diff --git a/src/lib/insights/derived/__tests__/chronotype.test.ts b/src/lib/insights/derived/__tests__/chronotype.test.ts
new file mode 100644
index 000000000..b607ddd4a
--- /dev/null
+++ b/src/lib/insights/derived/__tests__/chronotype.test.ts
@@ -0,0 +1,158 @@
+import { describe, it, expect } from "vitest";
+import {
+  computeChronotype,
+  bandForMSFsc,
+  type ChronotypeNight,
+} from "../chronotype";
+
+/** Minutes-of-day for HH:MM. */
+function hm(h: number, m = 0): number {
+  return h * 60 + m;
+}
+
+function freeNight(
+  day: number,
+  midpointMinutes: number,
+  asleepMinutes: number,
+): ChronotypeNight {
+  return {
+    night: `2026-06-${String(day).padStart(2, "0")}`,
+    midpointMinutes,
+    asleepMinutes,
+    dayType: "free",
+  };
+}
+
+function workNight(
+  day: number,
+  midpointMinutes: number,
+  asleepMinutes: number,
+): ChronotypeNight {
+  return {
+    night: `2026-06-${String(day).padStart(2, "0")}`,
+    midpointMinutes,
+    asleepMinutes,
+    dayType: "work",
+  };
+}
+
+describe("bandForMSFsc", () => {
+  it("bands MSFsc minutes-of-day into MCTQ classes", () => {
+    expect(bandForMSFsc(hm(2, 0))).toBe("extreme_early"); // 02:00
+    expect(bandForMSFsc(hm(3, 0))).toBe("early"); // 03:00
+    expect(bandForMSFsc(hm(4, 0))).toBe("intermediate"); // 04:00
+    expect(bandForMSFsc(hm(5, 30))).toBe("late"); // 05:30
+    expect(bandForMSFsc(hm(7, 0))).toBe("extreme_late"); // 07:00
+  });
+
+  it("normalises a midnight-straddling / corrected value onto the clock", () => {
+    expect(bandForMSFsc(hm(24, 0) + hm(1, 0))).toBe("extreme_early"); // 25:00→01:00
+    expect(bandForMSFsc(-30)).toBe("extreme_late"); // −0:30 → 23:30
+  });
+});
+
+describe("computeChronotype", () => {
+  it("computes MSF + band from a free-day mid-sleep set (no debt correction)", () => {
+    // Three free nights all centred at 04:00, equal-length → SDf == SDweek →
+    // MSFsc == MSF == 240 → intermediate.
+    const res = computeChronotype([
+      freeNight(1, hm(4), 480),
+      freeNight(2, hm(4), 480),
+      freeNight(3, hm(4), 480),
+    ]);
+    expect(res.state).toBe("ready");
+    expect(res.msfMinutes).toBeCloseTo(240, 6);
+    expect(res.msfScMinutes).toBeCloseTo(240, 6);
+    expect(res.band).toBe("intermediate");
+  });
+
+  it("applies the MSFsc sleep-debt correction when free days oversleep", () => {
+    // 3 free nights @ 04:00 / 9 h (540) + 3 work nights @ 03:00 / 6 h (360).
+    // SDf = 540; SDweek = (540·3 + 360·3)/6 = 450; oversleep = 90.
+    // MSFsc = 240 − 0.5·90 = 195 → "early" (< 03:30).
+    const res = computeChronotype([
+      freeNight(1, hm(4), 540),
+      freeNight(2, hm(4), 540),
+      freeNight(3, hm(4), 540),
+      workNight(4, hm(3), 360),
+      workNight(5, hm(3), 360),
+      workNight(6, hm(3), 360),
+    ]);
+    expect(res.msfMinutes).toBeCloseTo(240, 6);
+    expect(res.msfScMinutes).toBeCloseTo(195, 6);
+    expect(res.band).toBe("early"); // 195 → early (< 03:30)
+  });
+
+  it("computes social jetlag as circular |MSF_work − MSF_free|", () => {
+    // Free mid-sleep 04:00 (240), work mid-sleep 03:00 (180) → SJL = 60.
