General Adaptation Syndrome: Stages of the Body’s Stress Response

Every time you face a deadline, a conflict, or a sudden scare, your body does something remarkable – it launches a coordinated physiological response to help you survive the moment. But what happens when that moment stretches into days, weeks, or even months? Hans Selye, an Austrian-born endocrinologist working at McGill University in the 1930s, was the first scientist to map this out systematically. He called it the General Adaptation Syndrome (GAS) – a three-stage model describing exactly how the body responds to stress over time. Understanding GAS isn’t just academic; it’s a practical framework for recognizing what prolonged stress is doing to you, right now, at the biological level.

The origins of GAS: how a lab accident changed stress science 🔗

Selye didn’t set out to study stress. He was experimenting with ovarian extracts injected into laboratory rats when he noticed something unexpected – regardless of the substance injected or the type of stressor applied, the rats showed the same cluster of physical changes: enlarged adrenal glands, shrunken thymus and lymphatic tissue, and stomach ulcers. Selye recognized this as a nonspecific, triphasic pattern of physiological responses to injury or threat, and he published his findings in a landmark 1936 letter to the journal Nature. He termed the body’s predictable sequence of changes the General Adaptation Syndrome, and it fundamentally changed how science understood the relationship between stress and disease.

The three stages Selye identified – alarm, resistance, and exhaustion – are not random. They follow a logical progression: the body mobilizes, adapts, and eventually breaks down if the stressor doesn’t let up.

Stage one: the alarm reaction 🔗

The alarm stage is the body’s immediate, automatic response to a perceived threat. This is what most people know as the fight-or-flight response – a term originally coined by physiologist Walter Cannon to describe our hardwired reaction to danger. The moment a stressor is detected, the brain signals the autonomic nervous system (ANS), specifically the sympathetic branch, to kick into high gear.

Here’s what that looks like physiologically: the adrenal glands release a surge of cortisol and adrenaline (epinephrine). Heart rate climbs. Blood pressure rises. Breathing quickens. Blood is redirected away from non-urgent systems – like digestion and reproduction – and toward the large muscle groups that might need to fight or flee. Senses sharpen. Energy floods the system.

What the alarm stage is actually doing 🔗

The sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis work together during this phase to mobilize resources rapidly. The adrenal cortex releases cortisol, while the adrenal medulla releases catecholamines like adrenaline and noradrenaline. These hormones work in concert to sharpen focus, boost energy availability, and prepare the body to act.

Selye observed in his original experiments that this alarm reaction in rats appeared within 6 to 48 hours of exposure to a stressor, and was accompanied by measurable physical changes including reduced body fat, shrinkage of the thymus and lymph glands, and a drop in body temperature. In short-term situations – a near-miss car accident, a heated argument, a sudden emergency – the alarm response is entirely appropriate and even protective. The problem is when it doesn’t switch off.

A temporary dip in resistance 🔗

One counterintuitive aspect of the alarm stage is that, despite all the mobilization, the body’s overall resistance to stress actually drops briefly at the very start. The system is reallocating resources so rapidly that it temporarily lowers defenses. During this initial phase, there is a temporary decrease in the body’s resistance to stress as it marshals resources to deal with the immediate challenge. This is short-lived – the resistance stage follows quickly – but it explains why people sometimes feel physically run-down right after a major acute stressor, even before chronic stress sets in.

Stage two: the resistance stage 🔗

If the stressor persists beyond the initial alarm, the body doesn’t simply stay in crisis mode. Instead, it does something more sophisticated – it adapts. The resistance stage is the body’s attempt to restore a new kind of balance, often called homeostasis, while still operating under the continued presence of a stressor.

Cortisol levels drop slightly from their peak, and heart rate and blood pressure begin to normalize – but don’t return fully to baseline. The body remains on alert, running at a higher metabolic level than normal to offset the persistent stress. Physiologically, it’s a controlled burn: the system is working harder than usual, but in a way that looks, on the surface, like it’s coping fine.

Nonessential systems go offline 🔗

To sustain this heightened state, the body makes trade-offs. Cortisol suppresses the digestive system, the reproductive system, and growth processes – functions that are nonessential in a survival scenario. Immune responses are also partially dialed down during this phase. The body is essentially saying: reproduction and digestion can wait; right now, we need to stay alert and functional.

This is why people under chronic stress often report symptoms like poor concentration, irritability, and frustration – the brain and body are still running in a stress-adapted mode, even when there’s no obvious crisis at hand. The person may feel like they’re managing the situation, while their body is silently burning through reserves.

The illusion of coping 🔗

This is one of the most clinically important aspects of GAS – the resistance stage can mask how much damage is accumulating. The body continues to secrete stress hormones and blood pressure remains elevated, even if the person thinks they’re managing stress well. People in this stage often describe themselves as “stressed but functioning.” That’s technically accurate – but it doesn’t mean the body isn’t paying a cost. The longer the resistance stage continues without relief, the closer the body edges toward the third and most dangerous phase.

