The three stages of the stress response were first described in 1936 by the endocrinologist Hans Selye, who called the pattern the general adaptation syndrome. After subjecting laboratory rats to various harmful stimuli and observing the same physical changes every time, Selye proposed that the body moves through an alarm reaction, a stage of resistance, and finally a stage of exhaustion when stress persists too long.1Singapore Medical Journal. Hans Selye (1907-1982): Founder of the stress theory Nearly a century later, the model still serves as a useful map for understanding how your body handles pressure, even though researchers have refined and complicated the picture considerably.
The Alarm Reaction
The alarm stage is your body’s initial shock response. Within seconds of encountering a threat, two systems fire up almost simultaneously. The sympathetic nervous system triggers the adrenal glands to release adrenaline and noradrenaline, raising your heart rate, redirecting blood to your muscles, and sharpening your senses. At the same time, a slower hormonal cascade begins: the hypothalamus signals the pituitary gland, which signals the adrenal cortex to release cortisol.2PubMed Central. Regulation of the Hypothalamic-Pituitary-Adrenocortical Stress Response – Section: Abstract Cortisol floods the bloodstream and does the heavy metabolic lifting: it mobilizes glucose from storage, dampens non-essential processes like digestion and immune surveillance, and shifts energy toward whatever you need to survive in the next few minutes.
The alarm stage also involves a hormonal trade-off. Glucocorticoid production ramps up at the expense of other adrenal hormones, and the fraction of cortisol circulating freely in the blood increases, making it more biologically potent.3Best Practice & Research Clinical Endocrinology & Metabolism. The hypothalamic-pituitary-adrenal response to critical illness – Section: Abstract This is by design. When you are about to be hit by a car or confronted by a physical threat, your body does not need to worry about reproductive hormones or mineral balance. It needs fuel and focus, right now.
In Selye’s original rat experiments, the alarm reaction was marked by three physical signs: enlarged adrenal glands (working overtime), shrunken thymus and lymph tissue (immune resources being sacrificed), and deep stomach ulcers (gut lining breaking down under the cortisol surge).1Singapore Medical Journal. Hans Selye (1907-1982): Founder of the stress theory Humans rarely develop ulcers from a single stressful event, but the underlying biology is the same: the alarm reaction mobilizes everything at a cost to systems not needed for immediate survival.
The Resistance Stage
If the stressor does not go away, your body does not simply stay in emergency mode. It adapts. Cortisol levels remain elevated but often settle to a somewhat lower plateau than the initial spike. Your metabolism shifts to sustain the effort over a longer period. In animal studies, repeated stress changes how the body selects fuel: acute stress burns fat rapidly, but as stress becomes chronic, there is a gradual shift toward burning carbohydrates, suggesting the body is recalibrating its energy strategy for the long haul.4PubMed Central. The metabolic stress response: Adaptation to acute-, repeated- and chronic challenges in mice – Section: Results
From the outside, the resistance stage can look like successful coping. You are functioning, working, handling the situation. But the adaptations come with quiet trade-offs. Cortisol, which was so helpful in the alarm phase, starts to cause problems when it stays elevated. One of the most significant is immune suppression: chronic cortisol exposure alters how immune cells move through the body and changes which genes they switch on, reducing the body’s ability to fight infection and potentially setting the stage for autoimmune disorders.5PubMed Central. Immunology of Stress: A Review Article This is why people under sustained pressure seem to catch every cold that comes along. The immune system is not broken; it is being actively suppressed by the same hormones keeping you functional in other ways.
The resistance stage is often the longest of the three. Many people live here for months or years, especially when the source of stress is ongoing, things like a difficult job, financial insecurity, caregiving responsibilities, or a strained relationship. The body is managing, but each week of elevated cortisol and sympathetic nervous system activity accumulates a biological cost that eventually becomes harder to ignore.
The Exhaustion Stage
When the body’s adaptive resources are finally depleted, the system breaks down. This is the exhaustion stage, and it marks the point where stress begins to cause real, measurable damage. One of the key mechanisms involves something called glucocorticoid receptor resistance. Under prolonged stress, the receptors that cortisol is supposed to bind to become less sensitive, meaning cortisol can no longer do its job of reining in inflammation. The body keeps producing cortisol, but the signals it sends stop landing.6PubMed Central. Chronic stress, glucocorticoid receptor resistance, inflammation, and disease risk – Section: Abstract
This is a particularly insidious problem. The research supporting the receptor resistance model found that people who had been through prolonged threatening experiences developed measurably reduced cortisol sensitivity, and those with the greatest receptor resistance were more likely to develop upper respiratory infections when exposed to a cold virus. The link was not just hormonal but inflammatory: people with greater receptor resistance produced more inflammatory signaling molecules when they were sick.6PubMed Central. Chronic stress, glucocorticoid receptor resistance, inflammation, and disease risk – Section: Abstract In other words, the exhaustion stage does not just leave you tired. It rewires how your immune system communicates, tipping the balance toward chronic, low-grade inflammation.
