diff --git a/src/__tests__/spread-single-implementation-guard.test.ts b/src/__tests__/spread-single-implementation-guard.test.ts
index cf7191b6e..da69c33f5 100644
--- a/src/__tests__/spread-single-implementation-guard.test.ts
+++ b/src/__tests__/spread-single-implementation-guard.test.ts
@@ -30,6 +30,8 @@ const ALLOWED: Record<string, string> = {
     "The correlation engine's centred sums and the Welch t-test variances.",
   "lib/insights/mood-tag-influence.ts": "Welch t-test variances.",
   "lib/insights/mood-pattern-cards.ts": "A pooled variance for a t statistic.",
+  "lib/stats/segmented-regression.ts":
+    "The total sum of squares behind R² and the HAC standard errors of a segmented fit; neither is reported as a spread.",
   "lib/insights/derived/trajectory.ts":
     "The residual standard error of a regression fit (÷ n − 2).",
   "lib/insights/derived/sleep-score.ts":
diff --git a/src/lib/stats/__tests__/segmented-regression.test.ts b/src/lib/stats/__tests__/segmented-regression.test.ts
new file mode 100644
index 000000000..57001444b
--- /dev/null
+++ b/src/lib/stats/__tests__/segmented-regression.test.ts
@@ -0,0 +1,114 @@
+/**
+ * Exact statistics for the segmented (interrupted time series) regression.
+ *
+ * The reference values for the noisy fixtures were computed independently
+ * with statsmodels 0.14 (`OLS(...).fit(cov_type="HAC", cov_kwds={"maxlags":
+ * L, "use_correction": True}, use_t=True)`, L = floor(4·(n/100)^(2/9))) and
+ * scipy's `stats.t`. The fixture generator below is the same deterministic
+ * formula the reference script used, so a drift in the estimator, the HAC
+ * meat, the small-sample factor or the t quantile fails here.
+ */
+import { describe, expect, it } from "vitest";
+
+import {
+  fitSegmentedRegression,
+  neweyWestLag,
+  studentTCdf,
+  studentTQuantile,
+  type SegmentedPoint,
+} from "@/lib/stats/segmented-regression";
+
+/** y = 140 − 0.05·t − 8·D − 0.1·t·D + deterministic wobble. */
+function fixture(
+  nPre: number,
+  nPost: number,
+  opts: { gaps?: boolean; noise?: boolean } = {},
+): SegmentedPoint[] {
+  const out: SegmentedPoint[] = [];
+  for (let t = -nPre; t < nPost; t++) {
+    if (opts.gaps && ((t % 5) + 5) % 5 === 2) continue;
+    // Python's `%` is a floored modulo; JS `%` truncates. Match Python.
+    const pyMod = (((0.45 * t * t) % 7) + 7) % 7;
+    const noise =
+      opts.noise === false ? 0 : 3 * Math.sin(1.7 * t) + 2 * Math.cos(pyMod);
+    const d = t >= 0 ? 1 : 0;
+    out.push({ t, y: 140 - 0.05 * t - 8 * d - 0.1 * t * d + noise });
+  }
+  return out;
+}
+
+describe("studentT", () => {
+  it("matches scipy quantiles", () => {
+    expect(studentTQuantile(0.975, 10)).toBeCloseTo(2.228138851986274, 9);
+    expect(studentTQuantile(0.975, 86)).toBeCloseTo(1.98793420623902, 9);
+    expect(studentTQuantile(0.95, 3)).toBeCloseTo(2.3533634348018233, 9);
+  });
+
+  it("matches the scipy CDF", () => {
+    expect(studentTCdf(-2.1, 7)).toBeCloseTo(0.036935598106461326, 10);
+    expect(studentTCdf(0, 5)).toBeCloseTo(0.5, 12);
+  });
+});
+
+describe("fitSegmentedRegression", () => {
+  it("recovers a noise-free level and slope change exactly, with zero-width intervals", () => {
