diff --git a/fluid_sim.cpp b/fluid_sim.cpp
index 909cb7e..ba37f62 100644
--- a/fluid_sim.cpp
+++ b/fluid_sim.cpp
@@ -252,27 +252,44 @@ void mpuActiveMonitorMode() {
 
 
 
-struct GridCell {
-  uint8_t particles[10];
-  uint8_t count;
-};
-
-GridCell grid[GRID_SIZE][GRID_SIZE];
+// Optimized spatial grid using single array + cell indices
+uint8_t gridParticles[NUM_PARTICLES];  // All particles in grid order
+uint16_t gridCellStart[GRID_SIZE * GRID_SIZE];  // Start index for each cell
+uint8_t gridCellCount[GRID_SIZE * GRID_SIZE];   // Count for each cell
 
 #ifdef DEBUG
 bool blinkState = true;
 #endif
 
 void buildSpatialGrid() {
-  memset(grid, 0, sizeof(grid));
-
-  for(int i=0; i<NUM_PARTICLES; i++) {
+  // Clear cell counts
+  memset(gridCellCount, 0, sizeof(gridCellCount));
+  
+  // Count particles per cell
+  for(int i = 0; i < NUM_PARTICLES; i++) {
     int gx = constrain(particles[i].x / CELL_SIZE, 0, GRID_SIZE-1);
     int gy = constrain(particles[i].y / CELL_SIZE, 0, GRID_SIZE-1);
-    
-    if(grid[gx][gy].count < 10) {
-      grid[gx][gy].particles[grid[gx][gy].count++] = i;
-    }
+    int cellIndex = gy * GRID_SIZE + gx;
+    gridCellCount[cellIndex]++;
+  }
+  
+  // Calculate start indices (prefix sum)
+  gridCellStart[0] = 0;
+  for(int i = 1; i < GRID_SIZE * GRID_SIZE; i++) {
+    gridCellStart[i] = gridCellStart[i-1] + gridCellCount[i-1];
+  }
+  
+  // Reset counts for filling
+  memset(gridCellCount, 0, sizeof(gridCellCount));
+  
+  // Fill particle indices
+  for(int i = 0; i < NUM_PARTICLES; i++) {
+    int gx = constrain(particles[i].x / CELL_SIZE, 0, GRID_SIZE-1);
+    int gy = constrain(particles[i].y / CELL_SIZE, 0, GRID_SIZE-1);
+    int cellIndex = gy * GRID_SIZE + gx;
+    int insertPos = gridCellStart[cellIndex] + gridCellCount[cellIndex];
+    gridParticles[insertPos] = i;
+    gridCellCount[cellIndex]++;
   }
 }
 
@@ -336,6 +353,10 @@ void setup() {
 #endif
   attachInterrupt(digitalPinToInterrupt(EXT0_PIN), DMPDataReady, FALLING);
   MPUIntStatus = mpu.getIntStatus();
+  
+  // Initialize sleep timer (start counting from device boot/wake)
+  lastZMot = millis();
+  zMotInterrupt = false;
   // }
   
 
@@ -380,9 +401,12 @@ void applyPhysics() {
         if(gx+dx < 0 || gx+dx >= GRID_SIZE) continue;
         if(gy+dy < 0 || gy+dy >= GRID_SIZE) continue;
         
-        GridCell &cell = grid[gx+dx][gy+dy];
-        for(int c=0; c<cell.count; c++) {
-          const int j = cell.particles[c];
+        int cellIndex = (gy+dy) * GRID_SIZE + (gx+dx);
+        int cellStart = gridCellStart[cellIndex];
+        int cellCount = gridCellCount[cellIndex];
+        
+        for(int c = 0; c < cellCount; c++) {
+          const int j = gridParticles[cellStart + c];
           if(j <= i) continue; // Avoid duplicate pairs
 
           const float dx = particles[j].x - particles[i].x;
@@ -507,6 +531,10 @@ void reportIMU() {
         // Detect sudden changes (ignoring gravity component of ~16384)
         float accelDelta = abs(accelMagnitude - lastAccelMagnitude);
         maxAccelImpulse = max(maxAccelImpulse * 0.9f, accelDelta);
+        
+        // Reset sleep timer on ANY motion/DMP data (device is active)
+        lastZMot = millis();
+        zMotInterrupt = false;  // Clear zero motion flag since we have motion
     }
   }
   else {