+    const res = computeChronotype([
+      freeNight(1, hm(4), 480),
+      freeNight(2, hm(4), 480),
+      freeNight(3, hm(4), 480),
+      workNight(4, hm(3), 480),
+      workNight(5, hm(3), 480),
+    ]);
+    expect(res.socialJetlagMinutes).toBeCloseTo(60, 6);
+    expect(res.workNightsCounted).toBe(2);
+    expect(res.freeNightsCounted).toBe(3);
+  });
+
+  it("measures social jetlag across midnight by the shorter arc", () => {
+    // Work mid-sleep 23:30 (1410), free mid-sleep 00:30 (30) → circular 60,
+    // not 1380 — the midnight-straddle handling from circularMinuteDistance.
+    const res = computeChronotype([
+      freeNight(1, hm(0, 30), 480),
+      freeNight(2, hm(0, 30), 480),
+      freeNight(3, hm(0, 30), 480),
+      workNight(4, hm(23, 30), 480),
+    ]);
+    expect(res.socialJetlagMinutes).toBeCloseTo(60, 6);
+  });
+
+  it("fires the learning gate under the free-night threshold", () => {
+    const res = computeChronotype(
+      [freeNight(1, hm(4), 480), freeNight(2, hm(4), 480)],
+      { minFreeNights: 3 },
+    );
+    expect(res.state).toBe("learning");
+    expect(res.band).toBeNull();
+    expect(res.msfMinutes).toBeNull();
+    expect(res.msfScMinutes).toBeNull();
+    expect(res.freeNightsCounted).toBe(2);
+    expect(res.freeNightsUntilReady).toBe(1); // 3 − 2
+  });
+
+  it("clears the learning gate at the threshold", () => {
+    const res = computeChronotype(
+      [
+        freeNight(1, hm(4), 480),
+        freeNight(2, hm(4), 480),
+        freeNight(3, hm(4), 480),
+      ],
+      { minFreeNights: 3 },
+    );
+    expect(res.state).toBe("ready");
+    expect(res.band).not.toBeNull();
+  });
+
+  it("work-only data stays learning and reports no social jetlag", () => {
+    const res = computeChronotype([
+      workNight(1, hm(3), 480),
+      workNight(2, hm(3), 480),
+      workNight(3, hm(3), 480),
+    ]);
+    expect(res.state).toBe("learning");
+    expect(res.socialJetlagMinutes).toBeNull(); // no free side to compare
+    expect(res.freeNightsCounted).toBe(0);
+  });
+
+  it("ignores nights with zero asleep minutes", () => {
+    const res = computeChronotype([
+      freeNight(1, hm(4), 480),
+      freeNight(2, hm(4), 0), // dropped
+      freeNight(3, hm(4), 480),
+    ]);
+    expect(res.freeNightsCounted).toBe(2);
+    expect(res.state).toBe("learning");
+  });
+});
diff --git a/src/lib/insights/derived/__tests__/sleep-debt.test.ts b/src/lib/insights/derived/__tests__/sleep-debt.test.ts
new file mode 100644
index 000000000..5ff39fbf1
--- /dev/null
+++ b/src/lib/insights/derived/__tests__/sleep-debt.test.ts
@@ -0,0 +1,102 @@
+import { describe, it, expect } from "vitest";
+import {
+  computeSleepDebt,
+  type SleepDebtNight,
+} from "../sleep-debt";
+
+/** Build a contiguous run of nights ending YYYY-06-?? with given asleep mins. */
+function nights(asleepPerNight: number[]): SleepDebtNight[] {
+  return asleepPerNight.map((asleepMinutes, i) => ({
+    night: `2026-06-${String(i + 1).padStart(2, "0")}`,
+    asleepMinutes,
+  }));
+}
+
+const NEED = 480; // 8 h need, pinned so the deficit math is exact.
+
+describe("computeSleepDebt", () => {
+  it("sums a deficit sequence into the exact cumulative debt", () => {
+    // need 480; nights of 420 / 360 / 480 / 450 → deficits 60 / 120 / 0 / 30.
+    const res = computeSleepDebt(nights([420, 360, 480, 450]), NEED, {
+      minNights: 1,
+    });
+    expect(res.state).toBe("ready");
+    expect(res.debtMinutes).toBe(60 + 120 + 0 + 30); // 210
+    expect(res.perNight.map((n) => n.deficitMinutes)).toEqual([60, 120, 0, 30]);
+    expect(res.nightsCounted).toBe(4);
+    expect(res.needMinutes).toBe(NEED);
+  });
+
+  it("never mints negative deficit from catch-up sleep", () => {
+    // A 10 h night (600) is 120 over need — deficit floors at 0, not −120.
