Balance the autonomic nervous system for perfect wellbeing

In my 6 years' experience of working with people therapeutically with my Embodied Living programmes, I can honestly say that stress and disease happen when the autonomic nervous system is out of balance.Harmony, wellbeing, balance and performance happen when there is balance between the two sides of the nervous system: the sympathetic (SNA) and the parasympathetic (PNS).
These are like yin and yang. The SNS is the active, dynamic side and the PNS the passive, relaxed and expansive side. We need balance between the two for an optimal state of being where we are creative, open, relaxed yet active and engaged enough to get things done.
But the SNS gets a bad press: it is associated with the fear, flight or fight response and stress. But we need it.
Nevertheless for most of us today, our SNS is in overdrive to a greater or lesser degree.
We feel tired and wired. We have poor memory, short attention spans, feel fuzzy headed. We over react to situations and people. All sure signs that the SNS is hyper and all illustrative of how this 'out of balance' state affects the mind.

The SNS is easy and quick to engage: we perceive something (or someone!) that is challenging and instantaneously various muscle centres (particularly the eyes, jaw, throat, tongue, hands, feet, diaphragm and pelvic area) tighten and tense up. These muscle centres find it hard to subsequently let go; it takes time. Often we are in over drive so much of the time that they never fully relax (this leads to conditions such as chronic pain, chronic fatigue perhaps ME). The PNS invariably is under used, and often like anything that we don't use, has atrophed to such an extent that it is useless.

The very nature of the autonomic nervous system (involuntary) implies that we have no control. But there is a unique window into this system - not only to see how out of balance it is, but also to control it. Heart rate variability (HRV) is related to the frequency, regularity and responsivness of the changing beats of the heart. It is intimately linked with breathing and in particular, with the range of movement of the primary muscle of breathing: the diaphragm.

Yoga, though likely you will find no book that says this, is all about bringing the autonomic nervous system into balance through extending the range of the diaphragm (especially the exhalation phase, which is key to PNS engagement) to achieve high HRV. This brings about emotional and physiological coherence - a state of bliss, flow state where we are expansive and whole, ultimately the state of turya or samadhi.