30 days of Heart Rate Variability (HRV) Biofeedback

did you breathe today?

Marco Altini
8 min readJan 30, 2021
As calm as I can be

In this post, I’ll go over the basics of heart rate variability (HRV) biofeedback, and show changes in baseline physiology (resting HR, HRV) potentially linked to practicing deep breathing at resonant frequency (more on this later!). But first things first: why biofeedback?

Last year I went back to university for a master’s in Human Movement Sciences and High-Performance Coaching. During this process, I’ve developed an interest particularly in psychology and aspects of self-regulation that are tightly coupled with my work on HRV and stress. The ability to emotionally self-regulate is key outside of sport as well, with plenty of studies showing the positive effects of HRV biofeedback in both clinical and healthy populations facing stress and anxiety.

Now more than ever these aspects are relevant to all of us, and I’ve started putting together some resources on HRV Biofeedback (blog posts on the rationale, metrics, protocols, and expected outcomes that you can find here) and developed a biofeedback app: HRV4Biofeedback.

What’s biofeedback exactly?

In just a few words, HRV Biofeedback is a technique that consists of providing an individual with real-time feedback on instantaneous heart rate and respiration changes while being instructed to breathe at low frequencies (Lehrer and Gevirtz, 2014).

Basically, deep breathing

Morning (or night) measurements of HRV vs biofeedback: what’s the difference?

HRV analysis can be used for various applications. What we do at HRV4Training is to quantify baseline physiological stress (what we could call “chronic” stress), and how this changes in response to training and lifestyle over periods of weeks or longer. To quantify baseline physiological stress, our measurements need to be taken in a very precise moment, which is first thing in the morning (or using the entire night of data), so that we can avoid the effect of confounding factors. You can find a few examples here.

By capturing changes in resting physiology, we can provide useful feedback that helps individuals to make meaningful adjustments to better balance training and lifestyle. This is particularly relevant as we all respond differently even to the same stressors depending on various aspects, hence only by measuring our individual response we can figure out if it’s all proceeding according to our plans or not.

Over the years at HRV4Training we have always focused on assessing stress more than trying to improve our ability to deal with it via self-regulation, something that I am hoping to change with the new app

HRV Biofeedback is a technique that we use to improve self-regulation, and also strengthen the parasympathetic system. While our morning measurements should be done while resting and breathing naturally, during biofeedback we use deep breathing to elicit higher parasympathetic activity.

You can see your biofeedback session the same way you see your other training sessions, this is something you do so that in the longer term, there can be beneficial changes in health and performance. Biofeedback is just a positive stressor.

The HRV4Biofeedback app. Learn more, here

The science

As the body via the autonomic nervous system (ANS) responds to stressful stimuli in an attempt to maintain a state of balance, we can determine how effective this physiological self-regulation process is, by measuring the ANS.

In particular, the ANS is composed of two branches, the sympathetic and the parasympathetic one. The sympathetic branch is normally associated with the ‘fight or flight’ response and excitatory responses (Minns et al., 2018). On the other hand, the parasympathetic branch is characterized by inhibitory responses and restorative processes, such as lowering heart rate and breathing rate, so that the system can stay in balance after facing a stressor.

For these reasons, in the past fifty years, a vast body of research investigated the link between HRV and various mental and physical stressors, showing consistently reductions in parasympathetic activity when facing physical and psychological stressors (Hjortskov et al., 2004; Plews et al., 2012; Plews et al., 2014). Additionally, reduced parasympathetic activity has been associated with various clinical conditions (e.g. depression and anxiety disorders) as well as higher mortality risk (Stapelberg et al., 2012).

During HRV biofeedback, an individual is instructed to breathe at low frequencies. Breathing at low frequencies (or deep breathing) causes large oscillations in instantaneous heart rate, which synchronize with breathing rate

The influence of breathing on heart rate is mostly modulated by the parasympathetic branch of the ANS (Lehrer and Gevirtz, 2014). Hence, deep breathing results in training of the parasympathetic system, which might explain at least part of the positive effects of HRV biofeedback reported in the literature in the context of reducing stress and anxiety (Goessl, Curtiss, and Hofmann, 2017).

Strengthening the parasympathetic nervous system could also motivate using HRV Biofeedback in athletes, with the potential of improving emotional self-regulation, coping mechanisms, and performance (Khazan, 2016; Pusenjak et al., 2015).

Resonant frequency

As previously introduced, HRV Biofeedback requires the individual to breathe at low frequencies. Experimental studies have found the highest heart rate oscillations when breathing at approximately 0.1 Hz. This frequency is often called the resonant frequency of an individual and can vary by 0.5–1 breath/minute between individuals. The resonant frequency of an individual can be established with a protocol that consists of breathing at different frequencies for a few minutes until the frequency that elicits the maximal amplitude is found (Lehrer et al., 2003).

