
+  });
+
+  it("falls back to a null tz when the user row / timezone is unresolvable", async () => {
+    fetchCyclePhasesMock.mockResolvedValue([
+      { day: "2026-06-05", phase: "menstrual" },
+    ]);
+    findUserMock.mockResolvedValue(null);
+    await syncUserOuraCyclePhases("u1", "tok", 7);
+    expect(createMock).toHaveBeenCalledWith(
+      expect.objectContaining({ data: expect.objectContaining({ tz: null }) }),
+    );
+  });
+
+  it("creates one row per distinct detected day from a luteal→follicular transition", async () => {
+    fetchCyclePhasesMock.mockResolvedValue([
+      { day: "2026-06-08", phase: "luteal" },
+      { day: "2026-06-09", phase: "follicular" },
+    ]);
+    const created = await syncUserOuraCyclePhases("u1", "tok", 7);
+    expect(created).toBe(1);
+    expect(createMock).toHaveBeenCalledWith(
+      expect.objectContaining({
+        data: expect.objectContaining({ date: "2026-06-09", flow: "LIGHT" }),
+      }),
+    );
+  });
+});
diff --git a/src/lib/oura/__tests__/sync.test.ts b/src/lib/oura/__tests__/sync.test.ts
index a10533017..7738bc43a 100644
--- a/src/lib/oura/__tests__/sync.test.ts
+++ b/src/lib/oura/__tests__/sync.test.ts
@@ -12,6 +12,7 @@ const {
   fetchVo2MaxMock,
   fetchCardioAgeMock,
   fetchResilienceMock,
+  fetchCyclePhasesMock,
   refreshMock,
   upsertMock,
   updateManyMock,
@@ -31,6 +32,7 @@ const {
   fetchVo2MaxMock: vi.fn(),
   fetchCardioAgeMock: vi.fn(),
   fetchResilienceMock: vi.fn(),
+  fetchCyclePhasesMock: vi.fn(),
   refreshMock: vi.fn(),
   upsertMock: vi.fn(),
   updateManyMock: vi.fn(),
@@ -78,6 +80,7 @@ vi.mock("../client", async (importOriginal) => {
     fetchVo2Max: fetchVo2MaxMock,
     fetchCardiovascularAge: fetchCardioAgeMock,
     fetchResilience: fetchResilienceMock,
+    fetchDailyCyclePhases: fetchCyclePhasesMock,
     refreshAccessToken: refreshMock,
   };
 });
@@ -107,6 +110,7 @@ beforeEach(() => {
   fetchVo2MaxMock.mockReset().mockResolvedValue([]);
   fetchCardioAgeMock.mockReset().mockResolvedValue([]);
   fetchResilienceMock.mockReset().mockResolvedValue([]);
+  fetchCyclePhasesMock.mockReset().mockResolvedValue([]);
   refreshMock.mockReset();
   upsertMock.mockReset().mockResolvedValue({});
   updateManyMock.mockReset().mockResolvedValue({ count: 0 });
@@ -479,4 +483,51 @@ describe("syncUserOura", () => {
     expect(recordFailureMock).toHaveBeenCalled();
     expect(recordSuccessMock).not.toHaveBeenCalled();
   });
+
+  it("degrades gracefully when daily_cycle_phases 403s — the measurement sync still succeeds (F4/cycle)", async () => {
+    getConnMock.mockResolvedValue(CONN);
+    fetchReadinessMock.mockResolvedValue([
+      { id: "1", day: "2026-06-10", score: 70 },
+    ]);
+    // The endpoint is undocumented / gated for most connections — a 403 here
+    // must never surface as a whole-connection reauth prompt.
+    fetchCyclePhasesMock.mockRejectedValue(
+      new OuraApiError({
+        verb: "fetchDailyCyclePhases",
+        classification: "reauth_required",
+        httpStatus: 403,
+        reason: "http_403",
+      }),
+    );
+    const imported = await syncUserOura("u1");
+    expect(imported).toBe(1);
+    expect(recordSuccessMock).toHaveBeenCalledWith("u1", "oura");
+    expect(recordFailureMock).not.toHaveBeenCalled();
+  });
+
+  it("re-reads the connection for the cycle-phases fetch (picks up a rotated token)", async () => {
+    getConnMock.mockResolvedValue(CONN);
+    await syncUserOura("u1");
+    expect(fetchCyclePhasesMock).toHaveBeenCalledWith(
+      CONN.accessToken,
+      expect.objectContaining({
+        startDate: expect.any(String),
+        endDate: expect.any(String),
+      }),
+    );
+    // Once for the measurement-sync token, once more for the cycle-phases
+    // fetch's fresh read.
