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Heart Rate Variability: A Comprehensive Overview

Writer's picture: Rav BrarRav Brar



Heart rate variability (HRV) measures the variation in time between each heartbeat. It's a valuable indicator of the health and flexibility of your autonomic nervous system (ANS), which controls many bodily functions, including heart rate, breathing, and digestion1. The ANS comprises two branches: the sympathetic nervous system (SNS), responsible for the "fight-or-flight" response, and the parasympathetic nervous system (PNS), which promotes "rest-and-digest" functions(1). HRV reflects the balance between these two branches. Higher HRV generally indicates a healthier ANS and greater resilience to stress.


It's important to understand that your heart doesn't beat at a perfectly consistent rhythm. Instead, the time between beats varies slightly due to a phenomenon called sinus arrhythmia. This variation is usually linked to breathing (respiratory sinus arrhythmia) and is a normal part of how your heart and circulatory system function(2).


HRV and Stress


Stress significantly impacts HRV. When you experience stress, your body activates the SNS, leading to a more consistent heart rate and reduced HRV1. Conversely, relaxation, facilitated by the PNS, increases HRV, reflecting the body's ability to adapt and recover1. Research has established a clear link between low HRV and chronic stress. Individuals with lower HRV are more susceptible to stress and anxiety(1).


Interestingly, HRV can signal both acute (short-term) and chronic (long-term) stressors. These can range from temporary challenges like illness or lifestyle changes to more persistent issues like work pressure or relationship difficulties(4). Through your HRV metrics, you may notice signs of either acute (short-term) or chronic (long-term) stressors, such as illness, lifestyle changes, alcohol consumption, or longer-term sources of mental stress4. Factors like gender, personality, and the nature of the stressor can influence the impact of chronic stress on HRV(5). Notably, how you perceive stress can significantly affect your HRV, even more so than the actual stressor itself(5).


HRV and Sleep


HRV fluctuates throughout the sleep cycle. Studies have shown that parasympathetic indices, such as SDNN and RMSSD, are significantly higher during sleep(6). This suggests that analyzing HRV during sleep provides a more accurate representation of parasympathetic activity and might be a better indicator of peak parasympathetic activity than 24-hour HRV measurements(6). Research indicates a significant difference in HRV values between sleep cycles and stages. For example, mean NN (time interval between heartbeats) is lowest during pre-sleep and increases across slow-wave sleep stages(8). SDNN remains at pre-sleep levels until slow-wave sleep stage 3 and then increases in REM sleep stages 1 and 28.


HRV and Athletic Performance


HRV is a valuable tool for athletes to monitor training intensity and recovery. High HRV is associated with better athletic performance and a lower risk of injury(9). Conversely, low HRV may indicate fatigue and increased injury risk(9). Studies have shown that HRV-guided endurance training, where intensity and volume are adjusted based on HRV, leads to greater improvements in aerobic fitness(10). Athletes who demonstrate an increase in their weekly mean HRV or a decrease in weekly HRV coefficient of variation (CV) often show greater performance improvements(10).


It's important to note that age plays a role in what constitutes a "good" HRV score(11). While resting HRV might not reliably reflect overreaching, it can be a useful predictor of illness in athletes, such as upper respiratory tract infections(12). Furthermore, research with Olympic athletes has shown a link between HRV and healthy body composition, including lower body fat percentage(13). This suggests that HRV, used in combination with other tools, can be helpful in weight management by providing information about the body's reaction to daily nutrition(13).


Impact of Low HRV on Health


Low HRV is a critical marker for various health issues, including:

  • Cardiovascular diseases: Low HRV is a significant predictor of heart failure and coronary heart disease(14). Studies have shown that a decrease in variability correlates to an increase in mortality and morbidity in people with cardiovascular disease(16). It is associated with an increased risk of mortality from various causes(14).

  • Psychological disorders: Conditions like PTSD and schizophrenia show reduced HRV(14). Low HRV is also linked to higher self-reported scores of anxiety and depression(18).

  • Reduced cognitive function: Difficulty completing everyday cognitive tasks is another symptom of low HRV(18).


Benefits of High SDNN


SDNN, a key HRV metric, reflects the overall variability in heart rate over a specific period. High SDNN generally indicates better adaptation to stress and improved cardiovascular health(19). It suggests a strong influence of the PNS, allowing the body to effectively respond to different situations(20).


Other related metrics include NN50 (the number of pairs of successive intervals that differ by greater than 50ms) and pNN50 (the proportion of NN50 over all N-N intervals in a sample). These are highly correlated with RMSSD, and higher values are typically associated with greater cardiovascular health and performance(21). For both measures of NN50 and pNN50, they are highly correlated with RMSSD, and higher values are typically associated with greater cardiovascular health and performance(21).


Yeyro Ring and HRV Tracking


With the increasing awareness of HRV's importance, various wearable devices have emerged to help individuals monitor their HRV. The Yeyro Ring is one such device that tracks various health metrics, including HRV. It uses the RMSSD method to calculate HRV and filters out high-motion periods during sleep, improving accuracy(22). The Yeyro Ring X has shown perfect correlation with medical-grade electrocardiogram devices for recording heart rates at night(23). It provides detailed insights into sleep patterns and overall health(23).


