Well, it has been over a year since I last reported on my fascination with HRV! Why, the silence? I was busy! During the last year, I continued a pretty major data collection exercise in which each day I captured and logged as much as I could about my own HRV including the following:
Daytime HRV (captured on my Scoshe device using the Elite HRV app each am)
Nightime HRV (captured from my fitbit each night from the longest sleep period)
Stress Score (Fitbit)
Sleep Score (Fitbit)
Readiness Score (Fitbit)
Zone Minutes (Fitbit)
Resting Heart Rate Average (Fitbit)
Armed with this data, I processed it in several ways. I calculated correlation coefficients between each variable above. I also summarized the data by month to see the trends and even compared the monthly summaries to average daily high temperatures for each month (more on this later).
So, what did I learn? Mostly that trying to draw conclusions from the data is difficult! There are way too many variables at work making it challenging at best to tease out any strong conclusions! This might be a job better suited to AI than to an interested hobbyist with a spreadsheet! Even still, I did discover some interesting relationships.
First, I wondered how much agreement there was between the two methods I used to capture my daily HRV (night time readings captured automatically by my fitbit and day time readings captured during a 2 minutes sampling using the EHRV app each morning). When I calculated the correlation coefficient on the raw daily readings, the result was not that good – a result of -0.12. But when I averaged the readings by month and calculated the correlation, it was a much stronger correlation of 0.62. I suspect that there is more variability in the daytime readings as it is a small snapshot of time (2 mins) vs the longest sleep period at night that the fitbit algorithm uses. Further, HRV readings are highly susceptible to things like movement – so doing it at night time allows a more stable environment than doing it during the daytime (more than once, my dog would bark or something else would occur during the morning reading that could impact the calculation for that day). So – looking at the daily readings might not always give a true picture of what the body is really experience that given day – but the trends over a longer period of time may “even out” that variability. This seems to be in agreement with advice I have read by those who study HRV that it is difficult to use the results from day to day, or to compare results from one person to another. The trends can me meaningful though. Very subjectively, I found that the night time reading seemed to correlate to my own sense of how I was generally feeling (High HRV would seem to match up to feeling pretty good that day). So, If I had to pick one data collection method, I would probably opt for the fitbit nighttime system.
Speaking of trends, when I attended to first HRV international conference, I learned that HRV is suspected of being seasonal. Well, I can confirm that my data indicates this as well. As I mentioned, I also compared my monthly HRV results to the average daily high temp for each month. This was the strongest correlation I found (0.65)! See the chart below:
This is very interesting! It would seem HRV is higher with the warmer months and lower with the colder ones. Interesting – but troubling too! This makes it difficult to determine if HRV is being impacted by lifestyle changes, or just the natural seasonality effect! You may recall that I suspected my use of creatine may have negatively impacted my HRV and in fact, I stopped using it for that reason. But I collected that data in the fall when temps were starting to decline. How much of the reduction in my HRV was due to the creatine vs the normal seasonality? Difficult to say and difficult to control for in a limited experiment like mine!
The second strongest correlation I found was between HRV and resting heart rate. The result showed a negative correlation of -0.40. So – as my resting HR slowed, my HRV increased. What causes my RHR to slow? Again, this is difficult to answer as there are many possible causes. For me, I have noticed that the more cardio I get, the slower my RHR seems to be, which makes sense as athletes tend to have very low resting heart rates. There may be another factor at work in my case as well. In part 4 of this series, I had mentioned that my thyroid was starting to get a little lazy and I have in fact been prescribed some thyroid replacement meds (Synthroid). I have noticed anecdotally, as well as by my blood draws to manage the replacement meds, that as I go more hypothyroid (slower thyroid function) my RHR drops and when I am getting too much meds, my RHR elevates. Much to the chagrin of my Doctor, I have been watching my RHR / HRV and using it as an indicator of when I should either back off or increase my daily dosage of thyroid meds. It seems to be working for me! A win for the HRV analysis!
So, if I were to summarize all of my findings over the last year and a half of diving down the rabbit hole of HRV, here is what I would conclude. Note that this is based on my own experience and data, and does not constitute a sound research program at all! Your experiences might be different!
- HRV is directly related to eye-hand coordination. Increase your HRV and your coordination will improve. Anecdotally, I still see a pretty good correlation to my HRV on a given day and how well I perform on the shooting range!
- Red Light exposure seems to positively impact HRV – and also seems to improve short term visual acuity. I have two red light units now and use them both frequently.
- Sleep can improve trends in HRV.
- Using HRV to tailor workout schedules to avoid over-training is difficult – I have opted to just “listen to my body” instead of the data to tailor my work outs. I often joke that my fitbit is trying to kill me – pushing me to do more exercise on days when I am exhausted!
- Counter to the “HRV establishment”, I had no positive results in HRV by practicing resonance breathing. I was disappointed in this as I had high hopes!
- Creatine might have negatively impacted my HRV. I might reintroduce it again and see what happens. The internet is full of reports now indicating that creatine might be good for cognitive function in addition to its traditional uses in strength training and body building, so there are reasons to want to use it as long as it does not drop HRV.
- There are a ton of variables that impact HRV making it difficult to isolate impact of lifestyle changes – especially given the potential seasonality affect. But this is not a reason to avoid making those changes such as improving diet, reducing alcohol, increasing exercise, getting more sleep, etc.
- Monitoring HRV via night time data collection seems to provide a more stable picture if you are trying to look at day-to-day results. But daytime or night time results seem equally adept when looking at trends.
- HRV and resting heart rate have shown to be a good indicator of my thyroid function (even if my doctor is suspect). I will use it in between my blood draws to fine tune my med administration.
- The field of HRV is still very much in its infancy. I look forward to more “real” research coming out in the future!
So, there you have it! I have concluded my work on HRV, but not my interest! I hope you have found this of some use, or at least to be mildly entertaining!