+    const res = computeSleepDebt(nights([600, 600]), NEED, { minNights: 1 });
+    expect(res.debtMinutes).toBe(0);
+    expect(res.perNight.every((n) => n.deficitMinutes === 0)).toBe(true);
+  });
+
+  it("caps a single catastrophic night's deficit", () => {
+    // A 1 h night (60) is 420 short; the per-night cap (180) holds.
+    const res = computeSleepDebt(nights([60]), NEED, {
+      minNights: 1,
+      maxNightlyDeficitMinutes: 180,
+    });
+    expect(res.perNight[0].deficitMinutes).toBe(180);
+    expect(res.debtMinutes).toBe(180);
+  });
+
+  it("caps the cumulative total", () => {
+    // Ten 0 h nights × 180 cap = 1800 raw, clamped to 1200 total cap.
+    const res = computeSleepDebt(nights(Array(10).fill(0)), NEED, {
+      minNights: 1,
+      maxTotalDebtMinutes: 1200,
+    });
+    expect(res.debtMinutes).toBe(1200);
+  });
+
+  it("takes only the most recent windowNights and sorts oldest→newest", () => {
+    // 16 nights of 420 (60 short each); window 14 → 14 × 60 = 840.
+    const res = computeSleepDebt(nights(Array(16).fill(420)), NEED, {
+      windowNights: 14,
+    });
+    expect(res.nightsCounted).toBe(14);
+    expect(res.debtMinutes).toBe(14 * 60);
+    expect(res.windowNights).toBe(14);
+    // Oldest kept night is 2026-06-03 (06-01 and 06-02 aged out).
+    expect(res.perNight[0].night).toBe("2026-06-03");
+    expect(res.perNight.at(-1)?.night).toBe("2026-06-16");
+  });
+
+  it("debt drains as deficit nights age out of the rolling window", () => {
+    // Two short nights then twelve full ones, window 14 → only the short
+    // nights inside the window count; thirteen full + one short → drains.
+    const fourteen = nights([360, ...Array(13).fill(480)]); // 1 short, 13 full
+    const res = computeSleepDebt(fourteen, NEED, { windowNights: 14 });
+    expect(res.debtMinutes).toBe(120); // only the 360 night carries debt
+  });
+
+  it("returns a calm partial state under the night threshold", () => {
+    const res = computeSleepDebt(nights([300, 300, 300]), NEED, {
+      minNights: 7,
+    });
+    expect(res.state).toBe("partial");
+    expect(res.nightsCounted).toBe(3);
+    expect(res.nightsUntilReady).toBe(4); // 7 − 3
+    // It still reports the running figure so the UI can show a soft preview.
+    expect(res.debtMinutes).toBe(3 * 180); // each night 180 short
+  });
+
+  it("clears the partial state at or above the threshold", () => {
+    const res = computeSleepDebt(nights(Array(7).fill(420)), NEED, {
+      minNights: 7,
+    });
+    expect(res.state).toBe("ready");
+    expect(res.nightsUntilReady).toBe(0);
+  });
+
+  it("guards a non-positive need (no fabricated debt)", () => {
+    const res = computeSleepDebt(nights([300, 300]), 0, { minNights: 1 });
+    expect(res.state).toBe("partial");
+    expect(res.debtMinutes).toBe(0);
+    expect(res.needMinutes).toBe(0);
+  });
+});
diff --git a/src/lib/insights/derived/chronotype.ts b/src/lib/insights/derived/chronotype.ts
new file mode 100644
index 000000000..11d2fa052
--- /dev/null
+++ b/src/lib/insights/derived/chronotype.ts
@@ -0,0 +1,214 @@
+/**
+ * v1.17.0 — MCTQ chronotype + social jetlag (server-authoritative).
+ *
+ * Implements the Munich ChronoType Questionnaire derivation from observed
+ * sleep, not a survey: Roenneberg, Wirz-Justice & Merrow (2003), *J Biol
+ * Rhythms* 18(1):80-90; Roenneberg et al. (2007), *Sleep Med Rev* 11(6):429-
+ * 438. The chronotype marker is MID-SLEEP ON FREE DAYS (MSF) — the clock time
+ * halfway between sleep onset and wake on days without an alarm — corrected
+ * for the sleep debt people pay off by sleeping in on free days (MSFsc).
