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Every morning as part of my “morning report”

my Garmin watch includes my HRV status. It’s usually balanced (yay!) but when traveling, stressed, overtraining, or ill, it might show up as “unbalanced”. If you have Apple, Oura, Whoop, Fitbit, or another smart fitness tracker, your HRV data may be presented differently. How many of us have a smartwatch because we love to know our own health data even if we don’t always understand it? (Me too!)

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What is HRV?

Heart rate variability (HRV) measures the difference in time intervals between each heartbeat. It’s regulated by the autonomic nervous system (ANS) which is in charge of the involuntary, automatic bodily functions: breathing, digestion, heart rate, etc. The ANS has two components: the sympathetic nervous system (SNS) and parasympathetic nervous system (PNS). These work in tandem to prepare the body for action and calm the body down after work. HRV is essentially a report of how well these two components are working together.

How Do Smart Devices Capture HRV?

There are two ways of recording HRV, the majority of our smart devices utilize PPG (photoplethysmography). The other way to record HRV is with ECG (electrocardiogram). Some of these smart devices have an ECG function but it is a singular reading. We are more interested in continuous readings for HRV trends. My smartwatch has a sensor on the inside where the watch touches my wrist that lights up periodically. This light is reflected off my skin and back to the device and that information can measure the volume changes in our blood. These volume changes are in response to the beating of our heart (or heart rate). It’s through complex equations that the volume changes in response to our beating hearts are translated into HRV.

Is My Smart Device’s HRV Reliable?

Yes.

Apple Watches, Fitbit Watch/Band, Garmin Watches, Oura Ring, and Whoop Band all use PPG. As of 2023, Fitbit, Garmin, Oura, and Whoop record the entirety of HRV overnight. Garmin, Fitbit, and Whoop can record HRV during exercise when paired with an HRV-specific device like a chest strap. Oura and Apple capture HRV during activity or sessions, but in very different manners. Oura can record up to 180 minutes (3 hours) of continuous HRV while Apple records 1 minute at a time but can record up to 5 minutes.

I Don’t Have One Yet, What Should I Buy?

Honestly, it matters more which device you would use, if you would use it at all. All the wearables listed above use the same measuring system (PPG). They vary in how often and how long they track HRV in a single day or activity session, but again, we care about trends over time with our smart devices.

It isn’t necessary to track your HRV either, so don’t let this article pressure you into spending money. There are plenty of other metrics to gauge overall health if that’s something you want to gauge at all.

OK, and Now Tell Me What HRV Numbers are Best

No can do.

I can tell you generalities, but as with the majority of things, there is some nuance and expected differences per person from biological sex to age to existing health conditions to activity level and more. A higher HRV means your autonomic nervous system can adapt to stressors. This indicates overall good health and functional performance. With consistent training, especially for those newer to exercise, a gradual rise in HRV is likely to occur until it finds its “balanced” place.

A lower or reduced HRV is associated with stress, fatigue, and overtraining. A lower or reduced HRV also means your ANS is having a harder time adapting to stressors. If you notice your HRV was balanced or held stable at a particular number and starts to trend down, check in with your training, stress, sleep, fatigue, and more.

This doesn’t mean high HRV numbers are great and low HRV are bad, period. What is high to you might be low to someone else and still remain balanced for both of you. Nuance, my friends, is powerful.

TL;DR

HRV actually detects the gap between the beat of your heart rate. The trends of HRV are more important than an individual HRV metric. If you had a stable or balanced HRV that is now on the move, check in with what’s changed (if anything) in your life. If trending downwards, it may be a sign you’re getting sick or overtraining. If trending upwards, it may be a sign that your fitness and overall health is improving. Nothing is absolute and this is not medical-grade data. If you are concerned about overall health, please see a healthcare provider.

Primary Reference

Heart Rate Variability Measurement through a Smart Wearable Device: Another Breakthrough for Personal Health Monitoring?
By Li, Cardoso, Moctezuma-Ramirez, Elgalad, and Perin
Published 2023 in the International Journal of Environmental Research and Public Health

Post-Script

I am not a doctor. I am a nerd with multiple exercise and nutrition certifications plus a master’s degree in progress. This is not medical advice. This is information.