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PolyVagal Theory, developed by Stephen Porges, is a description of how the human nervous system evolved from reptiles and mammals. An understanding of the evolutionary history allows us to understand how the Vagus Nerve works - which has very profound significance for understanding human behaviour.
Your body's physiology - how it absorbs and conserves and uses energy, how it maintains its proper internal temperature, circulates blood, makes sure there is enough oxygen, etc - is what keeps you alive, and it is run by the Autonomic Nervous System (ANS). The ANS has two branches - one of which more or less deals with the outside world by using energy and moving. The other half of the ANS - the Vagus Nerve - organises the digestion of food and also determines how we use our facial expression and body language to communicate ("socialise").
Neuroception is the term used to describe the way that your body-mind (the more instinctive part) determines whether you are safe or not. When our body feels safe (which is NOT the same as the mind thinking you are safe), then the body and mind are working efficiently with an optimised use of energy, and you are able to be calm and alert, peaceful and energised. This is a particular balance of the Autonomic Nervous System - the "window of normal adaptation" (or in psychology the "Window of Tolerance").