+    const fit = fitSegmentedRegression(fixture(30, 30, { noise: false }));
+    expect(fit).not.toBeNull();
+    expect(fit!.intercept.estimate).toBeCloseTo(140, 9);
+    expect(fit!.preSlope.estimate).toBeCloseTo(-0.05, 9);
+    expect(fit!.levelChange.estimate).toBeCloseTo(-8, 9);
+    expect(fit!.slopeChange.estimate).toBeCloseTo(-0.1, 9);
+    expect(fit!.levelChange.standardError).toBeLessThan(1e-9);
+    expect(fit!.rSquared).toBeCloseTo(1, 12);
+  });
+
+  it("matches statsmodels HAC on a contiguous 90-day series", () => {
+    const fit = fitSegmentedRegression(fixture(40, 50))!;
+    expect(fit.n).toBe(90);
+    expect(fit.df).toBe(86);
+    expect(fit.hacLag).toBe(3);
+    expect(fit.intercept.estimate).toBeCloseTo(139.46950523140714, 9);
+    expect(fit.preSlope.estimate).toBeCloseTo(-0.09725785860840647, 10);
+    expect(fit.levelChange.estimate).toBeCloseTo(-7.336815685253282, 9);
+    expect(fit.slopeChange.estimate).toBeCloseTo(-0.03684505010294312, 10);
+    expect(fit.intercept.standardError).toBeCloseTo(0.3824128525477469, 9);
+    expect(fit.preSlope.standardError).toBeCloseTo(0.018189958785228614, 10);
+    expect(fit.levelChange.standardError).toBeCloseTo(0.5960977099276101, 9);
+    expect(fit.slopeChange.standardError).toBeCloseTo(0.02336733639963861, 10);
+    expect(fit.levelChange.ciLow).toBeCloseTo(-8.521818713079124, 8);
+    expect(fit.levelChange.ciHigh).toBeCloseTo(-6.151812657427441, 8);
+    expect(fit.slopeChange.ciLow).toBeCloseTo(-0.08329777744047885, 9);
+    expect(fit.slopeChange.ciHigh).toBeCloseTo(0.009607677234592618, 9);
+    expect(fit.rSquared).toBeCloseTo(0.8683372493940129, 10);
+  });
+
+  it("matches statsmodels HAC on a series with gaps", () => {
+    const fit = fitSegmentedRegression(fixture(30, 30, { gaps: true }))!;
+    expect(fit.n).toBe(48);
+    expect(fit.hacLag).toBe(3);
+    expect(fit.levelChange.estimate).toBeCloseTo(-6.123258149435133, 9);
+    expect(fit.levelChange.standardError).toBeCloseTo(0.9608527406825199, 9);
+    expect(fit.levelChange.ciLow).toBeCloseTo(-8.059729606822106, 8);
+    expect(fit.levelChange.ciHigh).toBeCloseTo(-4.18678669204816, 8);
+    expect(fit.slopeChange.estimate).toBeCloseTo(-0.11857109294626766, 10);
+    expect(fit.slopeChange.ciLow).toBeCloseTo(-0.23733292721328714, 9);
+    expect(fit.slopeChange.ciHigh).toBeCloseTo(0.0001907413207517944, 9);
+    expect(fit.rSquared).toBeCloseTo(0.8247246446176286, 10);
+  });
+
+  it("returns null when one side of the change has no points", () => {
+    expect(
+      fitSegmentedRegression(fixture(0, 30).filter((p) => p.t >= 0)),
+    ).toBeNull();
+  });
+
+  it("returns null without residual degrees of freedom", () => {
+    expect(fitSegmentedRegression(fixture(2, 2, { noise: false }))).toBeNull();
+  });
+
+  it("uses the automatic Newey–West lag", () => {
+    expect(neweyWestLag(90)).toBe(3);
+    expect(neweyWestLag(30)).toBe(3);
+    expect(neweyWestLag(10)).toBe(2);
+  });
+});
diff --git a/src/lib/stats/segmented-regression.ts b/src/lib/stats/segmented-regression.ts
new file mode 100644
index 000000000..b0451c5cd
--- /dev/null
+++ b/src/lib/stats/segmented-regression.ts
@@ -0,0 +1,285 @@
+/**
+ * Segmented regression for an interrupted time series.
