The Stress Response System: How Your Brain and Body React to Stress

Published: 04/28/2026 | Updated: 04/28/2026

Authored by: Sean Jackson, BA in Psychology, MSc in Counseling

Edited by: Michael Fegan, B.A.

We all experience stress, yet most of us don’t completely understand what’s happening in our brains and bodies when it occurs. Becoming familiar with that process, including what triggers it, how it works, and what happens when it malfunctions, is one of the most empowering things you can do for your mental health. It can help you recognize when your stress response is working as intended and when it may need attention.

Your stress response system might be overloading from everyday pressure. It might also be that you have anxiety, chronic stress, or burnout. In each case, what’s happening in your brain and body follows a predictable pattern, and once you understand that pattern, you can begin to change it.

The Stress Response System, Brain, and Body

Stress is a normal response to environmental stimuli to help us survive threats. In fact, the stress response system is one reason our species has thrived and evolved over thousands of years.^[1]

The stress response system developed as a short-term mechanism, and in this regard, it works remarkably well. But problems arise when that system remains active longer than it should.^[1],^[2] For example, in our human ancestors, the stress response informed them of predatory danger. Today, it might be triggered because of a fight with your significant other or a major deadline at work. Both are significant, but not life-threatening. It is this mismatch between modern stressors and an ancient response system that is at the root of many stress-related difficulties.

To understand why your body reacts to stress the way it does, it helps to start where the stress response begins: your brain.

The Fight, Flight, Freeze Response Explained

Your brain has a built-in alarm system involving numerous structures and processes. When your body’s alarm system (fight or flight) is triggered, this is what happens:

The amygdala detects threats and instantly sends a distress signal to the hypothalamus. This process happens so quickly that your brain doesn’t even fully process what’s going on before the signal is sent.^[1],^[3]
The hypothalamus is the command center. It sends signals throughout your nervous system to prepare your body to react to the threat.^[1]
Hormonal changes occur. Epinephrine and norepinephrine flood the bloodstream, which increases the heart rate, quickens your breathing, tenses your muscles, and sharpens your senses.^[4],^[3]

In some cases, your brain might detect a continuing threat. When that happens, a second stage of the fight-or-flight response is initiated in the hypothalamic-pituitary-adrenal axis (HPA axis).^[1],^[3]

You can think of this stage as a hormonal relay race: this cortisol and stress response connection helps explain why prolonged stress feels so physically exhausting.

The hypothalamus releases a hormone that signals the pituitary gland.
The pituitary gland releases another hormone that travels through the bloodstream to the adrenal glands.
The adrenal glands release cortisol, which helps sustain your body’s ability to react to the ongoing threat.^[5] It does so by keeping your blood pressure up, mobilizing your body’s reserve energy, and maintaining a heightened state of alertness. Where the epinephrine surge in fight or flight is like putting the gas pedal to the floor, cortisol keeps the engine revving, ready to hit the gas and take off.5 This cortisol and stress response connection is part of why prolonged stress feels so physically exhausting.

Sometimes, the danger is so acute that your fight or flight response freezes. You might feel paralyzed, have trouble speaking, or feel disconnected from your body. This is not a failure of the system. Instead, it’s your body keeping fight or flight ready for action while your brain rapidly assesses the situation.^[6]

How Stress Affects the Brain and Body

Under stress, your body experiences many different physiological changes:^[1],^[3],^[7]

Your heart rate increases, and your blood pressure rises. Meanwhile, blood is redirected from nonessential organs to your muscles.
Your airway opens to allow more oxygen in, and breathing becomes much more rapid and shallow.
Your muscles tense up, ready to act.
Digestion slows, and you might feel an upset stomach or nausea.
The immune system is suppressed, allowing your body to redirect critical resources to its threat response.

Your brain also changes its priorities. For example, the prefrontal cortex, which is responsible for thinking and reasoning, becomes less active. At the same time, the structures responsible for survival and threat response (e.g., the amygdala) take over.^[8] This explains why, in times of extreme stress, you might find it more difficult to regulate your emotions, make decisions, or think clearly. These cognitive effects are usually temporary, but when stress becomes chronic, they can persist and interfere with your daily functioning.

Nervous System Stress Regulation

Another layer of the stress response involves the nervous system’s two divisions: the sympathetic and parasympathetic. In fight, flight, or freeze, the sympathetic division is the gas pedal, compelling your body to act. The parasympathetic division is the brake that tells your body to rest.^[1],^[7]

When things are working as they should, the parasympathetic division takes charge once the threat has passed. However, chronic stress keeps the sympathetic division activated; that’s the gas-pedal-to-the-floor analogy from earlier. The result is a low-level state of alert, even when there are no actual threats to your well-being.^[1] This constant state of activation is referred to as allostatic load, which is what creates wear and tear on your body, both physiologically and psychologically.^[2] This accumulated wear and tear can begin to affect everything from immune function to mood regulation.

