During the alarm stage of stress, your body launches an immediate, full-system response designed to help you survive a threat. This is the first phase of what scientist Hans Selye called the General Adaptation Syndrome, a three-stage model he introduced in 1936 to describe how the body reacts to any significant demand. Within seconds of detecting danger, your nervous system triggers a cascade of hormonal and physical changes that redirect your body’s resources toward one goal: fighting the threat or escaping it.
How the Alarm Response Gets Triggered
The alarm stage begins in the brain. When you perceive something as dangerous, the threat-detection part of your brain signals the hypothalamus, which acts as a command center. The hypothalamus activates the sympathetic branch of your autonomic nervous system, the part that controls involuntary functions like heart rate and digestion. This activation happens almost instantly, well before you consciously decide how to respond.
Once the sympathetic nervous system fires, it sends signals to the adrenal glands, which sit on top of your kidneys. These glands release two key hormones into your bloodstream: epinephrine (adrenaline) and norepinephrine (noradrenaline). At the same time, the adrenal glands release cortisol, a slower-acting hormone that helps sustain the body’s energy supply for the duration of the stressor. Together, these three hormones produce the physical changes you actually feel during acute stress.
What Happens in Your Body
The physical changes during the alarm stage are dramatic and coordinated. Your heart rate increases, your heart contracts more forcefully, and your blood pressure rises. Blood vessels constrict in areas that aren’t immediately useful, like your skin and digestive organs, while blood flow increases to your large muscles, heart, and brain. Your airways widen so you can take in more oxygen with each breath. Your pupils dilate to let in more light, sharpening your vision.
These changes are not subtle. You may notice your heart pounding, your breathing becoming rapid and shallow, your muscles tensing, or your hands feeling cold (from blood being redirected away from your extremities). Your mental activity ramps up as well, making you feel hyper-alert and focused. Your blood’s ability to clot also increases, a built-in safeguard against potential injury.
How Your Body Fuels the Response
Mounting a fight-or-flight response takes significant energy, and the alarm stage has a built-in fueling system. Adrenaline triggers the release of stored glucose from your liver through a process called glycogenolysis, essentially breaking down your energy reserves and dumping sugar into your bloodstream. Fatty acids are also released from fat cells to serve as a secondary fuel source. Cortisol further supports this by increasing the rate at which your body produces new glucose and by slowing down processes that would compete for that energy.
The result is a sharp spike in blood sugar and available energy, directed specifically toward the muscles and brain that need it most. Your overall cellular metabolism increases across the body, and muscle strength temporarily improves. This is why people sometimes report feats of unusual strength or speed during moments of acute danger.
What Gets Shut Down
To free up resources for survival, your body temporarily depresses systems it doesn’t need in the moment. Intestinal motility slows significantly, which is why stress often causes nausea or a “knot” in your stomach. Blood flow to organs not involved in immediate motor activity drops. Processes like tissue repair and immune surveillance take a back seat. Selye originally observed that sustained alarm-level stress caused measurable shrinkage of immune tissue in lab animals, along with enlargement of the adrenal glands (from overuse) and even stomach ulceration.
These trade-offs are fine for short bursts. The alarm stage is designed to last minutes, not hours. Your body is temporarily operating outside its normal balance, burning through resources at an unsustainable rate.
What Comes After the Alarm Stage
If the stressor disappears quickly, your parasympathetic nervous system takes over and brings everything back to baseline. Heart rate slows, blood pressure drops, digestion resumes, and hormone levels normalize. You might feel drained or shaky afterward as your body recalibrates.
If the stressor persists, your body moves into the second phase of the General Adaptation Syndrome: the resistance stage. During resistance, your body attempts to adapt to the ongoing stress, maintaining elevated cortisol levels while trying to function as normally as possible. Hormone levels are still above baseline but not at the acute spike of the alarm phase. If the stress continues long enough without relief, the body eventually enters the exhaustion stage, where its adaptive resources become depleted and physical health begins to break down.
The alarm stage itself is not harmful. It’s a precisely engineered survival mechanism. The problems arise when the response fires too frequently, lasts too long, or gets triggered by stressors that don’t require a physical response, like work deadlines, financial worry, or social conflict. In those cases, the same cardiovascular changes, immune suppression, and metabolic shifts that protect you from a physical threat start working against your long-term health.