Key Terms
Concise definitions for terms used throughout this guide
The following definitions are provided for quick reference. Full context for each appears in the relevant section of the guide.
Allostatic Load
The cumulative physiological cost of repeated or prolonged activation of the body's stress-response systems — a wear-and-tear cost that builds up when the HPA axis is triggered too often, or for too long, without enough recovery in between. Explained in full in Section 2.
Amygdala
A brain structure central to threat detection, receiving sensory information through both a fast, imprecise pathway and a slower, more contextual one — the fast pathway is why physiological stress reactions can precede conscious recognition of a threat.
Cortisol
The body's main stress hormone, released by the adrenal glands as the final step of the HPA axis cascade. Adaptive in short bursts — mobilising energy and sharpening focus — but a well-documented driver of allostatic load when levels stay elevated for too long without adequate recovery.
HPA Axis (Hypothalamic-Pituitary-Adrenal Axis)
The hormonal cascade — hypothalamus to pituitary to adrenal glands — that releases cortisol in response to a perceived threat or demand.
HRV (Heart Rate Variability)
The variation in time between consecutive heartbeats. Higher variability generally reflects a nervous system that can flexibly shift between activated and calm states, and is tied to prefrontal-amygdala circuitry.
Parasympathetic Nervous System
The branch of the autonomic nervous system associated with calm, recovery, and "rest and digest" functions. The vagus nerve is its primary pathway.
Physiological Sigh
A double inhale (a sniff followed by a smaller second sniff) followed by one long exhale. The single technique in this guide with the strongest head-to-head controlled trial evidence for reducing acute physiological arousal.
Polyvagal Theory
A theoretical framework proposing that the vagal system has functionally distinct branches associated with social engagement versus immobilisation under extreme threat. Clinically influential; some specific anatomical claims remain debated among researchers.
Resonance Frequency Breathing
Breathing at approximately 5–6 breaths per minute, which aligns natural heart-rate oscillations from breathing and blood pressure regulation, producing the largest acute increases in heart rate variability of any technique in this guide.
Sympathetic Nervous System
The branch of the autonomic nervous system associated with alertness and activation — "fight or flight." Adaptive in short bursts; costly when chronically engaged without adequate recovery.
Vagal Tone
The general level of activity in the vagus nerve. Higher vagal tone is associated with faster physiological recovery after a stressor and is closely linked to HRV.
Vagus Nerve
The tenth cranial nerve and primary pathway of the parasympathetic nervous system, running from the brainstem to the heart, lungs, and digestive organs.