Heart Rate Variability
Heart Rate Variability (HRV) is the variation in time intervals between consecutive heartbeats. In research and clinical contexts, it is measured as the variability in R-R intervals (the time between successive R-waves in an ECG) or, for PPG-based devices, the analogous interbeat intervals (IBI). Higher HRV generally indicates greater parasympathetic (vagal) influence on heart rhythm; lower HRV indicates relative sympathetic dominance or reduced vagal tone. The metric reflects autonomic nervous system activity rather than heart rate itself.
Typical Adult Ranges
milliseconds (ms)Based on population studies. Individual needs vary by age and health status.
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Key Takeaways
Represents beat-to-beat variation in heart rhythm timing.
Reflects autonomic nervous system activity during sleep.
Higher values indicate greater parasympathetic influence.
Personal trends more informative than single values.
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How It's Measured
Oura measures HRV using photoplethysmography (PPG) sensors that detect blood volume changes in finger arteries.
Common Influences
Sleep quality: Fragmented or insufficient sleep is associated with reduced HRV.
Deep Dive
HRV represents the moment-to-moment flexibility of cardiac timing as modulated by the autonomic nervous system. It provides a window into the balance between sympathetic (activating) and parasympathetic (restorative) branches.
A useful framing is that HRV reflects the body's regulatory capacity. Higher variability suggests the cardiovascular system can adapt fluidly to changing demands; lower variability suggests the system is operating in a more constrained state, whether due to stress, fatigue, illness, or other factors affecting autonomic regulation.
The heart is not a metronome. Beat-to-beat timing is continuously modulated by the autonomic nervous system in response to respiratory patterns, blood pressure fluctuations, and central nervous system activity.
Parasympathetic input via the vagus nerve can rapidly slow heart rate; this input is reflected in respiratory sinus arrhythmia (heart rate increases during inhalation, decreases during exhalation). Sympathetic input generally increases heart rate and reduces variability. The interplay of these systems produces the variability measured as HRV.
HRV is influenced by many factors including fitness level, age, sleep quality, stress, inflammation, and acute physiological state. Changes in HRV reflect changes in autonomic balance, but the metric does not specify the underlying cause.