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 normalized HRV score (x10)?
Normalized HRV score (×10) is a scaled HRV metric that maps raw HRV values onto a standardized reference range, multiplied by 10 for resolution. It is used by consumer health platforms to present HRV data in a user-friendly numerical format.
Real-world uses
Normalized HRV scores are used by consumer apps and wearables to present HRV data on an easy-to-understand scale. Companies like Whoop, Garmin, and Oura each have proprietary scoring systems. The x10 scale maps raw HRV values to a 0–100 range for intuitive daily readiness assessment.
History
Normalized HRV scores emerged with the consumer wearable boom of the mid-2010s. Device makers developed proprietary algorithms to translate complex HRV metrics into simple scores that everyday users could understand without knowledge of the underlying physiology.
Common mistakes
Treating normalized scores from different platforms as equivalent—each vendor uses proprietary algorithms, baselines, and scaling. A score of 70 on one app does not mean the same thing as 70 on another. Always compare scores within the same platform.
When is this conversion used?
Converting SDNN to normalized HRV score (x10) 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 = 0.1 normalized HRV score (x10)
1 normalized HRV score (x10) = 10 RMSSD
How to convert SDNN to normalized HRV score (x10)
To convert SDNN to normalized HRV score (x10), multiply the value by 0.1.
To convert normalized HRV score (x10) back to SDNN, multiply by 10.
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 normalized HRV score (x10)
| SDNN | normalized HRV score (x10) |
|---|---|
| 0.1 | 0.01 |
| 0.5 | 0.05 |
| 1 | 0.1 |
| 2 | 0.2 |
| 5 | 0.5 |
| 10 | 1 |
| 25 | 2.5 |
| 50 | 5 |
| 100 | 10 |
| 250 | 25 |
| 500 | 50 |
| 1,000 | 100 |