The mechanisms behind this receptor resistance are varied and layered. They include changes in which version of the receptor the body produces, problems with getting receptors into the cell nucleus where they work, oxidative damage, and even epigenetic changes that silence the genes responsible for producing the receptors in the first place.7PubMed. Factors Influencing Glucocorticoid Treatment Response: Mechanism-Based Strategies to Overcome Glucocorticoid Resistance and Restore GRα Function In critical illness, this state has been described as “relative adrenal insufficiency,” where cortisol levels may actually be high but cannot overcome the tissue-level resistance, and it remains genuinely debated whether this resistance is a maladaptive breakdown or a last-ditch protective mechanism.8Comprehensive Physiology. Adrenocortical Stress Response during the Course of Critical Illness – Section: Abstract
What Chronic Stress Does to the Heart and Brain
The damage from prolonged stress does not stay confined to the hormonal system. Two organ systems that take a particularly heavy hit are the cardiovascular system and the brain. In the heart and blood vessels, chronic stress is an established risk factor for cardiovascular disease. The mechanisms converge on the endothelium, the thin inner lining of blood vessels. Excessive cortisol and adrenaline action damages this lining, triggering inflammation and oxidative imbalance that contribute to atherosclerosis.9PubMed. Chronic stress and endothelial dysfunction: mechanisms, experimental challenges, and the way ahead The autonomic nervous system, which spent months in overdrive during the resistance stage, also plays a role by keeping blood pressure elevated and heart rhythm less flexible.10PubMed. Chronic stress impacts the cardiovascular system: animal models and clinical outcomes
In the brain, the hippocampus, a region critical for memory and emotional regulation, is especially vulnerable. Both human and animal studies show that chronic stress changes the physical structure of neurons in this area, suppresses the growth of new brain cells, and can reduce the overall volume of the hippocampus.11PubMed Central. Stress effects on the hippocampus: a critical review – Section: Abstract Research on allostatic load, a concept that updated Selye’s model, found that the hippocampus and prefrontal cortex tend to shrink under repeated stress, while the amygdala, which processes fear, actually grows.12Annals of the New York Academy of Sciences. Protection and damage from acute and chronic stress: allostasis and allostatic overload and relevance to the pathophysiology of psychiatric disorders That is a troubling combination: the regions responsible for calm judgment and memory formation get weaker, while the region that triggers anxiety and hypervigilance gets stronger.
The gut is another casualty. Chronic psychological stress disrupts what researchers call the gut-brain-microbiome axis, causing shifts in gut bacteria, damage to the intestinal barrier, and neuroinflammation that impairs neurotransmitter signaling.13PubMed Central. Emerging Roles of Postbiotics in Gut-Brain-Microbiome Axis Modulation This helps explain why digestive problems, from irritable bowel symptoms to appetite changes, so often accompany periods of sustained stress.
Why Some People Break Down Faster Than Others
One of the most important details missing from Selye’s original model is individual variation. Two people facing the same stressor will not necessarily move through the stages at the same speed, and some people seem to resist the exhaustion stage far longer than others. Part of this comes down to genetics: specific gene variations affect how a person perceives and physiologically responds to stress, influencing everything from baseline cortisol sensitivity to susceptibility to addiction and mood disorders.14PubMed Central. The role of genetics in stress effects on health and addiction – Section: Abstract
Psychology matters just as much. Research on how people mentally appraise a stressful situation before it happens found that anticipatory cognitive appraisal, the way you think about a stressor in advance, accounts for a remarkable amount of the cortisol response, explaining roughly a third of the variation in one study. General personality traits and how you reflected on the stress afterward mattered far less.15Psychoneuroendocrinology. Psychological determinants of the cortisol stress response: the role of anticipatory cognitive appraisal – Section: Summary In practical terms, two people walking into the same difficult meeting can have meaningfully different hormonal responses based solely on how they framed the situation in their minds beforehand.
Exercise offers another lens on individual differences. In mouse studies modeling chronic social stress, voluntary running on a wheel produced strong behavioral recovery in stress-susceptible animals, restoring cellular repair, energy availability, and synaptic plasticity in a key brain region. But a subset of mice that were both stressed and physically active showed a different molecular signature, one that suggested structural adaptation without true functional resilience.16Scientific Reports. Differential behavior responses and genetic alteration underpinning exercise effectiveness in stress-susceptible mice – Section: Results Exercise helps, but it does not help everyone in the same way or to the same degree, and the biology behind that variability is just beginning to be mapped.
Early-Life Stress Programs the System
The stress response is not a fixed system you are born with. Early-life adversity can permanently recalibrate how the hormonal axis works. Evidence indicates that stressful experiences in childhood or infancy can induce lasting changes in how well cortisol binds to its receptors, effectively altering the thermostat for the entire stress response. First episodes of depression triggered by adult stress may begin normally, but if the cycle continues unchecked, the brain becomes sensitized, and future episodes begin to occur independently of any outside trigger, with increasing frequency and intensity.17PubMed. Early-life stress and HPA axis trigger recurrent adulthood depression
A meta-analysis of how early-life adversity affects cortisol responses to social stress in later life found large effect sizes at the peak of the cortisol response and during recovery, and a moderate effect even at baseline.18Translational Psychiatry. Early-life adversity and cortisol response to social stress: a meta-analysis – Section: Results People who went through early adversity do not just react more strongly to stress; they also take longer to come back down. That sluggish recovery means more total cortisol exposure over a lifetime, pushing the body toward the exhaustion stage sooner.
