Heart Rate Variability (HRV)

Monitor SDNN and RMSSD metrics to assess cardiovascular health and stress levels

What is Heart Rate Variability?

Heart Rate Variability (HRV) measures the variation in time between consecutive heartbeats. It's a marker of autonomic nervous system function and overall cardiovascular health.

HRV Metrics:

  • SDNN (Standard Deviation of NN intervals) - Measures overall HRV. Reflects total autonomic nervous system activity.
  • RMSSD (Root Mean Square of Successive Differences) - Measures short-term HRV. Primarily reflects parasympathetic (vagal) tone.

Why HRV Matters

Low HRV is linked to worse health outcomes and increased stress (Cleveland Clinic):

  • Lower HRV indicates reduced ability to adapt to stress
  • Associated with increased cardiovascular disease risk
  • Can indicate overtraining, illness, or chronic stress
  • Higher HRV generally indicates better cardiovascular fitness and resilience

📊 HRV is highly individual: Focus on your personal trends rather than absolute values. Compare today's HRV to your own baseline.

Interpreting HRV Values

HRV varies widely between individuals based on age, fitness, genetics, and other factors. There's no single "normal" range, but general patterns:

SDNN

  • >100 ms - Excellent HRV
  • 50-100 ms - Good HRV
  • 20-50 ms - Fair HRV
  • <20 ms - Low HRV (may indicate health concerns)

RMSSD

  • >50 ms - Excellent short-term HRV
  • 30-50 ms - Good short-term HRV
  • 15-30 ms - Fair short-term HRV
  • <15 ms - Low short-term HRV

⚠️ Use personal baseline: Your HRV may naturally be higher or lower than these ranges. What matters most is tracking changes from your own baseline.

Factors Affecting HRV

  • Age - HRV generally decreases with age
  • Fitness level - Athletes typically have higher HRV
  • Stress - Physical or mental stress lowers HRV
  • Sleep quality - Poor sleep reduces HRV
  • Alcohol - Consumption lowers HRV for 12-24 hours
  • Illness - Infections and illness decrease HRV
  • Dehydration - Can lower HRV
  • Overtraining - Excessive exercise without recovery lowers HRV

How Cardio Analytics Uses HRV Data

  • Displays nightly HRV trends - Apple Watch typically records HRV during sleep
  • Flags unusually low values - Alerts when HRV drops significantly below your baseline
  • Long-term trend analysis - Track HRV changes over weeks and months
  • Medication correlations - See how medications affect autonomic function
  • Symptom correlations - Identify patterns between low HRV and symptoms

HealthKit Data Types

Cardio Analytics reads HRV data from Apple HealthKit using these identifiers:

  • heartRateVariabilitySDNN - Overall HRV (SDNN in milliseconds) (Apple Docs)
  • heartRateVariabilityRMSSD - Short-term vagal tone (RMSSD in ms, where available) (Apple Docs)

Learn More About HealthKit Integration

Scientific References

  1. Cleveland Clinic. Heart Rate Variability (HRV): What It Is and How You Can Improve It. https://my.clevelandclinic.org/health/symptoms/21773-heart-rate-variability-hrv

View All References

Track Your HRV with Cardio Analytics

Monitor heart rate variability trends to assess stress levels and cardiovascular health.

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