It was recently proposed that respiratory sinus arrhythmia (RSA) reflects the ability of the organism to integrate behavioural and metabolic demands, improving its homeostasis efficiency. Since the various anatomical and functional levels of the vagus nerve provide the conceptual basis of this allostatic model, it was designed under the name of the polyvagal theory. Therefore, altered RSA responses to various challenges could help to detect some dysfunctional states. The putative homeostatic roles of this vagal loop i.e., afferent and efferent pathways are reviewed here, in the domain of various psychological and behavioural homeostasis. Evaluation of the autonomic activity was issued from the temporal and frequency domain analyses of heart rate variability (HRV). For the purpose of preventing over-training, a heuristic sequential psychological and sympathovagal evolution is proposed, called the multistage psycho-autonomic model of adaptation to training (MPAMAT).