HRV Tracker Pro is a heart rate variability tracker which lets you track HRV automatically with Apple Watch, or manually using the iPhone camera. The app uses the RMSSD formula for calculating HRV, which is the formula used by most established fitness brands. Apple Watch mode: Apple Watch measures heartbeats throughout the day. HRV Tracker Pro uses these measurements to calculate your sleeping HRV. By focusing exclusively on measurements during sleep, the app provides a more consistent baseline