HRV analysis shows the state of the autonomic nervous system (ANS). The heart is regulated by the sympathetic and parasympathetic nervous systems.
In healthy people, these two systems are in balance. This means their heart rate is more variable. A decline in heart rate variability is a sign that the body is not coping with stress well and is having trouble adapting to external demands.
The diagram above is a rhythmogram where every point shows the time interval between two heartbeats in milliseconds. The measurements were taken from a woman, 26 years old, at rest (average pulse 50-60 heartbeats per minute, the average interval between heart beats 1,000-1,300 ms) and during physical exercise (average pulse 100-120 heart beats per minute, the average interval between the heartbeats 400-500 ms).
Here is more information about what we measure and scientific data about heart rate variability.