We track your heartbeat through the phone camera & flash in order to measure heart rate variability (HRV). As your heart beats, blood flows in and out of your finger. These blood flows make your finger change color right below the skin’s surface. The phone’s flash illuminates your finger, and the camera takes a short clip of the color changes.

This non-invasive, light-based method of tracking the heartbeat is called photoplethysmography, or PPG. It’s how many smartwatches and fitness trackers track heart rate.

But what Welltory measures is heart rate variability, which is not the same thing as heart rate.