One lesser known improvement to the Health app in iOS 11 was the addition of Heart Rate Variability data, which can be obtained in a variety of ways, including via the Apple Watch. According to Apple’s description in the Health app:

Heart Rate Variability (HRV) is a measure of the variation in the time interval between heart beats. Apple Watch calculates HRV by using the standard deviation of these beat-to-beat intervals measured by the heart rate sensor (also known as SDNN).

While that offers a basic explanation of this data type, Serenity Caldwell of iMore just completed a deep dive into exactly what HRV is, how tracking it can be beneficial, and Apple’s current methods of tracking it. She writes:

Apple currently records HRV averages in your iPhone’s Health app through Apple Watch readings (as well as any third-party apps that have chosen to write data to the repository). When you first put your Apple Watch on for the day, you’ll trigger an HRV morning reading; the wearable monitors your heartbeat steadily for one minute, then uses under-the-hood calculations* to come up with your HRV average, displayed as ms (milliseconds) in the Health app for iPhone.

Caldwell also shares recommendations for different apps that can be paired with additional tracking methods – such as third-party heart monitors or even just your iPhone’s camera – to obtain more extensive data than the Apple Watch provides with its daily readings.

If you’re interested at all in Heart Rate Variability, and what your iPhone or Apple Watch can do to track it, Caldwell’s write-up is a fantastic resource.