Heart Rate Variability (HRV): The Signal Beneath the Pulse

EndoAxis Clinical Team

In a world where we can measure most things with exciting wearable gadgets, we become guardians of our bodies with an influx of information. And though these wearables can offer wonderful insight, it is important to understand what we are measuring and why. One of the metrics we tend to look for and give much credence to is HRV and there are important nuances to understanding how to interpret and utilize this data.

What is HRV, really?

Heart Rate Variability (HRV) is the beat-to-beat variation in time between successive heartbeats (the R–R interval on an ECG). Importantly, HRV is not about how fast your heart beats, but how flexibly it responds to internal and external demands. A healthy heart is not a metronome. In fact, a perfectly regular heart rhythm is often a sign of reduced physiologic adaptability. HRV captures the dynamic interplay between:

• The sympathetic nervous system (SNS: fight/flight)
• The parasympathetic nervous system (PNS: rest/digest, largely vagal tone)

At a mechanistic level, HRV reflects how effectively the autonomic nervous system

(ANS) integrates signals from the brain, baroreceptors, chemoreceptors, immune

mediators, hormones, and metabolic cues.

In short: HRV is a proxy for physiologic resilience.

Why HRV matters (and why clinicians should care)

HRV sits at the intersection of multiple systems that we already care deeply about:

1. Nervous system regulation

Higher HRV is generally associated with:

• Greater vagal tone
• Improved stress tolerance
• Faster recovery from physiologic insults

Low HRV, particularly when persistent, suggests autonomic imbalance with sympathetic dominance or impaired parasympathetic signaling.

2. Cardiometabolic health

Lower HRV has been associated with:

• Insulin resistance and impaired glucose tolerance
• Metabolic syndrome
• Hypertension
• Increased cardiovascular morbidity and mortality

Notably, these associations persist even when controlling for resting heart rate.

3. Inflammation and immune signaling

HRV inversely correlates with inflammatory markers (e.g., CRP, IL-6). This is thought to be mediated in part by the cholinergic anti-inflammatory pathway, where vagal efferent signaling dampens cytokine production.

This makes HRV particularly relevant in:

• Chronic inflammatory conditions
• Autoimmune disease
• Post-viral syndromes
• Overtraining or under-recovery states

4. Hormonal and circadian integration

HRV is exquisitely sensitive to:

• Sleep quality and circadian misalignment
• Cortisol rhythms
• Sex hormone transitions (peri/post-menopause, luteal vs follicular phase)
• Thyroid status

This is why HRV often changes before labs do.

When evaluating anyone, it is important to compare to their own HRV trends over time. Although “optimal” ranges are sometimes discussed in the fitness and functional health world, these HRV “optimal” reference ranges were largely derived from relatively small cohorts of young, healthy adults, often male and predominantly white. They, therefore, do not adequately account for the effects of age, sex, chronic disease burden, medications, fitness level, or ethnicity. As a result, many commonly cited HRV norms are not universally applicable across diverse or clinical populations. HRV is highly individualized and strongly influenced by:

• Age (declines with aging)
• Sex (women often have variable resting HRV patterns, partially influenced by
• where they are within their menstrual cycle)
• Ethnicity (data remains limited and underrepresented)
• Chronic morbidity (cardiometabolic disease, inflammation, autonomic dysfunction)
• Fitness, sleep, stress, and medications

Modern clinical and wearable-based HRV interpretation increasingly emphasizes longitudinal, within-person trends, rather than population-based cutoffs, which is key. HRV should be interpreted relative to an individual’s baseline and trajectory, not against generalized “optimal” ranges derived from homogeneous populations.

The upside of monitoring HRV

Pros

• Early warning signal: HRV often declines before overt symptoms appear
• Non-invasive and continuous: allows longitudinal pattern recognition
• Recovery intelligence: useful for adjusting training, workload, or stress exposure
• Behavioral feedback loop: sleep, alcohol, illness, overreaching, and underfueling all show up clearly

For motivated patients, HRV can become a powerful biofeedback tool.

The downside (and where HRV gets misused)

Cons and limitations

• Context matters: a single low reading is rarely meaningful
• Baseline dependency: inter-individual variability is large; absolute values matter less than trends
• Anxiety amplification: in some patients, HRV tracking increases stress rather than reducing it
• Not diagnostic: HRV reflects system load, not etiology

HRV should inform decisions… not dictate them.

