Show Notes

Resource 1:

Title: Pleasant odors specifically promote a soothing autonomic response and brain–body coupling through respiratory modulation

Authors: Valentin Ghibaudo, Matthias Turrel, Jules Granget, Maëlys Souilhol, Samuel Garcia, Jane Plailly & Nathalie Buonviso

Publication: Scientific Reports

Link: https://www.nature.com/articles/s41598-025-20422-x

Resource 2:

Title: Improved non-invasive detection of sleep stages when combining skin sympathetic nerve activity and heart rate variability analysis with AI

Authors: Md. Aktaruzzaman & Thomas H. Everett IV

Publication: Scientific Reports

Link: https://www.nature.com/articles/s41598-025-20282-5

Resource 3:

Title: Increased sleep apnea-specific hypoxic burden is independently associated with cardiovascular autonomic dysfunction in obstructive sleep apnea: A large-scale study

Authors: Chenyang Li, Zhenger Zhou, Xiaozhen Zhang, Enhui Zhou, Tianjiao Zhou, Jingyu Zhang, Xinyi Li, Jianyin Zou, Huajun Xu, Jian Guan, Yupu Liu, Suru Liu, Xiaoyue Zhu, Weijun Huang, Hongliang Yi, Shankai Yin

Publication: Sleep Medicine

Link: https://www.sciencedirect.com/science/article/abs/pii/S1389945725005386

Resource 4:

Title: Strengthening the heart by means of a gratitude intervention?

Authors: Andreas R. Schwerdtfeger, Claudia Traunmüller, Bernhard Weber & Christian Rominger

Publication: The Journal of Positive Psychology

Link: https://www.tandfonline.com/doi/full/10.1080/17439760.2025.2574048?af=R#abstract

Resource 5:

Title: Child and marital stress are associated with a psychophysiological index of self-regulatory capacities among parents of preschool children

Authors: Sasha MacNeil, Chelsea da Estrela, Warren Caldwell, Jean-Philippe Gouin

Publication: International Journal of Psychophysiology

Link: https://www.sciencedirect.com/science/article/pii/S0167876025007470?via%3Dihub

Resource 6:

Title: Factors influencing heart rate variability in nurses following night shifts: a prospective observational clinical study

Authors: Taihe Zhan, Ziying Zhang, Zhimin Shi, Hongyan Xie, Daogang Zha & Xiumei Wei

Publication: BMC Nursing

Link: httpsD://bmcnurs.biomedcentral.com/articles/10.1186/s12912-025-03975-0

Resource 7:

Title: Heart Rate Variability Biofeedback and Mental Stress Myocardial Flow Reserve: A Randomized Clinical Trial

Authors: Amit J. Shah, Paolo Raggi, Hua She, et al.

Publication: JAMA Network Open

Link: https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2840378

Resource 8:

Title: VCU research in action: Teaching patients to take a deep breath

Author: Olivia Trani

Publication: VCU News

Link: https://news.vcu.edu/article/2025/10/vcu-research-in-action-teaching-patients-to-take-a-deep-breath

In this solo episode of the Heart Rate Variability Podcast, Matt Bennett explores the science and practicality of resonance frequency breathing, a term he notices is often misused. He introduces his AI counterpart, AI Matt, to present the research on resonance frequency breathing, which involves finding a natural rhythm for breathing that aligns with the body's optimal pace. Matt aims to delve into the correct usage of this term, discuss its scientific basis, and share practical applications in the episode.

Matt discussed resonance frequency breathing, explaining that while 6 breaths per minute is often cited, the actual rate can vary based on factors like height. He noted that trained biofeedback practitioners have observed rates as low as 3.5 breaths per minute in special operators and professional athletes. Matt emphasized the importance of synchronization between the heart, breath, and nervous system, as well as the impact on brain functioning.

Matt discussed the concept of resonance frequency breathing, noting that while research suggests an average of 6 breaths per minute, individual rates can vary widely. He explained that resonance frequency assessments involve gradually reducing breathing rate from 7 to 3.5 breaths per minute to find the rate that maximizes HRV. Matt emphasized that while height may influence resonance frequency, factors like fitness and body mass could also affect it, particularly for former athletes. He recommended practicing paced breathing before taking the assessment and suggested that repeated testing might show small changes in breathing rate over time.

