RMSSD
A time-domain metric based on beat-to-beat changes. It is often used in short recordings and is sensitive to high-frequency variation related to respiratory cardiac modulation.
Learn / Heart rate variability
Heart rate variability (HRV): what it measures and how to read it
Heart rate variability describes variation in the time between successive heartbeats. It is widely used in cardiovascular, autonomic and psychophysiological research, but a single HRV number is not a diagnosis, a universal readiness score or a direct meter of the whole nervous system.
Short answer
HRV is about beat-to-beat timing, not how fast the heart beats
Two people can have the same average heart rate while the spacing between individual beats varies differently. HRV metrics describe patterns in those intervals. The pattern can change with breathing, posture, sleep, exercise, stress, illness, medication, recording length and measurement quality.
A useful mental model
HRV is a measurement layer. It can contribute information about cardiac autonomic regulation when the method and context are controlled, but it does not by itself explain why a person feels stressed, recovered, tired or well.
Common metrics
Different HRV metrics summarize different parts of the signal
SDNN
A time-domain measure of overall variability across the recording. Its meaning depends strongly on recording duration, so short and 24-hour values should not be treated as interchangeable.
Frequency-domain measures
Spectral methods describe how variability is distributed across frequency bands. Interpretation requires care, especially when labels are simplified into claims about sympathetic or parasympathetic balance.
Context matters
HRV can change even when nothing is “wrong”
Breathing pattern and respiratory rate
Body position and movement
Time of day and recording duration
Sleep, recent exercise and recovery context
Illness, medication and other physiological context
Signal quality, artifacts and the measurement method
HRV and vagal tone
HRV can inform cardiac vagal research without becoming a universal “vagus score”
Some HRV measures, especially under controlled conditions, are used as indicators of respiratory modulation of cardiac vagal activity. That does not make every wearable HRV score a direct measurement of vagal tone, sympathetic output or “nervous system balance.” Recent guidance specifically warns against that kind of overinterpretation.
Reading personal HRV
Consistency of measurement is often more useful than chasing someone else’s number
Compare like with like
A morning resting measurement should not be treated as directly interchangeable with an exercise recording or an overnight value from a different device.
Watch method and artifacts
Signal noise, missed beats and algorithm choices can change derived values. Research-grade interpretation starts with recording quality.
Use trends carefully
Repeated measurements under similar conditions can add context, but a trend still needs to be interpreted alongside sleep, training, illness and other factors.
Do not diagnose from one score
An unusually high or low consumer HRV value is not, by itself, a diagnosis or proof that one branch of the autonomic nervous system is malfunctioning.
References
Measurement standards and interpretation guidance
Task Force, 1996
Standards of measurement, physiological interpretation and clinical use
The foundational standards paper formalized many common HRV measurement and reporting conventions and remains an important methodological reference.
Open sourceLaborde, Mosley & Thayer, 2017
Recommendations for psychophysiological HRV research
A practical review of planning, recording, analysis and interpretation, emphasizing that HRV is easy to collect but easy to misinterpret without methodological control.
Open sourceQuigley et al., 2024
Updated publication guidelines for HR and HRV studies
A committee report covering ECG and photoplethysmographic measurement, physiological foundations, methodological strengths and weaknesses, and transparent reporting.
Open sourceHuman cardiovascular HRV guidelines, 2026
Rigor and reproducibility in HRV research
Recent evidence-based guidance cautions against treating HRV as a specific measure of cardiac sympathetic outflow or a simple sympathovagal-balance score.
Open sourceResearch boundary
General HRV education and VNS-specific HRV research have different jobs
This page owns the general “what is HRV?” question. The Research topic page owns the evidence layer around HRV, autonomic regulation and VNS-related studies.
Explore HRV research