HRV comparison: Polar vs. Oura Ring 3

We started these comparisons, to shed some light on the accuracy, and differences, of the same measurements from different wearables. For this week, we are comparing heart rate variability from Oura and Polar.

TL;DR

1. Polar HRV is consistently higher than Oura and Polar's HRV readings are more volatile than Oura's in general.
2. Both wearables record spikes in HRV data, up to a 30ms jump in HRV in Polar's case which is a huge change.
3. Not much consistency between Polar and Oura's measurements to compare HRV trends, unlike for heart rate.
4. The differences in HRV between Polar and Oura could be because Oura takes readings from your finger while Polar uses your wrist, especially for a metric as sensitive as HRV.

Heart rate variability (HRV) is a metric that people are increasingly looking at to create a holistic picture of one's health. HRV is related to how well your autonomic nervous system (ANS) is functioning, which is made up of your sympathetic and parasympathetic nervous systems. ANS regulates your heart rate, breathing, blood pressure, etc., and is influenced by a myriad of lifestyle choices. HRV allows for an understanding of whether your sympathetic or parasympathetic nervous system is more actively regulating your heart rate versus the other, so to speak.

A lower HRV likely indicates an overactive sympathetic nervous system. This is synonymous with being in a fight or flight response (due to a stressful situation, poor sleep, malnutrition, etc.). Higher HRV is thought to correlate with better cardiovascular fitness, being in a relaxed state, and in individuals who are physically more resilient to stress. This is why incorporating the right lifestyle habits such as meditating, sleeping well, exercising, and consistently eating nutrient-dense foods is found to increase HRV.

HRV is highly sensitive and unique to each individual so understanding your HRV is about looking for changes in your own trends over time as opposed to the value of a single HRV reading. That being said, typical values for adults range between 20ms - 200ms.

More and more wearable devices are beginning to record HRV. After comparing the heart rate measurements for the Polar Unite and Oura Ring 3 last week, we now turn our attention to how these wearables compare for HRV readings. Due to the sensitivity of HRV measurements, readings are most accurate when in deep sleep. Our Terra engineer tracked his HRV while sleeping using Polar and Oura for the same week that he tracked his resting heart rate. HRV readings for the whole week can be seen below.

What is immediately obvious from the above data is that the Polar HRV readings are consistently higher than the Oura readings. To understand things better, let's now look at a single night of sleep.

We can see how much higher the Polar HRV readings are versus Oura in the graph above. Also, Oura begins tracking sleep earlier and does not spike as much or appear as volatile, which we also found last week for heart rate readings. The HRV spikes present in the Polar readings (such as from 30ms to 60ms at 8:30AM) are extreme and likely meaningless in terms of their magnitude.

For this sleep session, changes in HRV do not match up between Polar and Oura at all unlike for the heart rate trends. Oura records extremely low HRV values in some cases, even less than 15ms. While every individual's acceptable HRV range is unique, this seems too low. We do know from our engineer's heart rate trends that they likely had a couple nights of restless sleep that week, which could also be causing the low HRV from Oura. The differences between HRV readings for Polar and Oura may also be due to Oura measuring via your finger and Polar via your wrist.

Let's look at a different night in the graph below.

In the sleep session above, Polar HRV again is much higher than Oura's, with up to a 30ms difference! Oura's HRV is higher and more volatile here than readings from other nights and both wearables show spikes in the data. The Polar readings are still very volatile, as per usual.

Unlike the April 14th data, both wearables pick up similar trends in HRV here. The increase and subsequent drop-off in HRV at 6:00AM is recorded by Polar and Oura, same at around 7:00AM. We know from the heart rate data of our engineer that he likely had restless sleep this night. This may be causing the volatility seen here, especially for the Oura ring, but it's hard to say for certain.

The significant difference in HRV between these wearables makes it difficult to analyse and compare their HRV readings. As a result, it's also still tricky, even for Oura's more consistent readings, to make concrete conclusions about one's health or sleep quality from HRV data alone. Whether we can attribute these spikes and the overall HRV trends seen during sleep to actual lifestyle choices is the big question for HRV readings obtained from wearables. For now, it's hard to say we can with much confidence, but the future looks bright.