The stress response has a specific biological pathway, and it starts at the top of the brain.
The HPA axis — hypothalamic-pituitary-adrenal axis — is a three-stage cascade that converts a perceived threat into a whole-body physiological response. Understanding how it works explains why stress can become self-perpetuating.
Step one: the hypothalamus detects a threat.
When the brain perceives danger — whether it's a physical threat or a worrying email — the hypothalamus releases corticotropin-releasing hormone (CRH). This is the alarm bell. It signals the next stage of the cascade to activate.
Step two: the pituitary gland amplifies the signal.
CRH reaches the anterior pituitary gland, which responds by releasing adrenocorticotropic hormone (ACTH) into the bloodstream. ACTH travels to the adrenal glands, which sit on top of the kidneys.
Step three: the adrenals release cortisol.
The adrenal glands respond to ACTH by producing cortisol — the body's primary stress hormone. Cortisol increases blood sugar, suppresses immune function, and redirects energy towards immediate survival. In the short term, this is adaptive and even beneficial. Cortisol sharpens focus, mobilises energy, and enhances short-term memory.
The feedback loop is supposed to shut itself off.
Under normal conditions, rising cortisol levels signal back to the hypothalamus and pituitary to reduce CRH and ACTH production. It's a negative feedback loop — the output inhibits the input. The system fires, does its job, and stands down.
Chronic stress breaks the feedback loop.
When the HPA axis fires repeatedly — day after day, without adequate recovery — the feedback mechanism becomes less effective. The hypothalamus becomes less sensitive to cortisol's "stand down" signal. The system stays partially activated, maintaining elevated cortisol as a new baseline. This is how stress becomes chronic: not because the threat persists, but because the off switch stops working properly.
The downstream effects are broad.
Research indicates that chronically elevated cortisol may impair hippocampal function (weakening memory), suppress immune activity, disrupt sleep architecture, increase abdominal fat storage, and contribute to mood disorders. It's not a single symptom — it's a systemic degradation that touches every major body system.
Chronic HPA activation has a recognisable pattern.
Research documents that people experiencing chronic stress often report sleep disturbances (such as early morning waking), persistent fatigue despite adequate sleep, increased susceptibility to minor infections, and difficulty sustaining focus on tasks that normally feel manageable. Persistent muscular tension in the shoulders and jaw is also frequently observed. These patterns appear consistently across populations with prolonged HPA axis activation — patterns that may be worth monitoring as markers of ongoing physiological stress.
The cortisol awakening response reveals the pattern.
Angela Clow and her team at the University of Westminster spent years studying the cortisol awakening response (CAR) — the sharp rise in cortisol that occurs within the first 30–45 minutes after waking. In a healthy system, this spike is pronounced and predictable: it primes the brain for the day ahead. But Clow's research showed that in people under chronic stress, the CAR becomes blunted or erratic. The body loses its natural rhythm, and cortisol stays elevated at times when it should be falling — particularly in the evening, when it disrupts sleep and prevents proper recovery.
Slow breathing directly targets the cascade.
The HPA axis doesn't operate in isolation — it's modulated by the autonomic nervous system. When you extend the exhale and slow the breathing rate, you activate the parasympathetic branch via the vagus nerve. This sends an inhibitory signal to the hypothalamus, reducing CRH output and effectively turning down the volume on the entire cascade. Ma et al. (2017) found that diaphragmatic breathing training significantly lowered salivary cortisol in healthy adults over an eight-week programme, suggesting that extended-exhale breathing patterns may help reduce cortisol levels over time with consistent practice. Regular practice may help recalibrate the feedback loop so that it responds to the "stand down" signal more effectively.
The feedback loop can be repaired.
The brain adapts in both directions. Just as chronic stress desensitises the feedback mechanism, consistent recovery practices can restore it. Research on meditation, regular physical activity, and sleep hygiene all point to the same conclusion: the HPA axis recalibrates when the body receives reliable signals that the threat has passed. The key word is consistent. One good night's sleep won't undo months of overactivation, but a sustained pattern of recovery — steady breathing, regular rest, reduced stimulation before bed — gradually restores the system's ability to switch off when it should.