+ *
+ * One change point, known in advance (a medication start), four parameters:
+ *
+ *   y(t) = b0 + b1·t + b2·D(t) + b3·t·D(t) + e(t)
+ *
+ * where `t` is the time in days relative to the change point and `D(t)` is 1
+ * at and after it, 0 before. `b1` is the trend before the change, `b2` the
+ * immediate shift in level, `b3` the change in slope. Fitted by ordinary
+ * least squares.
+ *
+ * Daily health series are autocorrelated: today's blood pressure says a lot
+ * about tomorrow's. Plain OLS standard errors ignore that and come out too
+ * narrow, so the covariance here is the Newey–West estimator (Bartlett
+ * kernel) with the usual automatic lag `floor(4·(n/100)^(2/9))` and the
+ * small-sample factor n/(n−k). Intervals use Student's t with n−k degrees of
+ * freedom. This matches `statsmodels` OLS `cov_type="HAC"` with
+ * `use_correction=True` and `use_t=True`; the unit tests pin it to values
+ * computed there.
+ *
+ * Observations are taken in the order given, which for a daily series with
+ * gaps treats a gap as adjacent days. That is a known approximation of the
+ * lag structure, not of the coefficients.
+ *
+ * Pure. No I/O, no rounding: callers round for display.
+ */
+
+/** One observation: time in days relative to the change point, and a value. */
+export interface SegmentedPoint {
+  t: number;
+  y: number;
+}
+
+/** A coefficient with its HAC standard error and two-sided interval. */
+export interface SegmentedEstimate {
+  estimate: number;
+  standardError: number;
+  ciLow: number;
+  ciHigh: number;
+}
+
+export interface SegmentedFit {
+  /** Observations used. */
+  n: number;
+  /** Residual degrees of freedom, n − 4. */
+  df: number;
+  /** Confidence level of every interval, e.g. 0.95. */
+  level: number;
+  /** Newey–West truncation lag that was applied. */
+  hacLag: number;
+  intercept: SegmentedEstimate;
+  preSlope: SegmentedEstimate;
+  levelChange: SegmentedEstimate;
+  slopeChange: SegmentedEstimate;
+  /** Coefficient of determination of the fit. */
+  rSquared: number;
+  /** Lag-1 autocorrelation of the residuals (a reader's sanity check). */
+  residualAutocorrelation: number;
+}
+
+const K = 4;
+
+/** Automatic Newey–West lag: floor(4·(n/100)^(2/9)). */
+export function neweyWestLag(n: number): number {
+  return Math.floor(4 * Math.pow(n / 100, 2 / 9));
+}
+
+/** Invert a small symmetric positive-definite matrix (Gauss–Jordan). */
+function invert(matrix: number[][]): number[][] | null {
+  const size = matrix.length;
+  const a = matrix.map((row, i) => [
+    ...row,
+    ...Array.from({ length: size }, (_, j) => (i === j ? 1 : 0)),
+  ]);
+  for (let col = 0; col < size; col++) {
+    let pivot = col;
+    for (let r = col + 1; r < size; r++) {
+      if (Math.abs(a[r][col]) > Math.abs(a[pivot][col])) pivot = r;
+    }
+    if (Math.abs(a[pivot][col]) < 1e-12) return null;
+    [a[col], a[pivot]] = [a[pivot], a[col]];
+    const p = a[col][col];
+    for (let j = 0; j < 2 * size; j++) a[col][j] /= p;
+    for (let r = 0; r < size; r++) {
+      if (r === col) continue;
+      const f = a[r][col];
+      if (f === 0) continue;
+      for (let j = 0; j < 2 * size; j++) a[r][j] -= f * a[col][j];
+    }
+  }
+  return a.map((row) => row.slice(size));
+}
+
+function designRow(p: SegmentedPoint): number[] {
+  const d = p.t >= 0 ? 1 : 0;
+  return [1, p.t, d, p.t * d];
+}
+
+/**
+ * Fit the four-parameter segmented model. Returns null when the design is
+ * singular (no points on one side, or every point at one instant) or when
+ * there are no residual degrees of freedom.
