HRV +

Requires a Polar H6/H7/H10 HRM monitor, although it may work with other BLE Bluetooth HRM monitors.

Heart Rate Variability (HRV) is a great indicator of what is going on inside your body. It's a measurement of how well the mind can slow heart rate (HR), which is an indicator of overall health and stress. In general, better health is indicated using HRV when there is more variability from an in-breath (HR speeding up) to an out-breath (HR slowing down). A simple one-minute measurement each day can be enough to predict when it is time to take it easy, to avoid sickness or injury.

This application has some nice features like export RAW RR files for Kubios testing as well as unlimited time testing if you would like to perform other types of HRV tests. Additionally, you can export all data including RR files to DropBox.