What is SDNN?
SDNN (standard deviation of all normal-to-normal RR intervals) is a comprehensive HRV metric capturing overall autonomic variability over a recording period. It is used in clinical cardiology and long-term stress and health assessments.
Real-world uses
SDNN is the gold-standard HRV metric for 24-hour Holter monitor recordings in clinical cardiology. It reflects overall autonomic nervous system function (both sympathetic and parasympathetic). SDNN below 50 ms in 24-hour recordings is associated with increased cardiac risk.
History
SDNN (Standard Deviation of NN intervals) was established as a key HRV metric in the 1996 Task Force guidelines. Early research by Wolf et al. (1978) showed reduced HRV predicted mortality after myocardial infarction, establishing SDNN's clinical importance.
Common mistakes
Comparing SDNN values from different recording durations—SDNN increases with longer recordings because it captures more variability. A 5-minute SDNN is always lower than a 24-hour SDNN and the two should not be directly compared.
What is ln(RMSSD) proxy score?
The natural logarithm of RMSSD is a derived HRV metric that normalizes the skewed distribution of raw RMSSD values. It provides a more statistically linear scale for comparing HRV across populations and monitoring physiological trends over time.
Real-world uses
The natural log of RMSSD is used in sports science and athlete monitoring to normalise the skewed distribution of raw RMSSD values. Coaches track ln(RMSSD) trends over weeks to detect overtraining or readiness for high-intensity sessions. Typical values range from about 2.5 to 4.5.
History
The use of ln(RMSSD) was popularised by Andrew Flatt and colleagues in sports science research around 2013–2017. The logarithmic transformation addresses the non-normal distribution of raw RMSSD data, making statistical analysis and trend detection more robust.
Common mistakes
Interpreting small changes in ln(RMSSD) as negligible—because it is a logarithmic scale, a change of 0.5 ln units represents a substantial change in the underlying RMSSD. Also, forgetting that the coefficient of variation of ln(RMSSD) over a week is more informative than single readings.
When is this conversion used?
Converting SDNN to ln(RMSSD) proxy score is useful in the heart rate variability domain when comparing values across different measurement standards or applying formulas that require a specific unit.
Worked examples
1 SDNN = 0.05 ln(RMSSD) proxy score
1 ln(RMSSD) proxy score = 20 RMSSD
How to convert SDNN to ln(RMSSD) proxy score
To convert SDNN to ln(RMSSD) proxy score, multiply the value by 0.05.
To convert ln(RMSSD) proxy score back to SDNN, multiply by 20.
Measurement standards
The 1996 Task Force guidelines define the standard HRV frequency-domain (LF/HF power bands), time-domain (SDNN, RMSSD, pNN50), and geometric (triangular index) measures. SDNN is typically measured over 24-hour Holter recordings; RMSSD over 5-minute or shorter epochs for short-term assessment.
Did you know?
Deep, slow breathing at around 6 breaths per minute resonates with the natural frequency of the baroreflex — the heart's pressure-regulation feedback loop — and measurably increases HRV, a phenomenon exploited in biofeedback therapy and meditation practices.
Quick reference: SDNN to ln(RMSSD) proxy score
| SDNN | ln(RMSSD) proxy score |
|---|---|
| 0.1 | 0.005 |
| 0.5 | 0.025 |
| 1 | 0.05 |
| 2 | 0.1 |
| 5 | 0.25 |
| 10 | 0.5 |
| 25 | 1.25 |
| 50 | 2.5 |
| 100 | 5 |
| 250 | 12.5 |
| 500 | 25 |
| 1,000 | 50 |