+    expect(getConnMock.mock.calls.length).toBeGreaterThanOrEqual(2);
+  });
+
+  it("a cycle-phases fetch error never blocks morning-refresh / rollup tail work", async () => {
+    getConnMock.mockResolvedValue(CONN);
+    fetchDailySleepMock.mockResolvedValue([
+      { id: "s", day: "2026-06-10", score: 80 },
+    ]);
+    fetchCyclePhasesMock.mockRejectedValue(new Error("boom"));
+    const imported = await syncUserOura("u1");
+    expect(imported).toBe(1);
+    expect(recordSuccessMock).toHaveBeenCalledWith("u1", "oura");
+  });
 });
diff --git a/src/lib/oura/client.ts b/src/lib/oura/client.ts
index 2081bfbc5..460fddfa4 100644
--- a/src/lib/oura/client.ts
+++ b/src/lib/oura/client.ts
@@ -15,6 +15,7 @@
  */
 import { getEvent } from "@/lib/logging/context";
 import { canonicalDailyTimestamp } from "@/lib/measurements/consolidation-tz";
+import { dayDiff } from "@/lib/cycle/day-math";
 import { safeFetch } from "@/lib/safe-fetch";
 import { OuraApiError, classifyOuraResponse } from "./response-classifier";
 
@@ -256,6 +257,37 @@ export interface OuraResilience {
   level?: string | null;
 }
 
+/**
+ * Oura `daily_cycle_phases` — the per-day Cycle Insights phase estimate
+ * (temperature-deviation-derived). UNDOCUMENTED in the published OpenAPI
+ * spec at `cloud.ouraring.com/v2/docs` as of this writing — the route was
+ * confirmed live against the production API (`GET
+ * /v2/usercollection/daily_cycle_phases` 400s "Token is missing" identically
+ * to every documented `daily_*` collection when called unauthenticated, vs a
+ * genuine 404 for an unknown path) but sits outside the spec dump alongside
+ * every other Oura reproductive-health resource (`period_start`,
+ * `pregnancy`, `fertile_window`, `ovulation_confirmed`, `blood_glucose` — all
+ * listed as valid webhook subscription data types, none with a documented
+ * read schema). These look gated behind an approval Oura does not extend to
+ * a self-registered OAuth app by default, so a 403/404 here is the EXPECTED
+ * outcome for most connections, not an anomaly — see `syncUserOuraCyclePhases`
+ * in `./cycle-sync` for how that is swallowed without touching the rest of
+ * the Oura connection's health status. Re-verify the `phase` field name +
+ * value set against `cloud.ouraring.com/v2/docs` once Oura documents it.
+ */
+export interface OuraCyclePhase {
+  id?: string;
+  day: string;
+  /**
+   * Oura's public Cycle Insights copy describes exactly two ring-observable
+   * phases (`follicular` / `luteal`, temperature-derived); some algorithm
+   * versions may additionally emit `menstrual` directly. Kept as a loose
+   * string — an unrecognised / missing value is skipped, never guessed (see
+   * `derivePeriodDaysFromCyclePhases`).
+   */
+  phase?: string | null;
+}
+
 export interface OuraDailyActivity {
   id: string;
   day: string;
@@ -429,6 +461,19 @@ export function fetchResilience(
   );
 }
 
+/** See the `OuraCyclePhase` docstring for the endpoint's documentation gap. */
+export function fetchDailyCyclePhases(
+  accessToken: string,
+  query: DateRangeQuery,
+): Promise<OuraCyclePhase[]> {
+  return fetchCollection<OuraCyclePhase>(
+    "/v2/usercollection/daily_cycle_phases",
+    accessToken,
+    "fetchDailyCyclePhases",
+    query,
+  );
+}
+
 // ─── Field → Measurement mapping ───────────────────────────────
 
 const SEC_TO_MIN = 1 / 60;
@@ -832,3 +877,69 @@ export function mapResilience(r: OuraResilience): MappedMeasurement[] {
     },
   ];
 }
+
+// ─── Cycle-phase → CycleDayLog mapping (v1.29.x) ───────────────────
+//
+// `daily_cycle_phases` does NOT feed `Measurement` like every collection
+// above — it feeds the cycle tracker's `CycleDayLog` via
+// `syncUserOuraCyclePhases` in `./cycle-sync`, which owns the write-side
+// "manual always wins" guard. This module only derives WHICH days look like
+// a period day; it never touches the database.
+
+const OURA_MENSTRUAL_PHASE = "menstrual";
+const OURA_FOLLICULAR_PHASE = "follicular";
+const OURA_LUTEAL_PHASE = "luteal";
+
+/**
+ * Derive the (sparse) set of `YYYY-MM-DD` days that look like a period day
+ * from a `daily_cycle_phases` series. The endpoint reports only the PHASE,
+ * not menstrual flow directly, so a period day is inferred two ways:
+ *
+ *   1. A record whose `phase` is literally `"menstrual"` — some algorithm
+ *      versions / accounts may emit this directly.