+ *
+ *   MSF      = circular mean of free-day sleep midpoints (minutes-of-day).
+ *   MSFsc    = MSF − 0.5·(SDf − SDweek)   when SDf > SDweek, else MSF
+ *              where SDf  = average free-day sleep DURATION,
+ *                    SDweek = average sleep duration across work + free days.
+ *   SJL      = social jetlag = circular |MSF_work − MSF_free|.
+ *
+ * The MSFsc correction (Roenneberg 2007) removes the morning "oversleep" that
+ * a sleep-debt rebound adds to the free-day midpoint, so a workweek-deprived
+ * sleeper is not mislabelled later than their true phase.
+ *
+ * ONE MIDPOINT ESTIMATOR
+ * ----------------------
+ * The per-night `midpointMinutes` is the SAME minutes-of-day midpoint the
+ * Sleep Score's Timing sub-score reads (centre of the canonical asleep span,
+ * in the user's wall clock). The caller supplies it; chronotype does NOT
+ * re-derive a second midpoint — and it reuses `circularMeanMinutes` /
+ * `circularMinuteDistance` from the Sleep Score so the midnight-straddle / DST
+ * handling is identical. Pure — no Prisma, no I/O; route, Coach, PDF, and the
+ * iOS serializer all reuse it.
+ *
+ * LEARNING GATE
+ * -------------
+ * Chronotype is a stable trait and a thin sample is noise. Below
+ * `minFreeNights` free-day nights the result is a calm "still learning — N of
+ * M nights" state and asserts NO type. This mirrors the Sleep Score's
+ * "< 3 nights → null" timing gate; the band only appears once the free-day
+ * sample is large enough to mean something.
+ *
+ * FREE vs WORK
+ * ------------
+ * The app has no work calendar, so the day-type signal is an INPUT per night
+ * (the caller maps weekday/weekend or a future shift schedule). A modelling
+ * assumption, surfaced as such — not hardcoded here.
+ */
+import {
+  circularMeanMinutes,
+  circularMinuteDistance,
+} from "./sleep-score";
+
+const MINUTES_PER_DAY = 1440;
+
+/** One night fed to the chronotype estimator. */
+export interface ChronotypeNight {
+  /** Wake-day key (YYYY-MM-DD), user timezone. */
+  night: string;
+  /**
+   * Sleep midpoint as minutes-of-day (0..1439) in the user's wall clock — the
+   * SAME estimator the Sleep Score Timing sub-score uses. The caller passes it;
+   * this module does not re-derive a midpoint.
+   */
+  midpointMinutes: number;
+  /** Canonical time-asleep minutes (`SleepNight.asleepMinutes`) for SD math. */
+  asleepMinutes: number;
+  /** Day type — caller-supplied (no work calendar is hardcoded here). */
+  dayType: "work" | "free";
+}
+
+export interface ChronotypeOptions {
+  /**
+   * Minimum free-day nights before a chronotype band is asserted. MCTQ needs a
+   * stable free-day estimate; below this the result stays in the calm learning
+   * state. 3 mirrors the Sleep Score's timing/consistency floor — enough to
+   * dampen a single anomalous lie-in without demanding a month of data.
+   */
+  minFreeNights?: number;
+}
+
+const DEFAULTS: Required<ChronotypeOptions> = {
+  minFreeNights: 3,
+};
+
+/**
+ * MCTQ chronotype band off MSFsc (minutes-of-day). The cut points follow the
+ * Roenneberg population percentiles (extreme-early through extreme-late around
+ * a ~04:00 mid-sleep mode). One banding helper — never re-thresholded per
+ * surface.
+ */
+export type ChronotypeBand =
+  | "extreme_early"
+  | "early"
+  | "intermediate"
+  | "late"
+  | "extreme_late";
+
+/**
+ * Band a corrected mid-sleep (MSFsc, minutes-of-day) into the MCTQ chronotype
+ * classes. Cut points (local clock): < 02:30 extreme-early, < 03:30 early,
+ * < 05:00 intermediate, < 06:00 late, else extreme-late — the standard MCTQ
+ * MSFsc deciles centred on the ~04:00 population mode.
