diff --git a/lib/compute/derivation_engine.dart b/lib/compute/derivation_engine.dart
index 16eb8578b..da8857c74 100644
--- a/lib/compute/derivation_engine.dart
+++ b/lib/compute/derivation_engine.dart
@@ -10269,7 +10269,13 @@ class DerivationEngine {
             'confidence': nap.confidence,
           },
       ];
-      candidates?.addAll(detected.where((n) => !napEdits.any((e) =>
+      // A nap longer than kMaxNapSec (3 h) is a sleep, not a nap — drop it
+      // before it can reach the suggestions, `nap_min`, sleep need or the
+      // timeline. Manual entries are already capped at entry time by
+      // manualNapWindowIsValid, which shares the same constant. The merge
+      // input below stays as-is; only the detected list is capped.
+      final capped = dropOversizedNaps(detected);
+      candidates?.addAll(capped.where((n) => !napEdits.any((e) =>
         e.overlaps(n['start'] as int, n['end'] as int))));
       final merged = applyNapEdits(const [], napEdits);
 
diff --git a/lib/compute/nap_edits.dart b/lib/compute/nap_edits.dart
index ccc49887d..2f55908c9 100644
--- a/lib/compute/nap_edits.dart
+++ b/lib/compute/nap_edits.dart
@@ -54,9 +54,14 @@ typedef NapMap = Map<String, dynamic>;
 /// scatter of two-minute entries would swamp the real ones in every total.
 const int kMinManualNapSec = 5 * 60;
 
-/// Maximum. Anything longer is a sleep, not a nap, and belongs in the main
-/// sleep window where the stager can actually say something about its stages.
-const int kMaxManualNapSec = 6 * 60 * 60;
+/// Maximum length of anything counted as a nap: 3 hours. Anything longer is a
+/// sleep, not a nap, and belongs in the main sleep window where the stager can
+/// actually say something about its stages. Applies to detected AND manually
+/// logged naps alike — one cap, one place.
+const int kMaxNapSec = 3 * 60 * 60;
+
+/// Maximum for a manually logged nap — the same cap as detected naps.
+const int kMaxManualNapSec = kMaxNapSec;
 
 /// Whether a window can be logged as a nap.
 bool manualNapWindowIsValid(int startSec, int endSec) {
@@ -129,6 +134,16 @@ List<NapMap> applyNapEdits(List<NapMap> detected, List<NapEdit> edits) {
   return out;
 }
 
+/// Drops every nap whose window exceeds [kMaxNapSec]. A detected bout longer
+/// than 3 h is a sleep fragment the detector misfiled as a nap; crediting it
+/// into `nap_min` would count a night's sleep as daytime rest. The filter is
+/// pure so the rule is testable without a derivation run.
+List<NapMap> dropOversizedNaps(List<NapMap> naps) => [
+      for (final n in naps)
+        if ((n['end'] as num).toInt() - (n['start'] as num).toInt() <= kMaxNapSec)
+          n,
+    ];
+
 /// Total asleep minutes across [naps] — the `nap_min` scalar.
 ///
 /// Always a number, including 0 for an empty list. The absent-versus-zero
diff --git a/test/nap_edits_test.dart b/test/nap_edits_test.dart
index 2b745b4bf..36bd4f9cf 100644
--- a/test/nap_edits_test.dart
+++ b/test/nap_edits_test.dart
@@ -218,12 +218,37 @@ void main() {
       expect(manualNapWindowIsValid(0, kMaxManualNapSec), isTrue);
     });
 
+    test('the cap is 3 hours — exactly 3h is a nap, 3h+1s is a night', () {
+      expect(kMaxNapSec, 3 * hour);
+      expect(kMaxManualNapSec, kMaxNapSec);
+      expect(manualNapWindowIsValid(0, 3 * hour), isTrue);
+      expect(manualNapWindowIsValid(0, 3 * hour + 1), isFalse);
+    });
+
     test('rejects a backwards or empty window', () {
       expect(manualNapWindowIsValid(100, 100), isFalse);
       expect(manualNapWindowIsValid(200, 100), isFalse);
     });
   });
 
+  group('dropOversizedNaps', () {
+    test('keeps naps up to 3h, drops longer ones', () {
+      final naps = [
+        {'start': 0, 'end': 2 * hour, 'duration_min': 120},
+        {'start': 3 * hour, 'end': 6 * hour, 'duration_min': 180}, // exactly 3h
+        {'start': 7 * hour, 'end': 11 * hour, 'duration_min': 240}, // 4h -> drop
+      ];
+      final kept = dropOversizedNaps(naps);
+      expect(kept.length, 2);
+      expect(kept[0]['end'], 2 * hour);
+      expect(kept[1]['end'], 6 * hour);
+    });
+
+    test('an empty list stays empty', () {
+      expect(dropOversizedNaps(const []), isEmpty);
+    });
+  });
+
   group('manualNapWindowHasEnded', () {
     test('accepts a nap that is over', () {
       expect(manualNapWindowHasEnded(14 * hour, 15 * hour), isTrue);