diff --git a/lib/GfxRenderer/Bitmap.cpp b/lib/GfxRenderer/Bitmap.cpp
index ad25ffc..63535ac 100644
--- a/lib/GfxRenderer/Bitmap.cpp
+++ b/lib/GfxRenderer/Bitmap.cpp
@@ -262,3 +262,90 @@ BmpReaderError Bitmap::rewindToData() const {
 
   return BmpReaderError::Ok;
 }
+
+bool Bitmap::detectPerimeterIsBlack() const {
+  // Detect if the 1-pixel perimeter of the image is mostly black or white.
+  // Returns true if mostly black (luminance < 128), false if mostly white.
+
+  if (width <= 0 || height <= 0) return false;
+
+  auto* rowBuffer = static_cast<uint8_t*>(malloc(rowBytes));
+  if (!rowBuffer) return false;
+
+  int blackCount = 0;
+  int whiteCount = 0;
+
+  // Helper lambda to get luminance from a pixel at position x in rowBuffer
+  auto getLuminance = [&](int x) -> uint8_t {
+    switch (bpp) {
+      case 32: {
+        const uint8_t* p = rowBuffer + x * 4;
+        return (77u * p[2] + 150u * p[1] + 29u * p[0]) >> 8;
+      }
+      case 24: {
+        const uint8_t* p = rowBuffer + x * 3;
+        return (77u * p[2] + 150u * p[1] + 29u * p[0]) >> 8;
+      }
+      case 8:
+        return paletteLum[rowBuffer[x]];
+      case 2:
+        return paletteLum[(rowBuffer[x >> 2] >> (6 - ((x & 3) * 2))) & 0x03];
+      case 1: {
+        const uint8_t palIndex = (rowBuffer[x >> 3] & (0x80 >> (x & 7))) ? 1 : 0;
+        return paletteLum[palIndex];
+      }
+      default:
+        return 128;  // Neutral if unsupported
+    }
+  };
+
+  // Helper to classify and count a pixel
+  auto countPixel = [&](int x) {
+    const uint8_t lum = getLuminance(x);
+    if (lum < 128) {
+      blackCount++;
+    } else {
+      whiteCount++;
+    }
+  };
+
+  // Helper to seek to a specific image row (accounting for top-down vs bottom-up)
+  auto seekToRow = [&](int imageRow) -> bool {
+    // In bottom-up BMP (topDown=false), row 0 in file is the bottom row of image
+    // In top-down BMP (topDown=true), row 0 in file is the top row of image
+    int fileRow = topDown ? imageRow : (height - 1 - imageRow);
+    return file.seek(bfOffBits + static_cast<uint32_t>(fileRow) * rowBytes);
+  };
+
+  // Sample top row (image row 0) - all pixels
+  if (seekToRow(0) && file.read(rowBuffer, rowBytes) == rowBytes) {
+    for (int x = 0; x < width; x++) {
+      countPixel(x);
+    }
+  }
+
+  // Sample bottom row (image row height-1) - all pixels
+  if (height > 1) {
+    if (seekToRow(height - 1) && file.read(rowBuffer, rowBytes) == rowBytes) {
+      for (int x = 0; x < width; x++) {
+        countPixel(x);
+      }
+    }
+  }
+
+  // Sample left and right edges from intermediate rows
+  for (int y = 1; y < height - 1; y++) {
+    if (seekToRow(y) && file.read(rowBuffer, rowBytes) == rowBytes) {
+      countPixel(0);              // Left edge
+      countPixel(width - 1);      // Right edge
+    }
+  }
+
+  free(rowBuffer);
+
+  // Rewind file position for subsequent drawing
+  rewindToData();
+
+  // Return true if perimeter is mostly black
+  return blackCount > whiteCount;
+}
diff --git a/lib/GfxRenderer/Bitmap.h b/lib/GfxRenderer/Bitmap.h
index 544869c..56cfe3e 100644
--- a/lib/GfxRenderer/Bitmap.h
+++ b/lib/GfxRenderer/Bitmap.h
@@ -37,6 +37,7 @@ class Bitmap {
   BmpReaderError parseHeaders();
   BmpReaderError readNextRow(uint8_t* data, uint8_t* rowBuffer) const;
   BmpReaderError rewindToData() const;
+  bool detectPerimeterIsBlack() const;
   int getWidth() const { return width; }
   int getHeight() const { return height; }
   bool isTopDown() const { return topDown; }
diff --git a/src/activities/boot_sleep/SleepActivity.cpp b/src/activities/boot_sleep/SleepActivity.cpp
index 91d75ab..8411294 100644
--- a/src/activities/boot_sleep/SleepActivity.cpp
+++ b/src/activities/boot_sleep/SleepActivity.cpp
@@ -12,6 +12,13 @@
 #include "images/CrossLarge.h"
 #include "util/StringUtils.h"
 