The HRV4Biofeedback app includes a resonant frequency test that determines the optimal breathing frequency for you.

Resonant frequency test in HRV4Biofeedback. Learn more, here

Building a habit

Building an app is certainly easier than building a habit. While some recent attempts used protocols as short as 3 minutes per day, the original biofeedback protocols recommend two sessions of 20 minutes per day. Obviously, that’s quite a difference.

Personally, I think that we should indeed try to match what is recommended in the original protocol if we hope to see some meaningful changes. Like everything else, committing and putting the time and effort is what leads to improved outcomes, which is hardly ever the case with simple hacks

Yet, starting with very short sessions can simply be a way in. Once I got to about 10 minutes per day, it started to feel different. I was feeling the “acute benefit” of the session, as it probably was a long enough time that I could relax and de-stress. From there, I built up to about 25 minutes per day, with the occasional double session that led to the recommended 40 minutes. While I used the app on and off for several months, only in the past 30 days I made it a proper habit:

In January 2021 I went from 5-10 minutes per day of practice a few times per week to a daily 15–40 minutes of practice. While still far from the original protocol, this seems to be a reasonable ‘dose’

Right now, regardless of the physiological changes shown below, I do enjoy the practice a lot. I look forward to it during the day, and it makes me feel much calmer (which wasn’t really the case the first months when doing less time).

The data: acute vs chronic changes

As covered above in the morning (or night) vs biofeedback section, it is important to measure outside of the biofeedback session, to assess any potential benefits.

This is one of the main issues with many biofeedback studies, where changes in physiology are assessed only acutely during the practice or right after, which typically means we are seeing an HRV change due to the fact that we are deep breathing, and this change is simply a transitory change that might not really impact our physiology positively

Here is an example of my HRV during a session, as you can see the rMSSD is about 140ms, while normally at rest is not even a half of that. This means that I am doing the exercise correctly (stimulating the parasympathetic system) but it does not mean that there will be any long term effect or chronic change in my physiology:

A good session

Combining biofeedback with morning measurements taken with HRV4Training (or night measurements taken with an Oura ring), you could see also potential changes in baseline chronic physiological stress as measured in a known context, as a result of your biofeedback sessions

This is exactly what I am showing here below, where you can see:

  • night HRV averages taken with the Oura ring (the value is transformed on a logarithmic scale as we often do in research when working with rMSSD, but what matters here are relative changes)
  • baseline: blue line, 7 days moving average to better highlight the recent trend, given the typical day to day variability in HRV
  • normal values: light blue band, built using the previous 60 days of data. This is key to understand if recent (baseline) physiological changes are within our typical normal range or if we are experiencing a significant change (either a reduction below normal, highlighting higher stress, or an increase, as shown here, potentially in response to deep breathing)
Baseline trending quite a bit higher than my normal. Biofeedback effect?

This is really fascinating to me.

I am quite skeptical about long term changes in resting physiology, in particular HRV. As our baseline is mostly linked to genetics, I think that the best way to use the data is typically as an assessment of the response to training and lifestyle stressors, so that we can make adjustments on a day to day basis and better balance stress. This means using the data as a feedback loop, and not necessarily aim for a higher score over time. What we aim at improving, are health and performance, not HRV itself. This is especially true if we already “check many boxes” (sleep hygiene, eating well, exercising etc.).

Yet, in the past month, I have seen quite a dramatic change in my baseline, and I am curious to see if the values will remain stably higher as I continue with my deep breathing practice.

Write up

Combining insights from biopsychosocial models and basic physiology, we can see how HRV biofeedback has been proposed as a technique that can help to improve emotional self-regulation and coping mechanisms via a strengthening of homeostasis, with the potential of resulting in better health and performance.

In particular, apart from the potential for direct improvements in health and performance, other pathways could be positively impacted, from a psychological (e.g. anxiety) and physiological (e.g. hormonal response, strengthening of the parasympathetic system) point of view. Such changes in psychological and physiological factors could then affect other health and performance-related outcomes such as injury risk and recovery.

Needless to say, everybody is different. I cannot help but think that depending on for example how anxious of a person you are, you might benefit more or less from deep breathing exercises, which might explain why it seems to work for me

Get breathing!

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 the co-founder of HRV4Training and loves running.

Twitter: @altini_marco



Marco Altini

Founder HRV4Training.com, Data Science @ouraring Lecturer @VUamsterdam. PhD in Machine Learning, 2x MSc: Sport Science, Computer Science Engineering. Runner