Cortisol is deeply involved in all of this. Understanding what it does tells us a lot about the damage chronic stress does to your body and mind.

Cortisol and Stress Response

Despite its reputation as the stress hormone, cortisol serves many different functions in the body, including regulating blood sugar and managing inflammation.^[5]

Like the rest of your body’s hormones, cortisol has a daily rhythm. It increases in the morning to help you wake up and feel alert, then decreases over the course of the day to help usher you into a wind-down time before bed.^[1],^[7] But when stress is prolonged, and the natural rhythm is disrupted, it can cause various issues like difficulty waking up, afternoon energy crashes, or trouble falling asleep at night.

Strss Hormones and Anxiety

One such cortisol-related issue is a stress response that doesn’t turn off. The issue centers on the amygdala; it remains on high alert, and over time, your body develops resistance to cortisol’s signals.8 The brakes don’t work as effectively, so feelings of anxiety and hypervigilance become more difficult to manage and easier to trigger.^[5]

Cortisol isn’t the only player, though. When you’re anxious, your brain releases elevated levels of norepinephrine. As norepinephrine increases, you might experience racing thoughts and feelings of being on edge.^[3],^[5] Experiencing these anxiety symptoms is often what chronic stress feels like in its early stages; you’re wired and too reactive. On the other end of the spectrum, burnout typically feels like you’re too depleted to care. In both cases, it’s a sign that the stress response system isn’t working as it should. Recognizing where you fall on this spectrum can help guide the type of support you may need.

Burnout and Stress Response

As the stress response continues, burnout becomes increasingly likely. Researchers call this progression General Adaptation Syndrome: your body moves from an initial alarm reaction to a resistance stage in which it adapts and keeps going.^[3] But as your body remains in that reactive stage, it eventually gets exhausted.

In your day-to-day life, burnout might look like:^[3],^[9]

Persistent fatigue
Poor sleep
Difficulty concentrating
Reduced stress tolerance
Emotional flatness
Growing sense of detachment

Additionally, cortisol patterns shift. Instead of experiencing the typical morning rush of cortisol followed by a gradual decline throughout the day, you might have a flat or dysregulated pattern, which directly affects your energy, cognitive function, mood, and motivation.^[5]

Burnout isn’t just being tired or feeling off. It’s a physiological change in how your body functions. Those changes get worse as low-level chronic stress persists, which diminishes the stress response system’s ability to cope. When chronic stress occurs, it doesn’t just make you feel burned out; it can have far-reaching impacts on your overall mental health.^[3]

Chronic Stress and Mental Health

Not everyone responds to chronic stress in the same way. Unique factors like genetics, social support, and early life experiences all influence how well you handle it.

But what we all experience is a gradual degradation in how our brains function. More specifically, chronic stress can affect how you think, your mood, and your ability to be emotionally resilient. It can also make mental health disorders more likely to occur.^[8] This connection between chronic stress and mental health is one of the most well-documented findings in psychological research.

Stress-Related Mental Health Disorders

Chronic stress has been extensively studied, and researchers have found links between it and mental health disorders like anxiety, depression, and PTSD. This is no coincidence; inflammation in the brain and dysregulation of the HPA axis are associated with each of these conditions.^[2],^[5]

For example, when depressed, your cortisol patterns change from the typical rise and fall to a flat and irregular pattern. You might experience a morning peak that’s way too high or too low, followed by a flat level throughout the day with little or no variation.^[5]

As another example, chronic stress compels the immune system to release molecules (cytokines) that cause inflammation. Studies of people with depression show elevated levels of cytokines, which directly affect energy, mood, and motivation. It’s why depression can be so physically exhausting; it’s not just an emotional process, but a biological one, too.^[5],^[8]

Trauma and Stress Response

When stress crosses into trauma, the baseline stress response in your body can fundamentally change. Early life adversity is especially impactful; a child’s developing brain is extremely sensitive to stress hormones, and trauma can alter stress reactivity in a way that continues throughout one’s life.^[6]

But trauma in this context doesn’t necessarily mean the worst of childhood experiences. Consistent exposure to an unsafe environment, for example, can alter the stress response system in much the same way as physical abuse. Likewise, emotional neglect and low-grade adversity (e.g., diminished economic opportunities from living in poverty) can also cause lifelong changes in how your body’s stress response system functions.^[2]

A good example of this is the freeze response discussed earlier. The emotional numbing, flat mood, and dissociation felt when the perceived danger is highly acute can become stuck in the on position in people who have experienced trauma.^[6]

These effects are not inevitable, however. With appropriate support, the brain can develop new patterns even after early adversity.

