Heart Rate Variability

General
Heart Rate Variability, or HRV for short, is a non-invasive measure of your autonomic nervous system, which is the body’s main control center. It is widely considered as one of the best objective metrics for physical fitness and determining your body’s readiness to perform. HRV is literally the variance in time between the beats (NN interval) of your heart. 

A commonly used statistical metric for representing short-time HRV (over a time duration of the order of 10s to 1 min) is the Standard Deviation of NN intervals (SDNN). The definition and additional contextual information of each output is outlined below in the table.
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Output
SDNN is returned as an integer on a scale of 0 - ∞ as a part of the output response JSON, with the key sdnn.

Output (milliseconds)
Definition
Example Color Indicator
Example User Text
0 - 35
Below average
Red
Your heart rate variability value is below average. This indicates your body is under pressure for some reason which could be due to exercise, psychological proceedings or any other external and internal stressors.
35 - 100 
Average - Healthy
Yellow
Your heart rate variability is in a normal range. To further increase your heart rate variability you can perform aerobic and/or breathing exercises or meditation.
>100
Above average – Excellent
Green
Your heart rate variability is above average which general states that the body has a stronger capability to cope up with stress or is recovering strongly from previously built-up stress.

TABLE OF CONTENTS

General

Output

Output (milliseconds)	Definition	Example Color Indicator	Example User Text
0 - 35	Below average	Red	Your heart rate variability value is below average. This indicates your body is under pressure for some reason which could be due to exercise, psychological proceedings or any other external and internal stressors.
35 - 100	Average - Healthy	Yellow	Your heart rate variability is in a normal range. To further increase your heart rate variability you can perform aerobic and/or breathing exercises or meditation.
>100	Above average – Excellent	Green	Your heart rate variability is above average which general states that the body has a stronger capability to cope up with stress or is recovering strongly from previously built-up stress.