+ *   2. The single day a `"luteal"` phase is immediately (calendar-adjacent)
+ *      followed by a `"follicular"` phase. Oura's own definition of the
+ *      follicular phase is "from the first day of menstruation to
+ *      ovulation", so this transition day is the best period-start signal
+ *      the two-phase (follicular/luteal) shape carries.
+ *
+ * Records are sorted by `day` internally (the collection's page order is not
+ * guaranteed to be chronological). Returns bare day strings — the caller
+ * writes each as a single-day conservative `flow: LIGHT` marker (the same
+ * "logged but unspecified intensity" boundary the HealthKit `unspecified`
+ * flow mapping uses), never a multi-day bleed span this endpoint gives no
+ * evidence for.
+ */
+export function derivePeriodDaysFromCyclePhases(
+  records: readonly OuraCyclePhase[],
+): string[] {
+  const sorted = [...records]
+    .filter((r) => typeof r.day === "string" && r.day.length > 0)
+    .sort((a, b) => dayDiff(a.day, b.day));
+
+  const days = new Set<string>();
+  let prevPhase: string | null = null;
+  let prevDay: string | null = null;
+
+  for (const r of sorted) {
+    const phase = typeof r.phase === "string" ? r.phase.toLowerCase() : null;
+
+    if (phase === OURA_MENSTRUAL_PHASE) {
+      days.add(r.day);
+    } else if (
+      phase === OURA_FOLLICULAR_PHASE &&
+      prevPhase === OURA_LUTEAL_PHASE &&
+      prevDay !== null &&
+      dayDiff(r.day, prevDay) === 1
+    ) {
+      days.add(r.day);
+    }
+
+    if (phase) {
+      prevPhase = phase;
+      prevDay = r.day;
+    }
+  }
+
+  return Array.from(days);
+}
diff --git a/src/lib/oura/cycle-sync.ts b/src/lib/oura/cycle-sync.ts
new file mode 100644
index 000000000..a750f3c0a
--- /dev/null
+++ b/src/lib/oura/cycle-sync.ts
@@ -0,0 +1,126 @@
+/**
+ * v1.29.x — Oura Cycle Insights (`daily_cycle_phases`) → HealthLog's cycle
+ * tracker, as an ADDITIONAL, SUBORDINATE source.
+ *
+ * Every other Oura collection (see `./sync`) writes `Measurement` rows,
+ * which support multiple source rows per `(type, day)` and resolve a
+ * canonical reading at READ time (`src/lib/analytics/source-priority.ts`).
+ * `CycleDayLog` has no such ladder — it is ONE canonical row per
+ * `(userId, date)` (`prisma/schema.prisma` — "the HealthKit sync unit + the
+ * manual log-day surface"), and the shared write helper
+ * (`src/lib/cycle/day-log-write.ts`) partial-merges a re-post's fields over
+ * whatever is already stored REGARDLESS of source. Calling it here would let
+ * an hourly Oura poll silently demote a user's own logged flow (e.g. a
+ * manual HEAVY re-tagged LIGHT on the next tick) — the exact opposite of
+ * "manual wins".
+ *
+ * So this module never touches the shared upsert helper. It only ever
+ * CREATEs a day that has NO existing `CycleDayLog` row at all (any source),
+ * and never updates a row it did not itself create. A day already owned by
+ * this module (source = OURA) is left alone on re-sync too — the marker is
+ * a fixed single-value hint (`flow: LIGHT`), not a corrigible reading, so
+ * there is nothing to refresh. The whole path degenerates to a cheap
+ * "create if absent, otherwise no-op".
+ *
+ * Gated behind the SAME two-layer cycle-module gate every other cycle
+ * writer respects (`isCycleAvailableForUser`) so an operator-disabled
+ * instance, or a user who turned cycle tracking off, never accumulates rows
+ * from a background Oura poll. No `MenstrualCycle` span is ever created or
+ * mutated here — mirrors the Apple Health importer precedent
+ * (`src/lib/cycle/import-accumulator.ts`), which attributes day-logs to an
+ * EXISTING cycle but never synthesises a new one from flow data either;
+ * cycle-span creation stays an explicit user action
+ * (`POST /api/cycle/period`).
+ */
+import { prisma } from "@/lib/db";
+import { getEvent } from "@/lib/logging/context";
+import { isCycleAvailableForUser } from "@/lib/cycle/gate";
+import {
+  fetchDailyCyclePhases,
+  derivePeriodDaysFromCyclePhases,
+} from "./client";
+
+/** The conservative "logged but unspecified intensity" flow level — mirrors
+ * the HealthKit `unspecified` → `LIGHT` boundary documented in
+ * `healthkit-mapping.ts`: a detected-but-unmeasured bleeding day is at least
+ * light flow, never invented as MEDIUM/HEAVY. */
+const OURA_PERIOD_FLOW = "LIGHT" as const;
+
+/** Prisma P2002 (unique constraint) detector, mirrored locally from
+ * `day-log-write.ts` rather than imported — a two-line duplicate is cheaper
+ * than coupling this module to that helper's internals. */
+function isUniqueViolation(err: unknown): boolean {
+  return (
+    typeof err === "object" &&
+    err !== null &&
+    "code" in err &&
+    (err as { code?: unknown }).code === "P2002"
+  );
+}
+
+/**
+ * Sync one user's Oura `daily_cycle_phases` window into `CycleDayLog`.
