We have a natural variability in the spacing between our heartbeats. This is called Heart Rat Variability (HRV)
The more variable your heart rate the more you are able to readily respond to your environment and switch gears between the parasympathetic and sympathetic systems.
HRV is used as a health marker and generally decreases with age.
High HRV is an indication of cardiovascular health, fitness, willpower, calm and positive emotions and your ability to handle stress and exercise.
Low HRV is related to inflammation, chronic stress, chronic pain, depression, cancer and low emotional flexibility.