diff --git a/flake.nix b/flake.nix
index 84a6b58..5fa02b2 100644
--- a/flake.nix
+++ b/flake.nix
@@ -59,7 +59,9 @@
         };
 
         devShells.default = pkgs.mkShell {
-          packages = [ pkgs.go pkgs.python3 ];
+          # nodejs is here for `node --check` on src/*.js — the frontend has no
+          # build step, so this is a syntax-check tool, not a dependency.
+          packages = [ pkgs.go pkgs.python3 pkgs.nodejs ];
           shellHook = ''
             echo "puppy-tracker dev shell"
             echo "  cd server && go run . -static ../src -data /tmp/puppy-events.json"
diff --git a/src/app.js b/src/app.js
index 9b1e0e9..b667da5 100644
--- a/src/app.js
+++ b/src/app.js
@@ -1989,17 +1989,30 @@
   }
 
   // Dim the quick actions that don't fit the current sleep state — a nudge
-  // toward the likely next tap, never a block: everything stays clickable so
-  // corrections (a missed sleep-end, a mid-nap pee) are always possible.
+  // toward the likely next tap. The one boundary that would just repeat the
+  // latest sleep event (a second "sleep start" while already asleep, or a
+  // second "sleep end" while already awake) is disabled outright, since it
+  // can only produce a zero-length window. Everything else stays clickable so
+  // corrections (a mid-nap pee) are never blocked, and a genuinely missed
+  // boundary is still fixable from the event log, which accepts any time.
   function renderActionHints(events) {
     const { state } = currentSleepState(events);
     document.querySelectorAll("button.action").forEach(btn => {
       const type = btn.dataset.type;
-      const unlikely =
+      const repeat =
+        state === "asleep" ? type === "sleep-start"
+        : state === "awake" ? type === "sleep-end"
+        : false; // no sleep history yet — either boundary is a fine first event
+      const unlikely = !repeat && (
         state === "asleep" ? type !== "sleep-end"
         : state === "awake" ? type === "sleep-end"
-        : false; // no sleep history yet — no hints to give
+        : false // no sleep history yet — no hints to give
+      );
       btn.classList.toggle("unlikely", unlikely);
+      btn.disabled = repeat;
+      btn.title = repeat
+        ? (type === "sleep-start" ? "Already asleep" : "Already awake")
+        : "";
     });
   }
 
diff --git a/src/changelog.json b/src/changelog.json
index 14cf02d..8733af7 100644
--- a/src/changelog.json
+++ b/src/changelog.json
@@ -1,4 +1,5 @@
 [
+  { "date": "2026-08-18", "text": "The sleep button that would just repeat the last one is now disabled: while asleep you can only tap ⏰ Sleep end, and while awake only 😴 Sleep start — no more accidental double taps creating zero-length sleep windows. If you did miss a boundary, you can still add it at the right time from the event log" },
   { "date": "2026-08-02", "text": "Added free-text notes: tap 📝 Note to jot down things that happened on a day — vaccinations, vet visits, milestones — with a date, optional photo, and any text. All your notes are collected in a new Notes section that stays visible whatever day you're viewing, so you can see at a glance when things like a tick vaccination were done" },
   { "date": "2026-08-02", "text": "The Daily counts chart now has Pees / Poos / Meals checkboxes so you can focus on just the metrics you care about — untick the rest to see, say, only poos; your choice is remembered" },
   { "date": "2026-08-01", "text": "Logging a pee or poo now sets off 💧/💩 fireworks that shoot up from the bottom of the screen — a little celebration you can switch off in Settings (and it honours a reduced-motion preference)" },
diff --git a/src/style.css b/src/style.css
index daac039..3cd45f9 100644
--- a/src/style.css
+++ b/src/style.css
@@ -272,6 +272,17 @@ button.action.note  { background: var(--note); }
    but fully tappable, so corrections are never blocked. */
 button.action.unlikely { opacity: 0.4; }
 
+/* The boundary that would repeat the latest sleep event is disabled outright
+   (see renderActionHints) — greyed out rather than merely dimmed, so it reads
+   as unavailable instead of just discouraged. */
+button.action:disabled {
+  opacity: 0.35;
+  filter: grayscale(1);
+  cursor: not-allowed;
+}
+button.action:disabled:active { transform: none; }
+button.action:disabled:hover { filter: grayscale(1); }
+
 button.ghost {
   background: transparent;
   color: var(--text);