+ */
+export function fitSegmentedRegression(
+  points: readonly SegmentedPoint[],
+  options: { level?: number; hacLag?: number } = {},
+): SegmentedFit | null {
+  const level = options.level ?? 0.95;
+  const n = points.length;
+  const df = n - K;
+  if (df < 1) return null;
+
+  const X = points.map(designRow);
+  const y = points.map((p) => p.y);
+
+  const xtx = Array.from({ length: K }, () => new Array<number>(K).fill(0));
+  const xty = new Array<number>(K).fill(0);
+  for (let i = 0; i < n; i++) {
+    for (let r = 0; r < K; r++) {
+      xty[r] += X[i][r] * y[i];
+      for (let c = 0; c < K; c++) xtx[r][c] += X[i][r] * X[i][c];
+    }
+  }
+  const bread = invert(xtx);
+  if (!bread) return null;
+  const beta = bread.map((row) => row.reduce((s, v, c) => s + v * xty[c], 0));
+
+  const residuals = y.map(
+    (yi, i) => yi - X[i].reduce((s, v, c) => s + v * beta[c], 0),
+  );
+
+  // Meat: Σ e_t² x_t x_tᵀ + Σ_l w_l Σ_t e_t e_{t−l} (x_t x_{t−l}ᵀ + x_{t−l} x_tᵀ)
+  const lag = Math.max(0, Math.min(options.hacLag ?? neweyWestLag(n), n - 1));
+  const meat = Array.from({ length: K }, () => new Array<number>(K).fill(0));
+  for (let l = 0; l <= lag; l++) {
+    const w = l === 0 ? 1 : 1 - l / (lag + 1);
+    for (let t = l; t < n; t++) {
+      const ee = residuals[t] * residuals[t - l] * w;
+      for (let r = 0; r < K; r++) {
+        for (let c = 0; c < K; c++) {
+          meat[r][c] +=
+            l === 0
+              ? ee * X[t][r] * X[t][c]
+              : ee * (X[t][r] * X[t - l][c] + X[t - l][r] * X[t][c]);
+        }
+      }
+    }
+  }
+  const correction = n / df;
+  const cov = Array.from({ length: K }, (_, r) =>
+    Array.from({ length: K }, (_, c) => {
+      let s = 0;
+      for (let i = 0; i < K; i++) {
+        for (let j = 0; j < K; j++) s += bread[r][i] * meat[i][j] * bread[j][c];
+      }
+      return s * correction;
+    }),
+  );
+
+  const tCrit = studentTQuantile(1 - (1 - level) / 2, df);
+  const estimateAt = (i: number): SegmentedEstimate => {
+    const se = Math.sqrt(Math.max(0, cov[i][i]));
+    return {
+      estimate: beta[i],
+      standardError: se,
+      ciLow: beta[i] - tCrit * se,
+      ciHigh: beta[i] + tCrit * se,
+    };
+  };
+
+  const meanY = y.reduce((s, v) => s + v, 0) / n;
+  let ssr = 0;
+  let sst = 0;
+  for (let i = 0; i < n; i++) {
+    ssr += residuals[i] ** 2;
+    sst += (y[i] - meanY) ** 2;
+  }
+  let acNum = 0;
+  for (let i = 1; i < n; i++) acNum += residuals[i] * residuals[i - 1];
+
+  return {
+    n,
+    df,
+    level,
+    hacLag: lag,
+    intercept: estimateAt(0),
+    preSlope: estimateAt(1),
+    levelChange: estimateAt(2),
+    slopeChange: estimateAt(3),
+    rSquared: sst > 0 ? 1 - ssr / sst : 1,
+    residualAutocorrelation: ssr > 0 ? acNum / ssr : 0,
+  };
+}
+