+ */
+export function bandForMSFsc(msfscMinutes: number): ChronotypeBand {
+  // Normalise onto [0,1440) so a value that the correction pushed below 0 or a
+  // late-night midpoint past midnight still bands on the wall clock.
+  const m = ((msfscMinutes % MINUTES_PER_DAY) + MINUTES_PER_DAY) %
+    MINUTES_PER_DAY;
+  if (m < 150) return "extreme_early"; // < 02:30
+  if (m < 210) return "early"; // < 03:30
+  if (m < 300) return "intermediate"; // < 05:00
+  if (m < 360) return "late"; // < 06:00
+  return "extreme_late";
+}
+
+export interface ChronotypeResult {
+  /**
+   * `learning` until `minFreeNights` free-day nights exist — no band asserted.
+   * `ready` once the free-day sample is large enough.
+   */
+  state: "learning" | "ready";
+  /** Mid-sleep on free days (minutes-of-day), or null while learning. */
+  msfMinutes: number | null;
+  /** Sleep-debt-corrected mid-sleep on free days (minutes-of-day), or null. */
+  msfScMinutes: number | null;
+  /** Chronotype band off MSFsc, or null while learning. */
+  band: ChronotypeBand | null;
+  /**
+   * Social jetlag in minutes = circular |MSF_work − MSF_free|. Null when there
+   * is no work-day OR no free-day midpoint to compare (one side missing).
+   */
+  socialJetlagMinutes: number | null;
+  /** Free-day nights counted. */
+  freeNightsCounted: number;
+  /** Work-day nights counted. */
+  workNightsCounted: number;
+  /** `minFreeNights` − `freeNightsCounted` while learning, else 0. */
+  freeNightsUntilReady: number;
+}
+
+function mean(values: readonly number[]): number | null {
+  if (values.length === 0) return null;
+  return values.reduce((s, v) => s + v, 0) / values.length;
+}
+
+/**
+ * Derive MCTQ chronotype, MSFsc, and social jetlag from per-night midpoints +
+ * day types. Pure. Holds the learning gate until enough free-day nights exist.
+ */
+export function computeChronotype(
+  nights: readonly ChronotypeNight[],
+  options: ChronotypeOptions = {},
+): ChronotypeResult {
+  const opts = { ...DEFAULTS, ...options };
+  const usable = nights.filter(
+    (n) =>
+      Number.isFinite(n.midpointMinutes) &&
+      Number.isFinite(n.asleepMinutes) &&
+      n.asleepMinutes > 0,
+  );
+  const free = usable.filter((n) => n.dayType === "free");
+  const work = usable.filter((n) => n.dayType === "work");
+
+  const freeNightsCounted = free.length;
+  const workNightsCounted = work.length;
+
+  // MSF = circular mean of free-day midpoints (handles the midnight straddle).
+  const msfMinutes = circularMeanMinutes(free.map((n) => n.midpointMinutes));
+  // MSW = circular mean of work-day midpoints — only used for social jetlag.
+  const mswMinutes = circularMeanMinutes(work.map((n) => n.midpointMinutes));
+
+  // Social jetlag = circular distance between the work and free mid-sleeps.
+  // Both sides must exist; otherwise the offset is undefined (not zero).
+  const socialJetlagMinutes =
+    msfMinutes != null && mswMinutes != null
+      ? circularMinuteDistance(mswMinutes, msfMinutes)
+      : null;
+
+  const ready = freeNightsCounted >= opts.minFreeNights && msfMinutes != null;
+  if (!ready) {
+    return {
+      state: "learning",
+      msfMinutes: null,
+      msfScMinutes: null,
+      band: null,
+      socialJetlagMinutes,
+      freeNightsCounted,
+      workNightsCounted,
+      freeNightsUntilReady: Math.max(
+        0,
+        opts.minFreeNights - freeNightsCounted,
+      ),
+    };
+  }
+
+  // MSFsc correction (Roenneberg 2007): subtract half the free-day oversleep
+  // (free-day duration minus the week-average duration), applied only when
+  // people sleep LONGER on free days (paying off workweek debt). When SDf ≤
+  // SDweek there is no debt rebound, so MSFsc = MSF.
