SDNN - Standard Deviation of NN Intervals

What is SDNN?

SDNN is the standard deviation of all normal-to-normal (NN) intervals in a recording. It is the simplest and most widely used measure of overall HRV.

Think of it this way: if your heartbeat intervals were all exactly the same (like a metronome), SDNN would be zero. The more your intervals vary, the higher the SDNN.

SDNN = √[ (1/(N−1)) · ∑(RR_i − RR̅)² ]

Where RR̅ is the mean of all RR intervals, and N is the total number of intervals. This is the sample standard deviation (N−1 denominator).

Visual Explanation

SDNN measures the spread of all your RR intervals around their average:

SDNN captures how spread out all your RR intervals are from the mean

Worked Example

Suppose we record 5 RR intervals (in milliseconds):

RR = [810, 830, 790, 850, 820]

Step 1: Calculate the mean:
RR̅ = (810 + 830 + 790 + 850 + 820) / 5 = 820 ms

Step 2: Calculate squared differences:
(810−820)² + (830−820)² + (790−820)² + (850−820)² + (820−820)²
= 100 + 100 + 900 + 900 + 0 = 2000

Step 3: SDNN = √(2000/4) = √500 ≈ 22.4 ms

Interpreting SDNN

SDNN (ms)	Interpretation
< 50	Low variability — may indicate stress or health risk
50 – 100	Moderate variability — compromised but functional
> 100	Healthy variability — good autonomic function

      Note: SDNN is highly dependent on recording length. A 5-minute SDNN cannot be directly compared to a 24-hour SDNN. Always compare recordings of the same duration.
    

Key Points

SDNN captures total variability — both short-term and long-term
It reflects the combined influence of sympathetic and parasympathetic activity
Best used for recordings of at least 5 minutes
Measured in milliseconds (ms)

ECG Basics RMSSD