diff --git a/apps/phystrax/app.js b/apps/phystrax/app.js
index 663652c14..ec2409c94 100644
--- a/apps/phystrax/app.js
+++ b/apps/phystrax/app.js
@@ -1,8 +1,15 @@
-let isMeasuring = false;
-let currentHeartRate = null;
-let lcdTimeout;
-let logData = [];
+var isMeasuring = false;
+var currentHeartRate = null;
+var lcdTimeout;
+var bpmValues = [];
+var logData = [];
+var lastHeartRateLogTimestamp = 0;
 
+// HRV calculation variables
+var hrvSlots = [10, 20, 30, 60, 120, 300];
+var hrvValues = {};
+
+// Heart rate monitor functions
 function startMeasure() {
     isMeasuring = true;
     Bangle.setLCDTimeout(0);
@@ -13,7 +20,7 @@ function startMeasure() {
     setTimeout(() => {
         Bangle.setHRMPower(1);
         Bangle.on('HRM', handleHeartRate);
-        Bangle.beep(400, 1000); // Buzz to indicate measurement start
+        Bangle.buzz(500, 20);
         drawScreen();
     }, 500);
 }
@@ -24,37 +31,22 @@ function stopMeasure() {
     Bangle.setLCDTimeout(10);
     Bangle.setHRMPower(0);
     Bangle.removeAllListeners('HRM');
-    saveDataToCSV(); // Save data to CSV when measurement stops
-    Bangle.beep(400, 800); // Buzz to indicate measurement stop
+    Bangle.buzz(500, 20);
     drawScreen();
 }
 
-function handleHeartRate(hrm) {
-    if (hrm.confidence > 90) {
-        currentHeartRate = hrm.bpm;
-        let date = new Date();
-        let dateStr = require("locale").date(date);
-        let timeStr = require("locale").time(date, 1);
-        let seconds = date.getSeconds();
-        let timestamp = `${dateStr} ${timeStr}:${seconds}`; // Concatenate date, time, and seconds
-        logData.push({ timestamp: timestamp, heartRate: currentHeartRate });
-        drawScreen();
-    }
-}
-
-
 function drawScreen(message) {
     g.clear(); // Clear the display
 
     // Set the background color
-    g.setColor('#95E7FF'); 
+    g.setColor('#95E7FF');
 
     // Fill the entire display with the background color
     g.fillRect(0, 0, g.getWidth(), g.getHeight());
 
     // Set font and alignment for drawing text
     g.setFontAlign(0, 0);
-    g.setFont('Vector', 15); 
+    g.setFont('Vector', 15);
 
     // Draw the title
     g.setColor('#000000'); // Set text color to black
@@ -65,9 +57,9 @@ function drawScreen(message) {
         g.setFont('6x8', 2);
         g.drawString('Measuring...', g.getWidth() / 2, g.getHeight() / 2 - 10);
 
-        // Draw current heart rate if available
-        g.setFont('6x8', 4);
-        if (currentHeartRate !== null) {
+        // Draw current heart rate if available and not zero
+        if (currentHeartRate !== null && currentHeartRate > 0) {
+            g.setFont('6x8', 4);
             g.drawString(currentHeartRate.toString(), g.getWidth() / 2, g.getHeight() / 2 + 20);
             g.setFont('6x8', 1.6);
             g.drawString(' BPM', g.getWidth() / 2 + 42, g.getHeight() / 2 + 20);
@@ -83,6 +75,8 @@ function drawScreen(message) {
         if (currentHeartRate !== null && currentHeartRate > 0) {
             g.setFont('6x8', 4);
             g.drawString(currentHeartRate.toString(), g.getWidth() / 2, g.getHeight() / 2 + 10);
+            g.setFont('6x8', 1.6);
+            g.drawString(' BPM', g.getWidth() / 2 + 42, g.getHeight() / 2 + 12);
         } else {
             g.setFont('6x8', 2);
             g.drawString('No data', g.getWidth() / 2, g.getHeight() / 2 + 10);
@@ -96,14 +90,78 @@ function drawScreen(message) {
 }
 
 function saveDataToCSV() {
-    let csvContent = "Timestamp,Heart Rate\n";
-    logData.forEach(entry => {
-        csvContent += `${entry.timestamp},${entry.heartRate}\n`;
-    });
-    require("Storage").write("heart_rate_data.csv", csvContent);
+    let date = new Date();
+    let dateStr = require("locale").date(date);
+    let timeStr = require("locale").time(date, 1);
+    let seconds = date.getSeconds();
+    let timestamp = `${dateStr} ${timeStr}:${seconds}`;
+
+    // Check if 10 seconds have passed since the last heart rate log
+    if (date.getTime() - lastHeartRateLogTimestamp >= 10 * 1000) {
+        logData.push({ timestamp: timestamp, heartRate: currentHeartRate });
+        lastHeartRateLogTimestamp = date.getTime(); // Update the last heart rate log timestamp
+
+        // Log data to CSV
+        let csvContent = "Timestamp,Heart Rate,HRV (ms)\n";
+        logData.forEach(entry => {
+            if (entry.heartRate > 0) {
+                let hrv = hrvValues[entry.timestamp] || ""; // Get HRV value for the timestamp
+                csvContent += `${entry.timestamp},${entry.heartRate},${hrv}\n`;
+            }
+        });
+        require("Storage").write("heart_rate_data.csv", csvContent);
+    }
 }
 
-setWatch(function() {
+function handleHeartRate(hrm) {
+    if (isMeasuring) {
+        currentHeartRate = hrm.bpm;
+        drawScreen();
+
+        // Log data to CSV
+        saveDataToCSV();
+
+        // Estimate RR intervals
+        let rrIntervals = estimateRRIntervals(bpmValues);
+
+        // Calculate HRV
+        calculateHRV(rrIntervals);
+    }
+}
+
+function estimateRRIntervals(rawData) {
+    let rrIntervals = [];
+    for (let i = 1; i < rawData.length; i++) {
+        let rrInterval = rawData[i] - rawData[i - 1];
+        rrIntervals.push(rrInterval);
+    }
+    return rrIntervals;
+}
+
+function calculateHRV(rawData) {
+    // Calculate HRV with SDNN method for each slot in hrvSlots
+    let sdnn = calcSDNN(rawData);
+    hrvValues[Date.now()] = sdnn; // Store HRV value with current timestamp
+
+    // Log HRV data if 5 minutes have elapsed
+    if (Date.now() - lastHeartRateLogTimestamp >= 5 * 60 * 1000) {
+        console.log("Recalculating HRV...");
+        saveDataToCSV(); // Save both heart rate and HRV data to the CSV file
+        lastHeartRateLogTimestamp = Date.now(); // Update the timestamp
+    }
+}
+
+function calcSDNN(rrIntervals) {
+    let sumOfDifferencesSquared = 0;
+    let meanRRInterval = rrIntervals.reduce((acc, val) => acc + val, 0) / rrIntervals.length;
+    rrIntervals.forEach(rr => {
+        sumOfDifferencesSquared += Math.pow(rr - meanRRInterval, 2);
+    });
+    let meanOfDifferencesSquared = sumOfDifferencesSquared / (rrIntervals.length - 1);
+    return Math.sqrt(meanOfDifferencesSquared);
+}
+
+setWatch(function () {
     if (!isMeasuring) {
         startMeasure();
     } else {