Stage three: the exhaustion stage 🔗

When a stressor is chronic and unresolved, the body eventually reaches its limit. The reserves that sustained the resistance stage are depleted. Adaptation energy – Selye’s term for the finite capacity the body has to maintain its stress response – runs out. This is the exhaustion stage, and it is where prolonged stress transitions from a strain into a serious health threat.

In the exhaustion stage, the body no longer has the energy to fight the stressor, which can lead to burnout, fatigue, depression, and anxiety. The sense of hopelessness that often accompanies burnout isn’t merely psychological – it reflects a genuine physiological depletion.

What exhaustion does to the immune system 🔗

The immune system bears some of the worst consequences of chronic stress. Prolonged cortisol elevation suppresses immune function by inhibiting the production of pro-inflammatory cytokines and reducing the activity of lymphocytes – the white blood cells that defend against infection. Chronic stress decreases lymphocyte levels, raising the risk of viruses and infection, and can trigger a state that researchers sometimes call “stress-induced sickness.”

This immune suppression isn’t trivial. Research has found that people under chronic stress had immune cells that produced significantly more inflammatory cytokines when exposed to bacterial toxins compared to low-stress individuals. And chronic exposure to elevated cortisol reduces T cell proliferation and activity, diminishing the body’s ability to mount effective immune responses.

Cardiovascular, metabolic, and mental health consequences 🔗

The damage doesn’t stop with the immune system. Chronic stress leads to dysfunctional responses resulting in heart disease, stomach ulcers, sleep dysregulation, and psychiatric disorders. The sustained activation of the sympathetic nervous system promotes atherosclerosis and raises the risk of stroke, coronary artery disease, and hypertension. Cortisol’s catabolic effects can also lead to muscle wasting and decreased bone density over time.

Impaired communication between the immune system and the HPA axis – a hallmark of chronic stress – has been linked to the development of metabolic disorders like diabetes and obesity, depression, and chronic fatigue. Selye himself referred to the diseases that emerged from prolonged stress responses as “diseases of adaptation” – conditions that are, in essence, the body’s own adaptive mechanisms turned against it.

The role of chronic stress in exhaustion 🔗

It’s worth being precise about what counts as “chronic” stress in this context. The exhaustion stage isn’t triggered by a bad week or a difficult month. It emerges from sustained, unresolved stress – the kind that comes from ongoing financial insecurity, a toxic work environment, long-term caregiving, or persistent relational conflict. Once in the exhaustion stage, prolonged stress raises the risk for chronic high blood pressure, stroke, heart disease, and depression, along with a higher risk for infections and cancer due to a weakened immune system.

The stage of exhaustion occurs when the body’s resistance breaks down with the loss of its adaptive capacity. Without intervention – whether through removing the stressor, behavioral change, or professional support – the body cannot self-correct at this stage.

Why GAS still matters today 🔗

Selye’s model was developed in the 1930s and 1940s, and stress science has grown enormously since then. Some of his specific claims – particularly about the role of hormones – have been refined or debated. But the core insight of GAS remains foundational: stress is not just a feeling. It is a biological process that unfolds in predictable stages, with measurable physiological consequences that accumulate over time.

Understanding which stage of GAS you might be in has real practical value. Someone in the alarm stage needs rest and short-term recovery. Someone in the resistance stage needs to take symptoms seriously before they worsen. Someone showing signs of exhaustion – chronic fatigue, emotional numbness, frequent illness, cardiovascular symptoms – needs meaningful intervention. The GAS framework also demonstrates how training and behavioral adaptation can modify the stress response, which is why stress management practices like exercise, mindfulness, and sleep hygiene are not just feel-good advice – they are physiologically grounded strategies for keeping the body from moving deeper into GAS.

What do you think? At which stage of the General Adaptation Syndrome do you think most people in high-pressure modern environments are operating – and what would it take for society to take the exhaustion stage as seriously as it does acute illness? If the body’s stress response was designed for short-term survival, how do you think our environments need to change to prevent chronic stress from becoming the norm rather than the exception?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC5915631/
2. https://www.ncbi.nlm.nih.gov/books/NBK349158/
3. https://www.apa.org/topics/stress/body
4. https://www.ncbi.nlm.nih.gov/books/NBK541120/
5. https://www.psychologistworld.com/stress/general-adaptation-syndrome
6. https://www.psychiatryofthepalmbeaches.com/general-adaptation-syndrome
7. https://www.healthline.com/health/general-adaptation-syndrome
8. https://www.mayoclinic.org/healthy-lifestyle/stress-management/in-depth/stress/art-20046037
9. https://psychcentral.com/stress/general-adaptation-syndrome
10. https://health.clevelandclinic.org/what-happens-when-your-immune-system-gets-stressed-out
11. https://www.thesupportivecare.com/blog/how-chronic-stress-impacts-the-immune-system
12. https://pmc.ncbi.nlm.nih.gov/articles/PMC11546738/
13. https://www.ebsco.com/research-starters/health-and-medicine/general-adaptation-syndrome-gas