General adaptation syndrome (GAS) is a three-stage model describing how your body responds to stress, moving from an initial shock response through adaptation and, if the stress persists long enough, into physical breakdown. Developed by Hans Selye in the 1950s, the model defines stress as “the non-specific response given by the body to any request made to it,” meaning the same biological pattern kicks in whether you’re fighting an infection, dealing with a difficult boss, or recovering from surgery. The three stages, alarm, resistance, and exhaustion, each have distinct physiological features that affect nearly every system in your body.
The Alarm Reaction Stage
The first stage is the one most people recognize instinctively. When your body detects a threat, it triggers the fight-or-flight response: a rapid, whole-body mobilization designed to help you survive immediate danger. Your adrenal glands release adrenaline and cortisol, flooding your system with energy and sharpening your focus. The physical changes are fast and dramatic:
- Heart rate and blood pressure increase
- Blood sugar levels rise to fuel muscles
- Breathing rate speeds up
- Muscles tense in preparation for action
- Pupils dilate to take in more visual information
- Sweating increases
At this point, your body is essentially borrowing resources from long-term maintenance (digestion, immune function, growth) and redirecting them toward short-term survival. That trade-off works well for brief threats. If the stressor disappears, your body calms down, hormone levels return to baseline, and normal functions resume. But if the stress doesn’t stop, you move into the next stage.
The Resistance Stage
During the resistance stage, your body tries to adapt to the ongoing stress rather than simply reacting to it. The initial surge of alarm symptoms fades. Your heart rate may come back closer to normal, and the acute adrenaline rush subsides. But your system isn’t truly at rest. It stays activated at a higher metabolic level, producing elevated amounts of cortisol and other hormones to keep you functioning under pressure.
Cortisol plays a dual role here. It acts as an anti-inflammatory agent, reducing the effects of physical injuries and helping your immune system continue fighting off infections. During this phase, your body can actually maintain relatively high efficiency: healing wounds, resisting illness, and staying mentally alert despite the persistent stressor. This is why people can function through weeks or months of difficult circumstances without immediately falling apart.
The catch is that this adaptation isn’t free. Your body is burning through its reserves to maintain this heightened state. Think of it like running a car engine at high RPMs. It works, but it wears out faster. If you resolve the source of stress during this stage, your body can recover fully. If you don’t, the cost eventually catches up.
The Exhaustion Stage
When stress persists long enough that your body’s adaptive reserves are depleted, you enter the exhaustion stage. This is where chronic stress translates into real health damage. The prolonged activation of your stress response system and excessive exposure to cortisol disrupt nearly every bodily process.
Cortisol, which was protective in the resistance stage, becomes harmful in excess. It suppresses your immune system, leaving you more vulnerable to infections and slower to heal. It interferes with digestion, reproduction, and growth. The wear and tear accumulates across multiple systems simultaneously, increasing your risk for a wide range of problems: heart disease, high blood pressure, stroke, diabetes, digestive disorders, chronic muscle pain, headaches, weight gain, sleep disruption, depression, anxiety, and difficulty with memory and concentration.
Selye’s original model described this stage bluntly: the body’s resistance breaks down with the loss of its adaptive response, a development that in extreme cases can lead to death. While most people don’t reach that extreme, the exhaustion stage explains why chronic, unrelieved stress is so strongly linked to serious illness. Your body simply runs out of the capacity to keep compensating.
How the Stages Connect
The key feature of GAS is that these stages aren’t random. They follow a predictable sequence driven by the same underlying biology. The alarm stage mobilizes resources. The resistance stage spends those resources to maintain function. The exhaustion stage is what happens when the account hits zero. You can’t skip stages or jump from alarm to exhaustion overnight. The transition depends on how long the stressor lasts and how intense it is.
One important nuance: the model applies to any type of stressor, not just psychological stress. A severe infection, a major surgery, extreme temperatures, or prolonged physical exertion all trigger the same three-stage pattern. That’s what Selye meant by “non-specific.” Your body doesn’t have separate stress programs for different threats. It runs the same biological sequence regardless of the cause.
Allostatic Load: The Modern Update
Since Selye’s original work, researchers have refined the model with a concept called allostatic load. This refers to the cumulative biological cost of adapting to stress over time. It captures something Selye’s model implied but didn’t fully spell out: the changes that happen during the resistance and exhaustion stages aren’t limited to cortisol. They include shifts in immune function, cardiovascular regulation, energy metabolism, and behavior patterns.
Modern research also clarifies that stress mediators like cortisol aren’t purely harmful. They have both protective and damaging effects depending on how long they’re elevated. In the short term, cortisol helps you adapt. Over the long term, it erodes the systems it was meant to protect. When these regulatory systems get pushed far enough from their normal operating range, they establish a new, less healthy baseline, a state researchers call an “allostatic state.” This is essentially the biological mechanism behind what Selye observed as the exhaustion stage: your body resets to a lower level of function because it can no longer sustain the effort of constant adaptation.
Understanding GAS gives you a practical framework for recognizing where you are in the stress cycle. The alarm symptoms are hard to miss. The resistance stage is trickier because you feel like you’re coping, even as your reserves quietly drain. By the time exhaustion sets in, the health consequences are already building. The earlier you can reduce or remove a chronic stressor, the less biological debt your body accumulates.