+namespace {
+// Perimeter cache file format:
+// - 4 bytes: uint32_t file size (for cache invalidation)
+// - 1 byte: result (0 = white perimeter, 1 = black perimeter)
+constexpr size_t PERIM_CACHE_SIZE = 5;
+}  // namespace
+
 void SleepActivity::onEnter() {
   Activity::onEnter();
   renderPopup("Entering Sleep...");
@@ -88,14 +95,14 @@ void SleepActivity::renderCustomSleepScreen() const {
       }
       APP_STATE.lastSleepImage = randomFileIndex;
       APP_STATE.saveToFile();
-      const auto filename = "/sleep/" + files[randomFileIndex];
+      const auto bmpPath = "/sleep/" + files[randomFileIndex];
       FsFile file;
-      if (SdMan.openFileForRead("SLP", filename, file)) {
+      if (SdMan.openFileForRead("SLP", bmpPath, file)) {
         Serial.printf("[%lu] [SLP] Randomly loading: /sleep/%s\n", millis(), files[randomFileIndex].c_str());
         delay(100);
         Bitmap bitmap(file, true);
         if (bitmap.parseHeaders() == BmpReaderError::Ok) {
-          renderBitmapSleepScreen(bitmap);
+          renderBitmapSleepScreen(bitmap, bmpPath);
           dir.close();
           return;
         }
@@ -107,11 +114,12 @@ void SleepActivity::renderCustomSleepScreen() const {
   // Look for sleep.bmp on the root of the sd card to determine if we should
   // render a custom sleep screen instead of the default.
   FsFile file;
-  if (SdMan.openFileForRead("SLP", "/sleep.bmp", file)) {
+  const std::string rootSleepPath = "/sleep.bmp";
+  if (SdMan.openFileForRead("SLP", rootSleepPath, file)) {
     Bitmap bitmap(file, true);
     if (bitmap.parseHeaders() == BmpReaderError::Ok) {
       Serial.printf("[%lu] [SLP] Loading: /sleep.bmp\n", millis());
-      renderBitmapSleepScreen(bitmap);
+      renderBitmapSleepScreen(bitmap, rootSleepPath);
       return;
     }
   }
@@ -136,13 +144,15 @@ void SleepActivity::renderDefaultSleepScreen() const {
   renderer.displayBuffer(EInkDisplay::HALF_REFRESH);
 }
 
-void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap) const {
+void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap, const std::string& bmpPath) const {
   int x, y;
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();
   float cropX = 0, cropY = 0;
   int drawWidth = pageWidth;
   int drawHeight = pageHeight;
+  int fillWidth = pageWidth;   // Actual area the image will occupy
+  int fillHeight = pageHeight;
 