+ * Returns the count of days newly created.
+ *
+ * Every error the Oura fetch throws propagates to the caller — this
+ * endpoint sits outside the scope most self-registered Oura apps are
+ * granted (see the `OuraCyclePhase` docstring in `./client`), so a 403/404
+ * here is the COMMON case, not an anomaly. `./sync` swallows it without
+ * touching the rest of the Oura connection's health status.
+ *
+ * The Oura fetch runs BEFORE the cycle-module gate / any database access on
+ * purpose: the overwhelmingly common outcome (no access to this endpoint, or
+ * no transition in the window) resolves without a single query.
+ */
+export async function syncUserOuraCyclePhases(
+  userId: string,
+  accessToken: string,
+  lookbackDays: number,
+): Promise<number> {
+  const now = new Date();
+  const start = new Date(now.getTime() - lookbackDays * 24 * 60 * 60 * 1000);
+  const records = await fetchDailyCyclePhases(accessToken, {
+    startDate: start.toISOString().slice(0, 10),
+    endDate: now.toISOString().slice(0, 10),
+  });
+  if (records.length === 0) return 0;
+
+  const periodDays = derivePeriodDaysFromCyclePhases(records);
+  if (periodDays.length === 0) return 0;
+
+  const available = await isCycleAvailableForUser(userId);
+  if (!available) return 0;
+
+  const user = await prisma.user.findUnique({
+    where: { id: userId },
+    select: { timezone: true },
+  });
+  const tz = user?.timezone ?? null;
+
+  let created = 0;
+  for (const day of periodDays) {
+    try {
+      await prisma.cycleDayLog.create({
+        data: {
+          userId,
+          date: day,
+          tz,
+          source: "OURA",
+          flow: OURA_PERIOD_FLOW,
+        },
+      });
+      created += 1;
+    } catch (err) {
+      if (isUniqueViolation(err)) {
+        // A row already exists for this date — the user's own entry, an
+        // Apple Health import, or this module's own prior write. Never
+        // overwrite: Oura is a subordinate signal here.
+        continue;
+      }
+      getEvent()?.addWarning(
+        `oura: cycle day-log create failed for ${userId} ${day}: ${err}`,
+      );
+    }
+  }
+  return created;
+}
diff --git a/src/lib/oura/sync.ts b/src/lib/oura/sync.ts
index b91d09197..dbafc4a37 100644
--- a/src/lib/oura/sync.ts
+++ b/src/lib/oura/sync.ts
@@ -78,6 +78,7 @@ import {
   storeOuraTokens,
 } from "./credentials";
 import { OuraApiError, classifyOuraError } from "./response-classifier";
+import { syncUserOuraCyclePhases } from "./cycle-sync";
 
 /** Default lookback window (days) for an incremental sync. Oura finalises a
  * night's scores hours after wake; 7 days re-fetches a handful of records (the
@@ -370,6 +371,28 @@ export async function syncUserOura(
   // sibling collection failed — one bad collection must not blank the source.
   const imported = await upsertOuraMeasurements(userId, result.readings);
 
+  // v1.29.x — best-effort Cycle Insights import. Fully isolated from the
+  // measurement sync's status ledger on purpose: `daily_cycle_phases` sits
+  // outside the scope most self-registered Oura apps are granted (see the
+  // `OuraCyclePhase` docstring in `./client`), so a 403/404 here is the
+  // COMMON case for a typical connection. Re-reads the connection so a
+  // reactive refresh `fetchAll` just performed is picked up rather than the
+  // (now possibly stale) token captured before it ran.
+  try {
+    const freshConn = await getOuraConnection(userId);
+    if (freshConn) {
+      await syncUserOuraCyclePhases(
+        userId,
+        freshConn.accessToken,
+        opts.lookbackDays ?? OURA_SYNC_LOOKBACK_DAYS,
+      );
+    }
+  } catch (err) {
+    getEvent()?.addWarning(
+      `oura: cycle-phases import skipped for ${userId}: ${err}`,
+    );
+  }
+
   // S4 — trigger the debounced morning refresh on a last-night segment landing
   // (mirrors the Withings / WHOOP / Apple seams). Fires whether or not a
   // sibling collection failed, since the sleep readings were still imported.