Humans have an internal nervous system which regulates heart rate, breathing, sleep, hormones and much more. In contrast to the peripheral nervous system, of which we are consciously in control (e.g. finger movement), there is also an autonomous nervous system, that functions independently from our consciousness. Just imagine if you had to tell your heart to beat 100.000 times a day, or if you constantly had to remember to tell your digestive system to start digesting. Quite an effort – and imagine the consequences if you were to forget!
The autonomous nervous system has two main systems, the sympathetic and the parasympathetic system. The sympathetic system dominates when we are under stress and active during the day. The parasympathetic system dominates when we sleep, laze around and recover.
When we are chronically overworked the parasympathetic system disappears more and more and we can no longer recover properly. If this chronic overload continues, the sympathetic system degenerates and our life-energy decreases. This is a typical process with patients who experience chronical stress and tend to burn out.
Intervals between heart beats become similar when the sympathetic system dominates. When the parasympathetic system dominates the intervals between heart beats become irregular, the heart just sort of sprawles along.
In measuring heart rate variability we make use of these characteristic changes dependent on the autonomous nervous system. The more varied your heart rate is, the more balanced you are and the more energy you have. The more monotonous your heart rate, the less energy you have and the more chronically stressed your body is. In this way we measure how relaxed or stressed you are.