Effects of Stress on Mood

Stress and emotional regulation go hand in hand. As stress persists, you might experience irritability, emotional sensitivity, and a reduced capacity to experience pleasure. Your sleep is also likely to be disturbed.^[2],^[10]

These symptoms have a compounding effect: stress causes you to sleep less or get a lower quality of sleep, which disrupts your brain’s ability to process emotional experiences and restore emotional equilibrium.^[10] This creates a reinforcing cycle. The good news is that change is possible. Your brain is incredibly resilient; it maintains its ability to recover and adapt, especially with the right interventions and support.

How to Calm the Stress Response: Stress Management, Therapy, and Proper Support

Recovery from chronic stress isn’t about reversing the damage it’s done. Rather, it’s about redirecting your brain and body toward healthier patterns.^[2]
By learning how to manage your stress response, you can build the capacity to return to equilibrium more effectively.^[2],^[11]

Stress Management and Mental Health

One of the most effective things you can do on a daily basis for stress management is moderate exercise. Aerobic activity increases your brain’s ability to form new neurons, reduces stress reactivity, lowers cortisol levels, and improves mood as well.^[9],^[1] Other techniques you can incorporate into your daily routine are also effective for stress management:

Get enough sleep. Seven or eight hours of sleep each night is recommended to ensure your brain and body can recover from the negative effects of stress.^[10]
Focus on a healthy diet. Prioritize healthy foods, minimize processed foods and sugars, and reduce alcohol and tobacco consumption.^[4],
^[1],^[6]
Take time for others. Social connection is critical for developing a buffer against chronic stress. Research shows that having supportive relationships reduces allostatic load and promotes a more regulated stress response system.^[9]

Recognizing your stress signals is important, too. For many people, stress includes physical, emotional, cognitive, and behavioral signals, such as:^[7],^[11]

Tension headaches
Tight muscles
Fatigue
Irritability
Difficulty concentrating
Persistent worry
Reduced enjoyment of pleasurable activities
Withdrawing from friends
Changes in eating habits
Feeling unable to slow down, even when exhausted

The difficulty is this: you might have lived so long with chronic stress, and your body might have adapted to it so thoroughly, that it can be difficult to even recognize it as stress. That’s where seeking professional help can make a profound difference.

How Therapy Reduces Stress

Therapy does so much more than help you process difficult emotions. Therapy also works at the neurological level, addressing the stress response at the source, and complementing the positive effects of self-care.6 Several therapies are especially useful for calming the stress response:

Cognitive-behavioral therapy (CBT) engages the prefrontal cortex in your brain and interrupts the automatic stress reactions it’s developed. Over time, new, more adaptive responses are generated to support a healthier stress response.^[5],^[6]
Mindfulness-based stress reduction (MBSR) reduces cortisol levels, decreases the intensity of the stress response, and improves emotional regulation by activating the prefrontal cortex to help regulate the amygdala’s stress reactivity.^[5],^[6]
Trauma-informed therapies like Eye movement desensitization and reprocessing (EMDR) are effective for addressing stress rooted in trauma. They work by helping your nervous system process stress and complete the defensive responses to stress.^[6]

Beyond the above, the relationship you develop with your therapist has a stress-regulating effect. A quality therapist will make you feel safe and heard, which helps reduce your brain’s allostatic load, much as having supportive connections with friends and family does. The result is a calmer autonomic nervous system that’s less reactive to stress-inducing triggers.^[6]

Mission Connection: Helping Your Brain Heal With Proper Support

Chronic stress isn’t a life sentence. With the proper support, your brain can regain its lost flexibility and grow new connections that facilitate recovery. This process looks different for everyone. For some, lifestyle changes might be all it takes. For others, extensive therapy might be required. Typically, a combination of interventions garners the best results.

You don’t have to wait until you’re deep in crisis to ask for help. If you’re feeling the effects of chronic stress, anxiety, or burnout, professional support can make all the difference, not in how you feel on a day-to-day basis, but in how your body and brain function, too.

Mission Connection offers compassionate, evidence-based care for stress and its impact on your mental health. Reach out to us today to learn more about how we can support you.