How HRV is measured (and what actually matters)

Gold standard

• ECG-based R–R interval analysis

Common consumer methods

• Chest strap ECG sensors (high fidelity)
• Photoplethysmography (PPG) via wearables (rings, watches)

Key metrics clinicians should recognize

RMSSD: Root mean square of successive differences

• Reflects short-term vagal activity (most commonly used in wearables). This is an important nuance as it is primarily driven by parasympathetic (vagal) shifts in activity.
• It responds quickly to changes in stress, sleep, illness, inflammation, and recovery and is relatively resistant to breathing pattern differences compared to other metrics.
• **It tracks trends in autonomic recovery well. It works well with short measurements and/or spot measurements as it creates a pattern for metrics.

SDNN: Standard Deviation of Normal-to-Normal Intervals

• Reflects overall autonomic variability – looking at sympathetic and parasympathetic activity.
• **Highly dependent on length of measurement – it is best when utilized for at least 24 hours (ie – 24 hr ECG, or holter monitoring). Short readings on this device undervalue the readings.

Frequency-domain metrics (LF/HF): historically popular, but often overinterpreted

• LF: low frequency (best for vagal tone)
• HF: high frequency (best for parasympathetic activity)
• LF/HF ratio: sympathovagal balance.
• **not reliable as very sensitive to posture, breath, movement changes.

For most clinical and lifestyle purposes, RMSSD trends are sufficient. And these are

usually what are found in wearable like the rings, bands, or watches (though Apple

watch uses SDNN).

What Improves HRV?

Sleep (highest impact) – poor sleep is one of the fastest ways to suppress HRV

• 7-9 hrs consistently
• Fixed bedtime/waketime
• Cool, dark room (~65-68˚F)
• No alcohol within 3-4 hrs of sleep
• Reduced late-night screen exposure

Breathing and Vagal Stimulation to increase parasympathetic tone

• Best methods
• Resonant Breathing:
  • ~6 breaths per minute: inhale 4-5 seconds, exhale 5-6 seconds 5-10 minutes, 1-2 times per day
• Long exhale Bias (exhale longer than inhale):
  • Optional add-ons: singing, gargling, humming, cold face immersion (activates diving reflex)

Aerobic Base Training (zone 2) – the most reliable long term builder of HRV

Target:

• 150-300 minutes per week at an easy conversational pace HR – ~60-70% max

Strength Training (but not to failure) – strength training improves HRV if

recovery is adequate

• Best practices: 2-4 times/wk emphasizing compound movements
• AVOID frequent training to absolute failure

Manage Chronic Stress – mental stress suppresses HRV just as much

as physical stress.

Helpful Tools:

• Mindful meditation (10-20 min daily), nature exposure, reducing cognitive overload, clear work/rest boundaries Rumination and unresolved stress keep HRV low even when fitness is high

Nutrition and Metabolic Health – blood sugar swings and inflammation

reduce HRV

Focus on:

• Adequate calories (undereating lowers HRV), stable blood glucose, adequate protein, Omega 3 intake, Magnesium

Avoid:

• Alcohol excess (HRV killer!), chronic low carb and + high stress, late heavy meals

Hydration and Electrolytes – low plasma volume reduces HRV

• Adequate Sodium, Potassium Rich foods
• Avoid dehydration

Temperature Exposure – short stressors can improve autonomic flexibility (resilience)

• Sauna: improves cardiovascular autonomic tone (if tolerated)
• Cold exposure: brief, not prolonged (end shower on cold as a starter then work up to further cold exposure)

Medication and supplements that can lower HRV:

• Stimulants
• Excess caffeine
• Certain antidepressants
• Poorly timed thyroid meds
• Alcohol

Clinical takeaways

• HRV is a systems-level biomarker, not a fitness score
• Trends over time matter far more than single values
  • Within the same person comparison
• Declining HRV is often a sign to reduce load, not push harder
• HRV integrates nervous system, metabolic, immune, and hormonal inputs

Think of HRV as a dashboard light – not the engine itself.

Used well, it offers remarkable insight into physiologic adaptability. Used poorly, it becomes noise.

EndoAxis offers unmatched supplemental support for nervous system needs. Outside of the lifestyle factors that impact our recovery, we can also look to supportive nutrients that encourage optimal recovery for either overstimulated, undernourished, or maladapted nervous system responses.