Lens HRV is an Android app that measures <b>heart-rate variability</b> — how much the time between your heartbeats changes from one beat to the next, measured in milliseconds. It uses Photoplethysmography (PPG), a simple optical technique that detects tiny changes in blood flow through your fingertip using only your phone’s camera and flash. Completely non-invasive and requires no extra sensors. <b>Why HRV matters</b> Heart-rate variability is more than just a heart measurement. The small changes between your heartbeats come from many processes happening in the body at the same time. Your heart does not work alone. It is constantly interacting with the rest of the body — the nervous system, the lungs, hormones, temperature, the immune system, digestion, circadian cycles, emotions and more. HRV works like a microscope that lets us observe that interconnected network. <b>What makes Lens HRV different</b> Most HRV apps today work like closed boxes. They are usually paid, require expensive extra sensors, and only show you the final numbers they decide to calculate. You rarely get access to the raw signal. Many of them also oversimplify the body’s state into a single score or color. Lens HRV is completely open source. Every algorithm it uses comes from publicly available, peer-reviewed research. The focus is on classic digital signal processing instead of heavy artificial intelligence. This keeps the app lightweight, understandable, and able to run on a wide range of devices. All data is processed directly on your phone — nothing is sent to the cloud. You keep full privacy and full control. The app does not offer “readiness” scores. That kind of metric is considered too complex and often oversimplified. Instead, Lens HRV focuses on clear and simple measurements. The goal is a frictionless tool that is accessible to everyone. <b>How it works</b> Place your index finger over the camera lens until the preview turns a deep red. A short calibration starts the recordi