diff --git a/lib/src/onehz/clinical/hrv_freq.dart b/lib/src/onehz/clinical/hrv_freq.dart
index 70dcb6e..d759248 100644
--- a/lib/src/onehz/clinical/hrv_freq.dart
+++ b/lib/src/onehz/clinical/hrv_freq.dart
@@ -231,7 +231,20 @@ double? _welchBandPower(
       ts.add(tSec[i]);
       ys.add(y[i]);
     }
-    if (ts.length < minPointsPerSegment) continue;
+    // A segment has to be BOTH beat-dense and time-complete: a dropout in
+    // the middle leaves few beats spanning the full window, and its
+    // periodogram is a window function, not a spectrum. Endpoint span alone
+    // misses an internal dropout that still leaves beats near both edges, so
+    // also reject on the single largest gap between consecutive beats.
+    if (ts.length < minPointsPerSegment || ts.last - ts.first < segSec * 0.8) {
+      continue;
+    }
+    var maxGap = 0.0;
+    for (var i = 1; i < ts.length; i++) {
+      final g = ts[i] - ts[i - 1];
+      if (g > maxGap) maxGap = g;
+    }
+    if (maxGap > segSec * 0.2) continue;
     final ls = lombScargle(ts, ys, grid);
     if (ls == null) continue;
     final p = ls.bandPower(loHz, hiHz);
diff --git a/test/onehz/clinical_test.dart b/test/onehz/clinical_test.dart
index af03c6e..aaa5d0e 100644
--- a/test/onehz/clinical_test.dart
+++ b/test/onehz/clinical_test.dart
@@ -114,6 +114,77 @@ void main() {
       expect(m.value!.hf!, greaterThan(0));
     });
 
+    test(
+        'a mid-segment recording gap is rejected, not averaged in as a '
+        'window function', () {
+      // HF's segment is 10 cycles of its 0.15 Hz floor = ~66.7 s. Beats
+      // clustered in the first ~27 s of that window, then a 45 s gap (an
+      // off-wrist moment, a BLE reconnect mid-drain), still total >=16
+      // points — enough to pass a beat-COUNT-only guard — but the window is
+      // no longer time-complete, so its periodogram is a window function,
+      // not a spectrum. resp_rate.dart's identical Welch loop already
+      // guards this with `span < segSec * 0.8`; hrv_freq's copy must too.
+      final rr = <double>[];
+      final times = <double>[];
+      var t = 0.0;
+      for (var i = 0; i < 28; i++) {
+        final v = 1000 + 40 * math.sin(2 * math.pi * 0.25 * (t / 1000));
+        rr.add(v);
+        t += v;
+        times.add(t);
+      }
+      t += 45000; // the gap
+      for (var i = 0; i < 4; i++) {
+        rr.add(1000);
+        t += 1000;
+        times.add(t);
+      }
+      final gapped = hrvFreq(rr, times, artifactFraction: 0.0);
+      expect(gapped.value?.hf, isNull,
+          reason: 'the only segment spanning the record is gap-corrupted');
+
+      // Same span, no gap: the segment is beat-dense AND time-complete.
+      final rrControl = <double>[];
+      final timesControl = <double>[];
+      t = 0.0;
+      while (t < 90000) {
+        final v = 1000 + 40 * math.sin(2 * math.pi * 0.25 * (t / 1000));
+        rrControl.add(v);
+        t += v;
+        timesControl.add(t);
+      }
+      final control = hrvFreq(rrControl, timesControl, artifactFraction: 0.0);
+      expect(control.value!.hf, isNotNull);
+    });
+
+    test(
+        'an INTERNAL gap with beats on both sides is rejected, not just an '
+        'endpoint-span shortfall', () {
+      // Beats near both edges of the ~66.7 s HF window, with a 40 s hole in
+      // the middle, can still clear the endpoint-span check (first-to-last
+      // beat still covers most of the window) while the window itself is
+      // half-empty. Guard on the single largest inter-beat gap too.
+      final rr = <double>[];
+      final times = <double>[];
+      var t = 0.0;
+      while (t < 20000) {
+        final v = 1000 + 40 * math.sin(2 * math.pi * 0.25 * (t / 1000));
+        rr.add(v);
+        t += v;
+        times.add(t);
+      }
+      t += 40000; // the internal gap
+      while (t < 66000) {
+        final v = 1000 + 40 * math.sin(2 * math.pi * 0.25 * (t / 1000));
+        rr.add(v);
+        t += v;
+        times.add(t);
+      }
+      final gapped = hrvFreq(rr, times, artifactFraction: 0.0);
+      expect(gapped.value?.hf, isNull,
+          reason: 'endpoints span the window but the middle is empty');
+    });
+
     test('GATES HF when artifact fraction exceeds the threshold', () {
       // RE-PINNED 2026-08: 400 beats, not 64. Band powers are now Welch-
       // averaged over segments long enough to RESOLVE the band (10 cycles of