Is there a meaningful way to interpret shifts of the HRV normal range over a longer period of time?
Is there a meaningful way to interpret shifts of the HRV normal range over a longer period of time?
some thoughts and a case study
I have received the question above in reply to a post in which I was covering the normal range and its importance in contextualizing daily or weekly changes in resting physiology (learn more, here).
What about the normal range itself? How does it change over time and what do those changes mean?
In broader terms, your normal range is your chronic physiology, and therefore it is impacted by different things.
An example is seasonality, if you live somewhere where summer and winter are very different, it is likely that you experience a stronger seasonality in the data (lower HRV in winter, higher in summer).
If you are making large changes in your lifestyle, e.g. you just started exercising or losing weight, then these changes will also impact pretty much anything in your body, including your HRV in the long term.
If you are more or less in control of your life (in terms of stress) and your behavior is not changing much (because you already take care of your sleep, exercise, etc.), and do not live somewhere where seasons are dramatically different, it could be that the normal range is pretty flat across the year, and across years (with reductions that would only be visible in the order of decades, due to aging).
As you can see, there is never an easy answer when it comes to our physiology, nor there is an ideal trend, but with the right context, we can try to understand what is happening and use the data effectively. We can learn something and we can make small adjustments, and see how things go.
This is why I am not a fan of "improving HRV" as a goal, it might be impossible if you already have a decent lifestyle (i.e. genetics determines your baseline) or it is very unlikely if winter is coming (is your new healthy habit useless just because it's October in the Northern hemisphere? obviously no, but that's how people are using HRV).
Note that I have just used "improve" and not "increase" when talking about HRV.
Improving HRV, typically means keeping things stable.
Keep in mind that stability means that you are bouncing back quickly, not that you are not facing stress, this is the case because we measure at a certain time, hours after stressors, according to a specific protocol. Protocols matter. Don’t fall for the rhetoric that more data is more insightful, because it is not the case when it comes to HRV. Collect meaningful data according to best practices, and you will be able to assess your individual responses to various stressors. Let’s look at an example.
Below is Alessandra's data, collected first thing in the morning using HRV4Training, for 1 minute, while sitting, after waking up. This is a protocol that makes the data more sensitive to stressors, with respect to measuring while lying down or sleeping, as I’ve discussed here and here.
In her data, we see how HRV has improved, but not really increased:
Here is the context for the data above, which is of course always key, as the physiology itself cannot tell us much about what was happening: Alessandra had a race on feb 11th, then was traveling to Japan to give a few talks on the following day (jetlag, work stress, etc.), then she came back to Europe and was driving from Spain to Italy at the beginning of March. Finally, she settled in Italy on March 5th.
The baseline (weekly average, blue line) is always within her normal range. However, in the past weeks, there have been fewer suppressions, and a lower coefficient of variation (less jumping around of daily values). Only using software that shows 1) daily values 2) baseline 3) normal ranges, you can derive the correct conclusions:
HRV has improved, because it is more stable.
Unfortunately, things are more complex than advertised when it comes to physiology, and our best use of HRV is probably in the relatively short term (days-weeks) in relation to our own normal range, aiming for a stable response.
Thank you for reading and for asking good questions!
Marco holds a PhD cum laude in applied machine learning, a M.Sc. cum laude in computer science engineering, and a M.Sc. cum laude in human movement sciences and high-performance coaching.
He has published more than 50 papers and patents at the intersection between physiology, health, technology, and human performance.
He is co-founder of HRV4Training, advisor at Oura, guest lecturer at VU Amsterdam, and editor for IEEE Pervasive Computing Magazine. He loves running.
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