Sleep, Stress, and the Vicious Cycle
Sleep disruption is both a consequence and a driver of the stress response. Poor sleep acts as a stressor in its own right, activating the same hormonal pathways and adding to the cumulative wear and tear on the body. Whether the cause is anxiety, shift work, long-distance travel, or simply a chaotic schedule, the result is the same: impaired brain function and accelerated biological aging.19PubMed Central. Sleep Deprivation and Circadian Disruption: Stress, Allostasis, and Allostatic Load – Section: Abstract This creates a feedback loop that is easy to fall into and hard to escape. Stress keeps you up at night, and the resulting sleep deprivation makes you more reactive to the next day’s stressors, which keeps you up the following night.
From an evolutionary perspective, this connection makes some sense. The alarm systems that keep you awake when a threat is nearby were useful when threats were physical and short-lived. One hypothesis argues that chronic insomnia represents an evolutionary mismatch: the ancient threat-detection system that once kept ancestors alive during brief dangerous periods is now chronically exposed to the psychosocial stressors of modern life, which never fully resolve.20PubMed. Insomnia as an evolved threat-response System: An evolutionary mismatch and kindling hypothesis
When Exhaustion Looks Like Depression
One of the most practically relevant questions about the exhaustion stage is where it ends and clinical depression begins. Sweden formally recognizes a diagnosis called exhaustion disorder, defined by identifiable stressors and characterized by cognitive difficulties, fatigue, and sleep problems. But when researchers directly compared symptom profiles between patients diagnosed with exhaustion disorder and those diagnosed with major depressive disorder, the overlap was striking. Both groups scored similarly on measures of perceived stress, depressed mood, and overall symptom severity.21PubMed Central. Exhaustion Disorder in Primary Care: A Comparison With Major Depressive Disorder and Adjustment Disorder – Section: Discussion Patients with major depression also scored high on exhaustion-specific measures, raising the question of whether exhaustion disorder might simply be a stress-triggered variety of depression rather than a fully separate condition.22PubMed Central. Comparison of exhaustion symptoms in patients with stress-related and other psychiatric and somatic diagnoses – Section: RESULTS
This matters for anyone trying to figure out what is wrong with them after a long period of stress. You might feel emotionally flat, struggle to concentrate, sleep poorly, and lose interest in things you used to enjoy. These could be signs of stress-related exhaustion, clinical depression, or both at once. The diagnostic label matters less than getting evaluated and treated, because regardless of category, the underlying biology, sustained cortisol exposure, inflammation, and brain changes, responds to many of the same interventions.
Measuring Stress Objectively
One frustration with the three-stage model is that it is easier to describe than to measure. You cannot take a blood test and be told, “You are in stage two.” But researchers are making progress on identifying biomarkers. In a longitudinal study of women with chronic stress-induced exhaustion, plasma levels of two growth factors, VEGF and EGF, were significantly higher compared to healthy women. Those levels dropped over two years of follow-up, suggesting they may track recovery from the exhaustion state.23PLOS ONE. Possible Biomarkers of Chronic Stress Induced Exhaustion – A Longitudinal Study – Section: Results These are early findings from a specific population and nowhere near ready for clinical use, but they point toward a future where stress-stage assessment could involve more than self-report questionnaires.
Tools That Target the Underlying Biology
Understanding the three stages suggests where interventions should focus. In the alarm stage, the goal is not to prevent the response entirely but to ensure it shuts off when the threat is gone. In the resistance stage, the aim is to reduce the total load so the body’s resources are not depleted. And in the exhaustion stage, the priority is reversing the inflammation and receptor resistance that have set in.
The vagus nerve, the long nerve connecting the brain to the heart, lungs, and gut, has become a focus of practical stress recovery tools. Techniques like controlled breathing, heart-rate variability biofeedback, and even transcutaneous vagus nerve stimulation work by boosting vagal tone, which shifts the nervous system away from the sympathetic “accelerator” and toward the parasympathetic “brake.” These approaches show promise for improving stress resilience, reducing fatigue, and supporting executive function under pressure.24PubMed Central. The vagus nerve: a cornerstone for mental health and performance optimization in recreation and elite sports Slow, paced breathing is probably the most accessible of these: it requires no equipment, costs nothing, and directly stimulates the vagal pathways that counteract the alarm response.
Sleep restoration is another underappreciated lever. Because sleep deprivation amplifies every stage of the stress response, improving sleep quality, whether through consistent scheduling, reduced light exposure at night, or treatment of an underlying sleep disorder, can meaningfully reduce cumulative stress load.19PubMed Central. Sleep Deprivation and Circadian Disruption: Stress, Allostasis, and Allostatic Load – Section: Abstract The three-stage model was built on lab rats exposed to cold and toxins, but for most people reading this article, the stressors are psychological and the recovery tools are behavioral. The biology, however, is the same. The alarm still fires, the resistance still costs you, and the exhaustion still comes if nothing changes.