General adaptation syndrome (GAS) is a three-stage model describing how your body responds to prolonged stress. First proposed in 1936 by endocrinologist Hans Selye, it explains why short-term stress can be protective while long-term stress breaks the body down. The three stages are the alarm reaction, the stage of resistance, and the stage of exhaustion.
How Selye Discovered the Model
Selye stumbled onto GAS by accident. He was injecting ovarian extracts into laboratory rats, hoping to discover a new hormone. Instead, he noticed a consistent pattern of physical damage: the outer tissue of the rats’ adrenal glands grew larger, their thymus glands (which help fight infection) deteriorated, and they developed stomach ulcers. Eventually, the rats died. When Selye tested other stressors, the same trio of changes appeared every time, regardless of the specific threat. He concluded that the body follows a predictable, universal sequence when confronting any sustained stressor, whether it’s an infection, an injury, or an intense emotional experience.
Stage 1: The Alarm Reaction
The alarm reaction is your body’s immediate “fight or flight” response. When your brain detects a threat, a region called the hypothalamus sends signals through the nervous system to your adrenal glands. Those glands pump adrenaline into your bloodstream, triggering a rapid cascade of changes: your heart beats faster, your blood pressure rises, and your breathing quickens. Small airways in the lungs open wide so you can take in more oxygen with each breath. That extra oxygen floods the brain, sharpening alertness and making sight, hearing, and other senses more acute.
At the same time, your body releases stored blood sugar and fats to fuel your muscles. Your muscles tense. You may notice beads of sweat. All of this happens within seconds, preparing you to either confront the threat or escape it. In Selye’s original experiments, this stage also caused the adrenal glands to enlarge, the thymus gland and lymphatic system to shrink, and stomach ulcers to appear. These changes reflect how aggressively the body redirects its resources toward immediate survival, temporarily pulling energy away from digestion and immune defense.
Stage 2: The Resistance Stage
If the stressor doesn’t go away, your body shifts into a longer-term coping mode. The initial adrenaline surge fades, but the stress response stays active at a lower, steadier level. The main player now is cortisol, a hormone that keeps blood sugar elevated, suppresses inflammation, and enhances the tone of your heart and blood vessels. In the short term, these adjustments are genuinely helpful. They keep you functioning at a higher-than-normal level so you can manage the ongoing challenge.
During resistance, you may feel like you’re handling things fine. Your body has adapted, and the obvious signs of alarm (pounding heart, sweating, muscle tension) have eased. But that adaptation comes at a hidden cost. Your stress hormones remain elevated, your immune system stays partially suppressed, and your body is burning through its reserves. Think of it like running a car engine at high RPMs: it works, but it’s not sustainable. The endocrine, nervous, and immune systems are all working overtime to maintain a sense of balance, and the longer this goes on, the greater the toll.
Stage 3: The Exhaustion Stage
Exhaustion sets in when the body’s adaptive reserves run out. Cortisol and other stress hormones have been circulating at high levels for so long that they start causing damage instead of providing protection. The body’s ability to cope diminishes, and in Selye’s original framework, this stage could lead to death.
In practical human terms, chronic stress that reaches this point raises your risk for a wide range of health problems. The Mayo Clinic lists these consequences of prolonged stress hormone exposure:
- Cardiovascular: heart disease, heart attack, high blood pressure, and stroke
- Mental health: anxiety, depression, and problems with memory and focus
- Digestive: stomach problems and changes in appetite
- Physical: headaches, muscle tension and pain, weight gain
- Sleep: insomnia and disrupted sleep patterns
The immune system takes a particularly hard hit. Prolonged cortisol secretion suppresses the body’s ability to fight infection, weakens muscle tissue, and raises blood pressure. During the exhaustion phase, the damage to the body’s stress response system can become difficult to reverse, because the physiological reserves needed for recovery have been depleted.
How Modern Science Has Updated the Model
Selye’s three-stage framework was groundbreaking, but researchers have refined it significantly since 1936. One key update is the concept of “allostatic load,” which describes the cumulative wear and tear on the body from repeated or chronic stress. Where Selye’s model treated the stress response as a simple on-off sequence moving through three predictable phases, allostatic load captures something messier and more realistic: stress mediators like cortisol can be both protective and damaging depending on context, timing, and duration.
The modern view also recognizes that the stress response involves far more than just the adrenal glands. It includes changes in immune function, cardiovascular metabolism, energy regulation, and even behavior. The biological changes Selye observed during the resistance and exhaustion stages are now understood as part of this broader network. Your body isn’t simply “adapting” or “failing.” It’s constantly recalibrating across multiple systems, and breakdown happens when that recalibration is demanded too often or for too long.
What GAS Means for Everyday Stress
The practical takeaway from GAS is that stress isn’t just a feeling. It’s a measurable, physical process that changes your hormones, your immune function, and your organ systems. A burst of acute stress (a near-miss in traffic, a tough presentation at work) triggers the alarm stage and then resolves. Your body returns to baseline, and no lasting harm is done. This kind of short-term stress response is normal and even useful.
The danger comes when stress persists for weeks, months, or years without adequate recovery. Ongoing financial pressure, a difficult relationship, chronic work demands, or untreated pain can keep your body locked in the resistance stage. You may not feel “stressed” in the dramatic, heart-pounding sense, because your body has adapted. But the adaptation itself is the problem: elevated cortisol, suppressed immunity, and disrupted sleep quietly accumulate until they manifest as one of the health conditions associated with the exhaustion stage. The GAS model’s core insight, now supported by decades of additional research, is that there’s a biological limit to how long your body can sustain that elevated coping state before something gives.