   Serial.printf("[%lu] [SLP] bitmap %d x %d, screen %d x %d\n", millis(), bitmap.getWidth(), bitmap.getHeight(),
                 pageWidth, pageHeight);
@@ -160,6 +170,8 @@ void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap) const {
     // Don't constrain to screen dimensions - drawBitmap will clip
     drawWidth = 0;
     drawHeight = 0;
+    fillWidth = bitmap.getWidth();
+    fillHeight = bitmap.getHeight();
     Serial.printf("[%lu] [SLP] ACTUAL mode: centering at %d, %d\n", millis(), x, y);
   } else if (coverMode == CrossPointSettings::SLEEP_SCREEN_COVER_MODE::CROP) {
     // CROP mode: Scale to fill screen completely (may crop edges)
@@ -176,6 +188,8 @@ void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap) const {
     // After cropping, the image should fill the screen exactly
     x = 0;
     y = 0;
+    fillWidth = pageWidth;
+    fillHeight = pageHeight;
     Serial.printf("[%lu] [SLP] CROP mode: drawing at 0, 0 with crop %f, %f\n", millis(), cropX, cropY);
   } else {
     // FIT mode (default): Scale to fit entire image within screen (may have letterboxing)
@@ -188,18 +202,28 @@ void SleepActivity::renderBitmapSleepScreen(const Bitmap& bitmap) const {
       // Image is taller than screen ratio - fit to height
       scale = static_cast<float>(pageHeight) / static_cast<float>(bitmap.getHeight());
     }
-    const int scaledWidth = static_cast<int>(bitmap.getWidth() * scale);
-    const int scaledHeight = static_cast<int>(bitmap.getHeight() * scale);
+    fillWidth = static_cast<int>(bitmap.getWidth() * scale);
+    fillHeight = static_cast<int>(bitmap.getHeight() * scale);
 
     // Center the scaled image
-    x = (pageWidth - scaledWidth) / 2;
-    y = (pageHeight - scaledHeight) / 2;
+    x = (pageWidth - fillWidth) / 2;
+    y = (pageHeight - fillHeight) / 2;
     Serial.printf("[%lu] [SLP] FIT mode: scale %f, scaled size %d x %d, position %d, %d\n", millis(), scale,
-                  scaledWidth, scaledHeight, x, y);
+                  fillWidth, fillHeight, x, y);
   }
 
+  // Detect perimeter color and clear to matching background
+  const bool isBlackPerimeter = getPerimeterIsBlack(bitmap, bmpPath);
+  const uint8_t clearColor = isBlackPerimeter ? 0x00 : 0xFF;
+
   Serial.printf("[%lu] [SLP] drawing to %d x %d\n", millis(), x, y);
-  renderer.clearScreen();
+  renderer.clearScreen(clearColor);
+
+  // If background is black, fill the image area with white first so white pixels render correctly
+  if (isBlackPerimeter) {
+    renderer.fillRect(x, y, fillWidth, fillHeight, false);  // false = white
+  }
+
   renderer.drawBitmap(bitmap, x, y, drawWidth, drawHeight, cropX, cropY);
   renderer.displayBuffer(EInkDisplay::HALF_REFRESH);
 