+// ── Student's t ────────────────────────────────────────────────────────
+
+/** ln Γ(x), Lanczos approximation (g = 7, n = 9). */
+function logGamma(x: number): number {
+  const c = [
+    0.99999999999980993, 676.5203681218851, -1259.1392167224028,
+    771.32342877765313, -176.61502916214059, 12.507343278686905,
+    -0.13857109526572012, 9.9843695780195716e-6, 1.5056327351493116e-7,
+  ];
+  if (x < 0.5) {
+    return Math.log(Math.PI / Math.sin(Math.PI * x)) - logGamma(1 - x);
+  }
+  const z = x - 1;
+  let a = c[0];
+  const t = z + 7.5;
+  for (let i = 1; i < 9; i++) a += c[i] / (z + i);
+  return (
+    0.5 * Math.log(2 * Math.PI) + (z + 0.5) * Math.log(t) - t + Math.log(a)
+  );
+}
+
+/** Continued fraction for the incomplete beta (Numerical Recipes betacf). */
+function betaContinuedFraction(x: number, a: number, b: number): number {
+  const tiny = 1e-300;
+  let c = 1;
+  let d = 1 - ((a + b) * x) / (a + 1);
+  if (Math.abs(d) < tiny) d = tiny;
+  d = 1 / d;
+  let h = d;
+  for (let m = 1; m <= 300; m++) {
+    const m2 = 2 * m;
+    let aa = (m * (b - m) * x) / ((a + m2 - 1) * (a + m2));
+    d = 1 + aa * d;
+    if (Math.abs(d) < tiny) d = tiny;
+    c = 1 + aa / c;
+    if (Math.abs(c) < tiny) c = tiny;
+    d = 1 / d;
+    h *= d * c;
+    aa = (-(a + m) * (a + b + m) * x) / ((a + m2) * (a + m2 + 1));
+    d = 1 + aa * d;
+    if (Math.abs(d) < tiny) d = tiny;
+    c = 1 + aa / c;
+    if (Math.abs(c) < tiny) c = tiny;
+    d = 1 / d;
+    const delta = d * c;
+    h *= delta;
+    if (Math.abs(delta - 1) < 1e-15) break;
+  }
+  return h;
+}
+
+/** Regularised incomplete beta I_x(a, b). */
+function incompleteBeta(x: number, a: number, b: number): number {
+  if (x <= 0) return 0;
+  if (x >= 1) return 1;
+  const front = Math.exp(
+    logGamma(a + b) -
+      logGamma(a) -
+      logGamma(b) +
+      a * Math.log(x) +
+      b * Math.log(1 - x),
+  );
+  if (x < (a + 1) / (a + b + 2)) {
+    return (front * betaContinuedFraction(x, a, b)) / a;
+  }
+  return 1 - (front * betaContinuedFraction(1 - x, b, a)) / b;
+}
+
+/** Student's t cumulative distribution function. */
+export function studentTCdf(t: number, df: number): number {
+  const x = df / (df + t * t);
+  const tail = 0.5 * incompleteBeta(x, df / 2, 0.5);
+  return t >= 0 ? 1 - tail : tail;
+}
+
+/** Student's t quantile, by bisection on the CDF (monotone, so it converges). */
+export function studentTQuantile(p: number, df: number): number {
+  if (p <= 0) return Number.NEGATIVE_INFINITY;
+  if (p >= 1) return Number.POSITIVE_INFINITY;
+  if (p === 0.5) return 0;
+  let lo = -1e3;
+  let hi = 1e3;
+  for (let i = 0; i < 200; i++) {
+    const mid = (lo + hi) / 2;
+    if (studentTCdf(mid, df) < p) lo = mid;
+    else hi = mid;
+    if (hi - lo < 1e-12) break;
+  }
+  return (lo + hi) / 2;
+}