Matt discussed the importance of regularly reassessing one's resonance frequency breathing to ensure accuracy, especially if consistent readings are obtained over several months without significant lifestyle changes. He shared his personal experience with adjusting his breathing patterns, including experimenting with a 4.5 to 3.5 breaths per minute rate and a 2:1 inhale-to-exhale ratio, inspired by recent research suggesting extended exhales can improve low-frequency heart rate variability. Matt emphasized the significance of personalized assessments using tools like the OptimalHRV app to determine optimal breathing patterns and encouraged others to explore different breathing techniques for maximum impact.

Matt discussed the importance of practicing residence frequency breathing assessment and emphasized the value of this technique in maximizing breathwork practices. He noted that while the concept is gaining recognition among fitness influencers and in peer-reviewed research, the full impact of the science is often overlooked in blog posts. Matt planned to include relevant research in the show notes and promised to cover this topic further in the next episode.

Resource 1:

Title: The relationship between heart rate variability and affective disorders: associations with symptomatic improvement and therapeutic alliance

Authors: Alexandra F. Gonçalves, Eugénia Ribeiro, Adriana Sampaio, Natividade S. Couto-Pereira, Pedro Moreira & Joana F. Coutinho

Publication: BMC Psychology

Link: https://bmcpsychology.biomedcentral.com/articles/10.1186/s40359-025-02960-1

Resource 2:

Title: Development and validation of the socio-evaluative N-back task to investigate the impact of acute social stress on working memory

Authors: Matthias Haucke, Sabrina Golde & Stephan Heinzel

Publication: Scientific Reports

Link: https://www.nature.com/articles/s41598-025-22611-0

Resource 3:

Title: Panic Attack Prediction for Patients With Panic Disorder via Machine Learning and Wearable Electrocardiography Monitoring: Model Development and Validation Study

Authors: Hayoung Oh, Hunmin Do, Chaehyun Maeng, Jinsuk Park, Taejun Yoon, Jihwan Kim, Hyeran Hwang, Seoin Choi, & Piao Huilin

Publication: Journal of Medical Internet Research

Link: https://www.jmir.org/2025/1/e69045

Resource 4:

Title: Non-invasive cardiovascular risk stratification in type 2 diabetes: a pulse wave and pulse rate variability analysis with machine learning

Authors: Saurav Kumar, Apakrita Tayade, Amber Shrivastava, & Ravi Bhallamudi

Publication: Biomedical Signal Processing and Control (via Science Direct)

Link: https://www.sciencedirect.com/science/article/abs/pii/S1746809425014491

Resource 5:

Title: How Tracking Your Health Metrics Can Help You Live Longer

Author: Alice Park (Interview with Zahi Fayad)

Publication: TIME

Link: https://time.com/7324741/health-metrics-tracking-live-longer/

Resource 6:

Title: Stressed at Work? Your Heart Disease Risk Just Jumped 50%. Here’s the One Number That Shows How to Fight Back

Author: Julien Raby

Publication: BoxLife Magazine

Link: https://boxlifemagazine.com/boost-heart-resilience-by-tracking-hrv/

Resource 7:

Title: “Resonance Breathing” Is The Anti-Stress Hack You Can Do Anywhere

Author: Carolyn Steber

Publication: Bustle

Link: https://www.bustle.com/wellness/resonance-frequency-breathing-stress-hack

Below are the links to the studies and articles discussed in this episode:

In this episode of the Heart Rate Variability Podcast, host Matt Bennett sits down once again with Dr. Inna Khazan, a leading expert in HRV biofeedback and applied psychophysiology. Together, they unpack one of the most fascinating and misunderstood aspects of heart rate variability — low-frequency HRV — and its connection to self-regulation, stress resilience, and overall wellness.

Dr. Khazan begins by breaking down the concept of frequency domains in HRV. Just as white light contains multiple colors, the heart rate signal is composed of several distinct frequency components. Using tools like the Fast Fourier Transform, researchers can separate heart signals into high-frequency, low-frequency, and very-low-frequency ranges — each linked to specific physiological processes and parts of the autonomic nervous system.

The conversation dives deep into low-frequency power, which represents the interplay between the baroreflex (the body’s blood pressure regulation system) and the vagus nerve. Dr. Khazan explains how resonance-frequency breathing—typically practiced for 20 minutes a day—acts like strength training for these systems. Over time, this practice enhances emotional regulation, stress recovery, and overall heart-brain coherence.

Listeners learn that breathing too long in a low-frequency state can actually suppress other important HRV components, such as high-frequency and very-low-frequency power. Instead, Dr. Khazan recommends brief, consistent training sessions to balance all aspects of the nervous system. She also clarifies common misconceptions, including the outdated idea that low-frequency HRV measures sympathetic activity, emphasizing instead its parasympathetic and baroreflex origins.