+  const sdFree = mean(free.map((n) => n.asleepMinutes)) ?? 0;
+  const sdWeek = mean(usable.map((n) => n.asleepMinutes)) ?? sdFree;
+  const oversleep = Math.max(0, sdFree - sdWeek);
+  const msfScRaw = msfMinutes - 0.5 * oversleep;
+  const msfScMinutes =
+    ((msfScRaw % MINUTES_PER_DAY) + MINUTES_PER_DAY) % MINUTES_PER_DAY;
+
+  return {
+    state: "ready",
+    msfMinutes,
+    msfScMinutes,
+    band: bandForMSFsc(msfScMinutes),
+    socialJetlagMinutes,
+    freeNightsCounted,
+    workNightsCounted,
+    freeNightsUntilReady: 0,
+  };
+}
diff --git a/src/lib/insights/derived/sleep-debt.ts b/src/lib/insights/derived/sleep-debt.ts
new file mode 100644
index 000000000..573bc4232
--- /dev/null
+++ b/src/lib/insights/derived/sleep-debt.ts
@@ -0,0 +1,158 @@
+/**
+ * v1.17.0 — cumulative sleep debt (server-authoritative timing math).
+ *
+ * Sleep debt is the running shortfall between how much sleep a person NEEDS
+ * and how much they actually got, accumulated over a rolling window. Per
+ * night the deficit is `max(0, need − asleep)` — only undersleep accrues
+ * debt; a night of catch-up sleep does NOT mint a negative deficit that
+ * silently erases yesterday's shortfall (the body does not bank surplus sleep
+ * minute-for-minute; Van Dongen et al. 2003, *Sleep* 26(2):117-126 show the
+ * recovery curve is sub-linear). Catch-up instead pays the running debt DOWN
+ * through the rolling window: as a deficit night ages out of the window it
+ * stops contributing, so a stretch of full nights drains the debt toward zero.
+ *
+ * ONE ENGINE, NO DUPLICATION
+ * --------------------------
+ * This module is PURE — it consumes the canonical per-night `asleepMinutes`
+ * the shared sleep engine (`reconstructSleepNights`) already produces and
+ * takes the sleep `need` as a parameter (the caller resolves it via the one
+ * existing `sleepNeedMinutes(ageYears)` table — there is no second need table
+ * here). "actual" MUST be the canonical asleep total, never a divergent
+ * reconstruction, so this figure can never contradict the dashboard / Sleep
+ * Score / hypnogram for the same night. No Prisma, no I/O — the route, Coach
+ * snapshot, doctor PDF, and iOS serializer all reuse it.
+ *
+ * Composition with the Sleep Score: Sufficiency (`scoreSufficiency`) is the
+ * SINGLE-NIGHT view of `need − actual` expressed 0–100; this is the ROLLING
+ * CUMULATIVE of the same `need − actual`. Same inputs, two framings — a
+ * timing/trend signal, not a second sleep-adequacy score.
+ */
+
+/** One night's canonical asleep total, keyed by its wake day. */
+export interface SleepDebtNight {
+  /** Wake-day key (YYYY-MM-DD), user timezone — from the canonical engine. */
+  night: string;
+  /** Canonical time-asleep minutes for the night (`SleepNight.asleepMinutes`). */
+  asleepMinutes: number;
+}
+
+/** Tunable knobs; every default is documented inline. */
+export interface SleepDebtOptions {
+  /**
+   * Rolling window length in nights. 14 is the standard two-week sleep-debt
+   * horizon used across the consumer-sleep literature — long enough that a
+   * single bad night does not dominate, short enough that month-old debt no
+   * longer shapes today's signal.
+   */
+  windowNights?: number;
+  /**
+   * Minimum tracked nights before the cumulative debt is asserted. Below this
+   * the result is a calm PARTIAL state ("still learning") rather than a
+   * confident total off a near-empty window — a one-night window would read a
+   * single rough night as a fortnight of debt.
+   */
+  minNights?: number;
+  /**
+   * Per-night deficit cap (minutes). One catastrophic night (a 2-hour sleep,
+   * or a missing-stage artefact) must not swamp a fortnight of otherwise-fine
+   * sleep; 180 min (3 h) is the largest single-night shortfall that still
+   * reads as "a rough night" rather than a data glitch.
