Stress Physiology 101 |KfitWell Coaching
An infographic of the HPA axis: Hypothalamus–Pituitary–Adrenal system.
Stress Physiology 101: How Chronic Stress Shapes Your Body, Mood & Performance
Stress is not “in your head” — it is a full-body physiological response controlled by a powerful internal network known as the HPA axis. When this system is balanced, stress helps you perform, focus, and survive. But when activated too frequently or for too long, it begins to reshape your hormones, metabolism, digestion, sleep, and emotional resilience (McEwen, 2007) [1].
In a world where constant pressure, digital overload, and lifestyle demands never switch off, understanding the science of stress is no longer optional — it’s essential for maintaining long-term well-being.
1. How the Stress System Works: The HPA Axis
When your brain detects a threat — physical, emotional, or even imagined — it activates the HPA axis. This initiates a cascade involving the hypothalamus, pituitary gland, and adrenal glands.
Step 1: The hypothalamus releases CRH (Corticotropin-Releasing Hormone).
Step 2: The pituitary gland responds with ACTH (Adrenocorticotropic Hormone).
Step 3: The adrenal glands release cortisol — your main stress hormone (Sapolsky, 2015) [2].
These reactions occur within seconds, priming your body for survival by increasing alertness, blood sugar availability, and muscular readiness. In short bursts, this system is adaptive and protective. In chronic activation, however, it becomes harmful.
2. Cortisol: Helpful vs. Harmful
Cortisol is often misunderstood. It is essential for energy regulation, metabolic function, inflammation control, and waking you up in the morning. The issue arises when cortisol remains elevated for extended periods due to chronic stress, poor sleep, or emotional strain (Miller et al., 2007) [3].
Healthy cortisol: follows a natural 24-hour rhythm (high in morning, lower at night).
Chronic cortisol: remains high throughout the day, flattening this rhythm and disrupting essential functions.
Persistent cortisol elevation influences:
- Increased abdominal fat storage due to higher glucose availability
- Reduced thyroid function and slower metabolism
- Increased hunger signals (especially cravings for sugary foods)
- Impaired digestion and gut motility
- Higher inflammation levels due to immune system suppression
- Disrupted sleep-wake cycles
Over time, this can progress into what researchers call “allostatic load” — the physiological wear and tear caused by chronic stress (McEwen & Seeman, 1999) [4].
3. Stress and the Body: What Actually Happens?
Chronic stress affects nearly every system in the human body. Unlike acute stress (running late, tight deadlines), long-term stress begins to reshape biological function.
✔ Digestion & Gut Health
Stress diverts energy away from digestion — a survival mechanism. This reduces stomach acid, slows motility, and alters gut microbiota composition (Foster & McVey Neufeld, 2013) [5].
✔ Metabolism & Weight Regulation
Cortisol increases glucose release into the bloodstream. When unused, this is stored as fat, primarily around the abdomen. Stress also impacts leptin and ghrelin — hormones that regulate appetite and satiety.
✔ Sleep & Circadian Rhythms
A flattened cortisol curve disrupts melatonin signalling, making it harder to fall asleep or stay asleep. Poor sleep then further increases cortisol, forming a harmful cycle.
✔ Mood, Anxiety & Cognition
High cortisol over long periods reduces prefrontal cortex function — the area responsible for planning, reasoning, and focus. This increases anxiety, reduces emotional control, and impairs decision-making (Arnsten, 2009) [6].
✔ Immunity & Inflammation
While cortisol is anti-inflammatory in the short term, chronic exposure weakens immune response. This makes you more susceptible to infections and slows recovery.
4. The Difference Between “Good Stress” and “Bad Stress”
Not all stress is negative. Researchers refer to eustress (positive stress), which stimulates growth, motivation, and resilience. Exercise, cold exposure, learning new skills, and performance challenges all trigger eustress beneficially (Dienstbier, 1989) [7].
The issue is distress — stress that is excessive, chronic, or outside your perceived coping abilities.
The key goal is not to eliminate stress, but to build stress tolerance and regulate the HPA axis effectively.
5. Evidence-Based Stress Regulation Strategies
Below are science-supported methods proven to calm the stress system and rebalance physiology.
✔ Diaphragmatic Breathing
Slow controlled breathing activates the parasympathetic nervous system, reducing heart rate and lowering cortisol within minutes (Ma et al., 2017) [8].
✔ Regular Exercise
Physical activity is one of the strongest modulators of stress hormones. Moderate-intensity exercise improves mood, reduces cortisol, and enhances brain-derived neurotrophic factor (BDNF), supporting resilience.
✔ Sleep Prioritisation
Quality sleep stabilises cortisol rhythm. Even a single night of poor sleep increases cortisol by up to 37% the next evening (Leproult et al., 1997) [9].
✔ Nature Exposure
Multiple studies show that green environments reduce sympathetic nervous system activation and improve psychological recovery.
✔ Cognitive Reframing
Changing how you interpret stress — rather than the amount of stress you have — significantly reduces physiological reactivity (Crum et al., 2013) [10].
✔ Social Support & Connection
Human connection lowers cortisol and increases oxytocin, which buffers stress responses. Even brief positive interactions can create measurable physiological changes.
Conclusion
Stress is a natural biological mechanism designed to protect you — but today it is overactivated more than ever. By understanding how your HPA axis works and what chronic stress does to your body, you can begin taking control through targeted, science-backed strategies that restore balance, resilience, and long-term health.
References
- McEwen, B. S. (2007). Physiology and neurobiology of stress and adaptation. Physiological Reviews.
- Sapolsky, R. (2015). Why Zebras Don't Get Ulcers. Henry Holt.
- Miller, G. E., Chen, E., & Zhou, E. S. (2007). Chronic stress and inflammation. Psychological Bulletin.
- McEwen, B. S., & Seeman, T. (1999). Protective and damaging effects of stress. Annals of the New York Academy of Sciences.
- Foster, J. A., & McVey Neufeld, K. (2013). Gut–brain axis: How the microbiome influences anxiety and depression. Trends in Neurosciences.
- Arnsten, A. F. (2009). Stress signalling pathways that impair prefrontal cortex structure and function. Nature Reviews Neuroscience.
- Dienstbier, R. A. (1989). Arousal and physiological toughness. Psychological Review.
- Ma, X. et al. (2017). Influence of diaphragmatic breathing on stress. Frontiers in Psychology.
- Leproult, R., Copinschi, G., Buxton, O., & Van Cauter, E. (1997). Sleep loss results in an elevation of evening cortisol. Sleep.
- Crum, A. J., Salovey, P., & Achor, S. (2013). Rethinking stress. Journal of Personality and Social Psychology.
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