@@ -281,7 +305,7 @@ void SleepActivity::renderCoverSleepScreen() const {
   if (SdMan.openFileForRead("SLP", coverBmpPath, file)) {
     Bitmap bitmap(file);
     if (bitmap.parseHeaders() == BmpReaderError::Ok) {
-      renderBitmapSleepScreen(bitmap);
+      renderBitmapSleepScreen(bitmap, coverBmpPath);
       return;
     }
   }
@@ -293,3 +317,80 @@ void SleepActivity::renderBlankSleepScreen() const {
   renderer.clearScreen();
   renderer.displayBuffer(EInkDisplay::HALF_REFRESH);
 }
+
+std::string SleepActivity::getPerimeterCachePath(const std::string& bmpPath) {
+  // Convert "/dir/file.bmp" to "/dir/.file.bmp.perim"
+  const size_t lastSlash = bmpPath.find_last_of('/');
+  if (lastSlash == std::string::npos) {
+    // No directory, just prepend dot
+    return "." + bmpPath + ".perim";
+  }
+  const std::string dir = bmpPath.substr(0, lastSlash + 1);
+  const std::string filename = bmpPath.substr(lastSlash + 1);
+  return dir + "." + filename + ".perim";
+}
+
+bool SleepActivity::getPerimeterIsBlack(const Bitmap& bitmap, const std::string& bmpPath) const {
+  const std::string cachePath = getPerimeterCachePath(bmpPath);
+
+  // Try to read from cache
+  FsFile cacheFile;
+  if (SdMan.openFileForRead("SLP", cachePath, cacheFile)) {
+    uint8_t cacheData[PERIM_CACHE_SIZE];
+    if (cacheFile.read(cacheData, PERIM_CACHE_SIZE) == PERIM_CACHE_SIZE) {
+      // Extract cached file size
+      const uint32_t cachedSize = static_cast<uint32_t>(cacheData[0]) |
+                                  (static_cast<uint32_t>(cacheData[1]) << 8) |
+                                  (static_cast<uint32_t>(cacheData[2]) << 16) |
+                                  (static_cast<uint32_t>(cacheData[3]) << 24);
+
+      // Get current BMP file size
+      FsFile bmpFile;
+      uint32_t currentSize = 0;
+      if (SdMan.openFileForRead("SLP", bmpPath, bmpFile)) {
+        currentSize = bmpFile.size();
+        bmpFile.close();
+      }
+
+      // Validate cache
+      if (cachedSize == currentSize && currentSize > 0) {
+        const bool result = cacheData[4] != 0;
+        Serial.printf("[%lu] [SLP] Perimeter cache hit for %s: %s\n", millis(), bmpPath.c_str(),
+                      result ? "black" : "white");
+        cacheFile.close();
+        return result;
+      }
+      Serial.printf("[%lu] [SLP] Perimeter cache invalid (size mismatch: %lu vs %lu)\n", millis(),
+                    static_cast<unsigned long>(cachedSize), static_cast<unsigned long>(currentSize));
+    }
+    cacheFile.close();
+  }
+
+  // Cache miss - calculate perimeter
+  Serial.printf("[%lu] [SLP] Calculating perimeter for %s\n", millis(), bmpPath.c_str());
+  const bool isBlack = bitmap.detectPerimeterIsBlack();
+  Serial.printf("[%lu] [SLP] Perimeter detected: %s\n", millis(), isBlack ? "black" : "white");
+
+  // Get BMP file size for cache
+  FsFile bmpFile;
+  uint32_t fileSize = 0;
+  if (SdMan.openFileForRead("SLP", bmpPath, bmpFile)) {
+    fileSize = bmpFile.size();
+    bmpFile.close();
+  }
+
+  // Save to cache
+  if (fileSize > 0 && SdMan.openFileForWrite("SLP", cachePath, cacheFile)) {
+    uint8_t cacheData[PERIM_CACHE_SIZE];
+    cacheData[0] = fileSize & 0xFF;
+    cacheData[1] = (fileSize >> 8) & 0xFF;
+    cacheData[2] = (fileSize >> 16) & 0xFF;
+    cacheData[3] = (fileSize >> 24) & 0xFF;
+    cacheData[4] = isBlack ? 1 : 0;
+    cacheFile.write(cacheData, PERIM_CACHE_SIZE);
+    cacheFile.close();
+    Serial.printf("[%lu] [SLP] Saved perimeter cache to %s\n", millis(), cachePath.c_str());
+  }
+
+  return isBlack;
+}
diff --git a/src/activities/boot_sleep/SleepActivity.h b/src/activities/boot_sleep/SleepActivity.h
index 283220c..5bcb1d0 100644
--- a/src/activities/boot_sleep/SleepActivity.h
+++ b/src/activities/boot_sleep/SleepActivity.h
@@ -1,6 +1,8 @@
 #pragma once
 #include "../Activity.h"
 
+#include <string>
+
 class Bitmap;
 
 class SleepActivity final : public Activity {
@@ -14,6 +16,10 @@ class SleepActivity final : public Activity {
   void renderDefaultSleepScreen() const;
   void renderCustomSleepScreen() const;
   void renderCoverSleepScreen() const;
-  void renderBitmapSleepScreen(const Bitmap& bitmap) const;
+  void renderBitmapSleepScreen(const Bitmap& bitmap, const std::string& bmpPath) const;
   void renderBlankSleepScreen() const;
+
+  // Perimeter detection caching helpers
+  static std::string getPerimeterCachePath(const std::string& bmpPath);
+  bool getPerimeterIsBlack(const Bitmap& bitmap, const std::string& bmpPath) const;
 };