+   */
+  maxNightlyDeficitMinutes?: number;
+  /**
+   * Cumulative-debt cap (minutes). Beyond a point more debt is not
+   * meaningfully more actionable — the message is the same ("you are deeply
+   * under-slept"). 1200 min (20 h) caps the headline so the bar / number
+   * stays legible and a long gap of untracked-then-rough nights cannot mint
+   * an absurd total.
+   */
+  maxTotalDebtMinutes?: number;
+}
+
+const DEFAULTS: Required<SleepDebtOptions> = {
+  windowNights: 14,
+  minNights: 7,
+  maxNightlyDeficitMinutes: 180,
+  maxTotalDebtMinutes: 1200,
+};
+
+/** Per-night deficit detail, oldest → newest, for the trend strip / tooltip. */
+export interface SleepDebtNightDetail {
+  night: string;
+  asleepMinutes: number;
+  /** `max(0, need − asleep)` after the per-night cap, in minutes. */
+  deficitMinutes: number;
+}
+
+export interface SleepDebtResult {
+  /**
+   * `partial` until `minNights` tracked nights exist — the calm "still
+   * learning" state. `ready` once the window has enough nights to assert a
+   * cumulative debt.
+   */
+  state: "partial" | "ready";
+  /** Cumulative sleep debt in minutes over the window, after caps. */
+  debtMinutes: number;
+  /** The sleep need (minutes) used for the deficit — echoed for transparency. */
+  needMinutes: number;
+  /** Nights actually counted (≤ windowNights). */
+  nightsCounted: number;
+  /** The configured rolling window length in nights. */
+  windowNights: number;
+  /** `minNights` − `nightsCounted` while partial, else 0 — drives the nudge. */
+  nightsUntilReady: number;
+  /** Per-night deficits over the window, oldest → newest. */
+  perNight: SleepDebtNightDetail[];
+}
+
+/**
+ * Compute cumulative sleep debt over the trailing window. Pure.
+ *
+ * `nights` may arrive in any order and span more than the window — the most
+ * recent `windowNights` (by wake-day key) are taken. `needMinutes` is the
+ * caller-resolved age-based need (`sleepNeedMinutes`); passing it in keeps
+ * this module free of the need table and lets a test pin an exact value.
+ */
+export function computeSleepDebt(
+  nights: readonly SleepDebtNight[],
+  needMinutes: number,
+  options: SleepDebtOptions = {},
+): SleepDebtResult {
+  const opts = { ...DEFAULTS, ...options };
+  const safeNeed = Number.isFinite(needMinutes) && needMinutes > 0
+    ? needMinutes
+    : 0;
+
+  // Take the most recent `windowNights` by wake-day key. Sort ascending so the
+  // perNight strip reads oldest → newest like every other trend surface.
+  const windowed = [...nights]
+    .filter((n) => Number.isFinite(n.asleepMinutes) && n.asleepMinutes >= 0)
+    .sort((a, b) => (a.night < b.night ? -1 : a.night > b.night ? 1 : 0))
+    .slice(-opts.windowNights);
+
+  const perNight: SleepDebtNightDetail[] = windowed.map((n) => {
+    const rawDeficit = Math.max(0, safeNeed - n.asleepMinutes);
+    // Cap one catastrophic night so it cannot swamp the fortnight.
+    const deficitMinutes = Math.min(rawDeficit, opts.maxNightlyDeficitMinutes);
+    return {
+      night: n.night,
+      asleepMinutes: n.asleepMinutes,
+      deficitMinutes,
+    };
+  });
+
+  const nightsCounted = perNight.length;
+  const rawDebt = perNight.reduce((sum, n) => sum + n.deficitMinutes, 0);
+  const debtMinutes = Math.min(Math.round(rawDebt), opts.maxTotalDebtMinutes);
+
+  const ready = nightsCounted >= opts.minNights && safeNeed > 0;
+  return {
+    state: ready ? "ready" : "partial",
+    debtMinutes,
+    needMinutes: safeNeed,
+    nightsCounted,
+    windowNights: opts.windowNights,
+    nightsUntilReady: ready ? 0 : Math.max(0, opts.minNights - nightsCounted),
+    perNight,
+  };
+}