HRV Comparison: Eightsleep vs. WHOOP

TL;DR

1. Average HRV readings during sleep from Eightsleep and WHOOP were measured over the course of a week.
2. Eightsleep's HRV readings on the whole are higher than WHOOPs. The trends match on some days, especially when HRV is higher/trending up.
3. Both devices pick up on a higher HRV over the weekend, likely due to better quality sleep, followed by a drop-off during the week.
4. When looking at average HRV readings, the volatility and spikes typically seen for 5-minute interval HRV readings are smoothed out - except in cases of large outliers in the data. We also saw this last week for Oura vs. WHOOP averages.
5. Due to the volatility of HRV readings, the trends at times disagreed with each other. The Eight sleep readings over the course of a single night for have a lot of spikes in the HRV when the trends disagree. This may have been a poor measurement night or due to improper setup. Since WHOOP only stores HRV averages for a sleep session, the level of volatility in readings over the course of a night could not be looked into.

Continuing with our weekly HRV comparisons of different wearables, We now turn our attention to WHOOP vs. Eightsleep's HRV readings during sleep. As WHOOP only stores the average HRV for each sleep session, our comparison will focus on looking at HRV averages as we did for our last article for Oura vs. WHOOP (check it out here).

When analyzing HRV averages/night, we miss out on the finer details as well as the volatility associated with HRV readings for a single night. That being said, there are still valuable insights to gain from looking at longer-term trends. To collect Eightsleep and WHOOP HRV readings, the same team member who wore an Oura and WHOOP 4.0 for a week also slept on an Eightsleep Pod Pro 2 cover.

During a single sleep session, Eightsleep records HRV every 15 minutes. We took a look at Eightsleep's HRV readings when we compared it to a Garmin Epix 2 (see here). We found that Eightsleep and Garmin matched trends on good measurement days. In general, however, the HRV readings for both devices were volatile, with spikes in the data. This volatility is a common theme for HRV readings, but it appears to be smoothed out when looking at HRV averages. However, as we will see here, in the case of extreme outliers, the averages will still be impacted. A graph of Eightsleep and WHOOP's HRV averages during sleep can be seen below.

There is not as much agreement here as compared with the HRV readings for WHOOP and Oura for the same week. Some trends are still picked up by WHOOP and Eightsleep, but Eightsleep estimates HRV at higher values than WHOOP for the entire week. We can also see that from August 31st to September 1st, both devices record a drop in HRV. However, unlike WHOOP, Eightsleep does not measure a decrease in HRV between the 2nd and 3rd of September.

Since WHOOP's data also matches with Oura's HRV average for September 2nd - 3rd, we can conclude that Eightsleep's readings are likely the outlier as opposed to WHOOP. However, Eightsleep and WHOOP both measure an increase in average HRV/night for the weekend (September 3rd–5th), and a drop-off in HRV for the work week right after.

On September 5th, the averages are nearly identical. Because these readings match, this was likely a good measurement night in general. Our team member seems to have gotten a night of restful sleep on Sunday. It's interesting that for the highest recorded HRV, the readings match best. This is likely because high HRV is indicative of restful sleep, which involves less movement and less fluctuation in heart rate. This in turn makes it easier for devices to accurately measure HRV. After September 5th, Eigthsleep's drop-off in average HRV is not as large as for WHOOP.

When comparing HRV averages, we saw in our last article that we lose the fine grain detail and volatility associated with HRV readings on a 5–15 minute timescale. Let's observe a single night of HRV readings for Eightsleep to see what is causing the differences in trends for some of Eightsleep's HRV averages compared to WHOOP.

Comparing HRV nightly averages between wearables allows for a bigger-picture analysis and comparison of HRV trends. We see here that Eightsleep and WHOOP both record an increase in average HRV over the weekend, but the magnitude is larger for Eightsleep. Eightsleep's trends deviate from WHOOP before and after the weekend.

Averaging HRV readings allows for comparisons between wearables without being too critical of the data at shorter timescales, which can appear volatile due to the sensitivity of measuring HRV. However, for HRV data with extreme volatility or lots of spikes/dips, the HRV average will still be impacted.