Conclusion


HRV is a valuable indicator of overall health and well-being. It reflects the balance of the ANS, providing insights into stress levels, sleep quality, and athletic performance. Maintaining a healthy HRV is crucial for preventing chronic diseases and mental health problems. Techniques like meditation, deep breathing, and yoga have been shown to be effective in reducing stress and improving HRV(5). Wearable devices like the Yeyro Ring offer a convenient way to monitor HRV and gain a deeper understanding of your health.


Beyond simply tracking HRV, it's essential to understand its implications. By monitoring your HRV, you can gain valuable insights into how your body responds to stress, exercise, and recovery. This awareness can empower you to make informed decisions about your lifestyle, training regimen, and stress management techniques. Furthermore, emerging research suggests the potential of HRV biofeedback as a therapeutic tool to improve ANS function and overall health. By actively engaging in practices that promote HRV, you can take proactive steps towards a healthier and more resilient life.



Citations


  1. Relationship Between HRV and Stress | Apollo Neuro, accessed January 21, 2025, https://apolloneuro.com/blogs/news/hrv-and-stress

  2. Heart Rate Variability (HRV): What It Is and How You Can Track It - Cleveland Clinic, accessed January 21, 2025, https://my.clevelandclinic.org/health/symptoms/21773-heart-rate-variability-hrv

  3. Stress and heart rate variability: Relationship and management - MedicalNewsToday, accessed January 21, 2025, https://www.medicalnewstoday.com/articles/stress-and-heart-rate-variability

  4. HRV and Stress: What HRV Can Tell You About Your Mental Health - Oura Ring, accessed January 21, 2025, https://ouraring.com/blog/hrv-and-stress/

  5. Stress and HRV - What's The Connection | Welltory App, accessed January 21, 2025, https://welltory.com/stress-and-hrv/

  6. www.frontiersin.org, accessed January 21, 2025, https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2024.1386425/pdf

  7. Exploring sleep heart rate variability: linear, nonlinear, and circadian rhythm perspectives, accessed January 21, 2025, https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2024.1386425/full

  8. The Inconsistent Nature of Heart Rate Variability During Sleep in Normal Children and Adolescents - PubMed Central, accessed January 21, 2025, https://pmc.ncbi.nlm.nih.gov/articles/PMC7046589/

  9. Maximizing Athletic Performance Through Heart Rate Variability ..., accessed January 21, 2025, https://neupttech.com/blog/maximizing-athletic-performance-through-heart-rate-variability-training

  10. Interpreting HRV Trends in Athletes: High Isn't Always Good and Low Isn't Always Bad, accessed January 21, 2025, https://simplifaster.com/articles/interpreting-hrv-trends-athletes/

  11. HRV: The Endurance Athlete's Complete Guide - FasCat Coaching, accessed January 21, 2025, https://fascatcoaching.com/blogs/training-tips/hrv-heart-rate-variability

  12. Heart Rate Variability (HRV) - Science for Sport, accessed January 21, 2025, https://www.scienceforsport.com/heart-rate-variability-hrv/

  13. Heart Rate Variability (HRV): How Olympic athletes can use the heart-mind connection to boost their performance | Magazine issue 6/2024, accessed January 21, 2025, https://www.in-mind.org/article/heart-rate-variability-hrv-how-olympic-athletes-can-use-the-heart-mind-connection-to-boost

  14. What Are The Health Effects Of A Low HRV? - Consensus Academic Search Engine, accessed January 21, 2025, https://consensus.app/questions/what-health-effects/

  15. HRV Biofeedback for Stress – Low Heart Rate Variability Measurement | Drake Institute, accessed January 21, 2025, https://www.drakeinstitute.com/heart-rate-variability

  16. How does heart rate variability affect your health? - PinnacleCare, accessed January 21, 2025, https://www.pinnaclecare.com/blog/heart-rate-variability-health-impact/

  17. Low Heart Rate Variability in a 2-Minute Rhythm Strip Predicts Risk of Coronary Heart Disease and Mortality From Several Causes | Circulation, accessed January 21, 2025, https://www.ahajournals.org/doi/10.1161/01.cir.102.11.1239

  18. 6 Low HRV Symptoms (And What Causes Them) - WHOOP, accessed January 21, 2025, https://www.whoop.com/ca/en/thelocker/low-hrv/

  19. RMSSD, pNN50, SDNN and other HRV measurements | Welltory, accessed January 21, 2025, https://welltory.com/rmssd-and-other-hrv-measurements/

  20. Heart Rate Variability - Caring Medical, accessed January 21, 2025, https://caringmedical.com/hauser-neck-center/heart-rate-variability/

  21. Measuring HRV: SDNN and RMSSD - Terra API, accessed January 21, 2025, https://tryterra.co/blog/measuring-hrv-sdnn-and-rmssd-3a9b962f7314

  22. Ultra-Human Ring vs. Other Fitness Trackers: Discrepancies in Heart Rate Variability : r/Ultrahuman - Reddit, accessed January 21, 2025, https://www.reddit.com/r/Ultrahuman/comments/1dorpsr/ultrahuman_ring_vs_other_fitness_trackers/

  23. Yeyro Ring & Yeyro Ring X The Most Accurate Health Tracker Ring, accessed January 21, 2025, https://www.yeyroring.com/blogs/recommended-1/yeyro-ring-health-tracker

  24. Unfold The Value of Yeyro Ring In Your Lives, accessed January 21, 2025, https://www.yeyroring.com/blogs/recommended-1/unfold-the-value-of-yeyro-smart-ring

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