Matt and Dr. Khazan discuss Optimal HRV’s “Optimal Zone” scale, which tracks the percentage of time users spend in low-frequency dominance during a session. They also unpack metrics like Max-Min and total low-frequency power, explaining how they interact and what each reveals about training efficiency and day-to-day readiness.

Practice resonance-frequency breathing for 20 minutes a day to enhance self-regulation.

Avoid over-training in the low-frequency zone — balance is key.

Understand that low-frequency HRV is not a measure of stress or sympathetic activity, but rather a reflection of vagal and baroreflex strength.

Leverage your Optimal HRV app metrics to track progress, focus, and nervous-system adaptability.

Explore more insights from Dr. Khazan and Matt Bennett on heart rate variability, stress regulation, and biofeedback science.

Visit OptimalHRV.com for resources, show notes, and upcoming episodes, including the "This Week in HRV" series, which highlights the latest HRV research and applications.

Welcome to the show notes for this week’s episode – This Week in HRV Edition. Below, you’ll find direct links to all the studies and articles discussed in this episode. These sources represent the latest research connecting HRV to mental health, resilience, environmental design, and leadership.

Research Studies:

News and Features:

Each of these studies and stories offers a unique perspective on how HRV connects the heart, brain, and body. For more insights, visit our website and subscribe to weekly updates on the evolving science of heart rate variability.

In this episode, we review seven new studies published between September 19 and September 25, 2025. Together, they highlight the many ways HRV intersects with brain activity, breathing practices, cardiac recovery, exercise environments, emergency medicine, environmental stressors, and new multimodal measurement approaches.

We’ll explore:

As always, I’ll connect the research to practical, client-ready strategies you can use right away.

This podcast is sponsored by Optimal HRV. Learn more at optimalhrv.com.

Refrences:

In this episode, Matt Bennett explores how to apply the research on HRV training through muscle contraction into personal practice.  

References for this episode. 

Fred Shaffer's work: https://youtu.be/ajwgwzn2ZUo

Sources for this summary: Vaschillo/Lehrer et al., laboratory demonstration that 0.1-Hz rhythmic skeletal muscle tension increases 0.1-Hz power in heart rate, systolic pressure, and pulse transit time and strengthens HR–BP coupling, consistent with baroreflex resonance; includes with- and without-leg-crossing conditions (N≈37). ResearchGate Shaffer, Moss, & Meehan (2022) randomized study showing that one and six contractions per minute increased multiple HRV metrics versus twelve per minute, with a ~0.10-Hz peak at six per minute. SpringerLink Meehan & Shaffer (2023) within-subjects study showing greater HR, HR max–min, and low-frequency power when adding core contraction and leg-crossing versus wrist-ankle alone at six contractions per minute. SpringerLink Frontiers in Neuroscience review describing the resonance-frequency model and noting that rhythmic skeletal muscle tension, like slow breathing, can stimulate the baroreflex at ~0.1 Hz. Frontiers Clinical and physiological studies on leg-crossing and muscle-tensing maneuvers increasing cardiac output, mean arterial pressure, and cerebral oxygenation during orthostatic stress and vasovagal reactions. Europe PMC+2Physiology Journals+2 Systematic reviews and trials on isometric handgrip training showing blood-pressure reductions with mixed HRV effects, distinct from contraction-based biofeedback. Nature+1

This week’s episode of the Heart Rate Variability Podcast explores three new publications from September 5–11, 2025. We look at advances in contactless HRV monitoring using millimeter-wave radar, how physical fatigue alters time-domain and nonlinear HRV metrics, and a review of how accurate consumer wearables really are when tracking HRV.

We dive into what these findings mean for clinicians, researchers, and anyone using HRV to track resilience, recovery, or performance.

References

Cui, Y., Zhang, W., Xu, J., Liu, Y., & Chen, H. (2025). Non-Contact Heart Rate Variability Monitoring with FMCW Radar via a Novel Signal Processing Algorithm. Sensors, 25(17), 5607. Read here

Zhang, J., Niu, X., Wei, X., Ma, J., & Du, W. (2025). Effects of Induced Physical Fatigue on Heart Rate Variability in Healthy Young Adults. Sensors, 25(17), 5572. Read here

Pramanik, P. (2025, September 8). Wearable health trackers reveal how accurate your smartwatch really is. News-Medical. Read here