The Biology of Stress

Stress is the body’s instinctive response to pressure, demand, or perceived danger. It sets off a cascade of changes, including rising hormone levels, heightened alertness, and shifts in metabolism and blood flow. These reactions are designed to help the body face challenges and recover.

This system originally evolved to protect against physical threats like predators or injury. Today, it often responds to emotional and psychological pressures such as deadlines, financial worries, and caregiving demands among them. The body doesn’t differentiate between physical and emotional stressors. It reacts the same way, as if survival is on the line.

To truly understand what stress does to the body, it is important to look beyond isolated moments of tension, as the more serious effects emerge over time. When stress becomes constant, persisting for weeks or months, it can start to leave a lasting imprint. One of the earliest scientific models, the General Adaptation Syndrome, of stress helps explain this process by mapping how the body shifts from short-term responses to long-term strain.

General Adaptation Syndrome Origins

In the 1930s, Canadian endocrinologist Hans Selye introduced the General Adaptation Syndrome (GAS), a three-stage model describing the body’s reaction to prolonged stress ^[1].

Selye’s work helped show that stress isn’t just a feeling, but rather a biological process that unfolds in stages. When Selye introduced the General Adaptation Syndrome, he offered a way to understand what happens in the body when we’re under pressure. Instead of thinking of stress as a single event or reaction, the GAS model helps us see it as a pattern the body follows when demands keep coming.

By recognizing these stages, we can better understand why we might feel wired and alert at first, but then slowly burn out over time. It gives language to a cycle many people experience but don’t always know how to explain, and that understanding makes it easier to respond with care, not judgment.

The Three Phases of Stress

According to the General Adaptation Syndrome model, the body experiences stress in three distinct stages. Each phase involves different responses and has unique impacts on the body and mind.

Stage 1: Alarm 

The alarm stage begins the moment the brain detects a stressor. The hypothalamus signals the adrenal glands to release stress hormones, primarily adrenaline and cortisol ^[2]. These hormones prepare the body for immediate action by increasing heart rate, redirecting blood to muscles, and sharpening focus. At the same time, cortisol helps regulate energy and manage inflammation to support the short-term response.

This burst of sensationis known as the fight-or-flight response. It’s effective in moments of true danger but puts strain on the body when triggered repeatedly.

Stage 2: Adaptation 

If the source of stress doesn’t go away, the body moves into the adaptation stage. It continues functioning under pressure, but this comes at a cost. Adrenaline levels drop, while cortisol levels remain elevated, helping to maintain energy and alertness.

On the surface, daily routines may carry on as usual. A person might feel alert but notice an increase in fatigue or irritability. Internally, however, the body is working overtime to stay balanced. Immune responses weaken, recovery slows, and energy reserves begin to thin. During this stage, the body may also show signs such as:

• Weakened immune defenses.
• Slower recovery from illness or exertion.
• Persistent fatigue or low energy.
• Difficulty concentrating or staying focused.
• Heightened emotional reactivity or irritability.
• Sleep disturbances or restless nights.
• Changes in appetite, digestion, or overall metabolism.

This stage can last for a prolonged period, depending on how persistent the stressor is and whether there are chances for rest and reset. Over time, the effort to maintain balance becomes more challenging to sustain.

Stage 3: Exhaustion

When stress continues without relief, the body enters the exhaustion stage. This is when its ability to cope becomes depleted. People might find themselves constantly tired, struggling to concentrate, or feeling emotionally low. The immune system becomes more vulnerable, making it easier to get sick and more challenging to recover ^[3].

This stage is not just about feeling tired or emotionally drained. It’s a full body message that the systems designed to help us adapt are now under strain. Without a chance to rest and recover, stress begins to affect not only mood and energy, but also immune function, digestion, and long-term mental clarity.

Recognizing when this stage is approaching creates a critical window for care. That might mean stepping away from nonstop demands, getting more sleep, reaching out for help, or simply acknowledging that the body is signaling a need for relief. These moments of awareness are not signs of failure. They are the first steps toward protecting health and well-being before deeper strain sets in.

Coping doesn’t always require a dramatic reset. What matters most is taking action, even in small ways. Prioritizing rest, cutting back on nonessential tasks, spending time with people who bring comfort, or creating small moments of calm can all make a meaningful difference. These choices help the body recalibrate and can prevent short-term stress from becoming long-term harm.

Taking a Stress-Informed Approach

In the short term, stress can help the body respond quickly and effectively. It sharpens focus, increases energy, and improves reaction time. But when stress becomes constant, it begins to affect cognitive function, immune response, and emotional balance.

People may find it harder to concentrate or remember things. They might sleep less soundly, experience stomach issues, or feel run down more often. Emotionally, chronic stress can make people more irritable, anxious, or overwhelmed.

Recognizing these signs early allows for small, meaningful changes that can reduce the long-term toll and help restore a sense of balance. People may benefit from:

• Taking short, regular breaks during high-stress periods.
• Talking with a therapist, counselor, or trusted friend.
• Creating routines that support consistent sleep and balanced nutrition.
• Setting clear limits on work and personal obligations.
• Engaging in calming activities like stretching, walking, or journaling.
• Making time for connection with others, even in small ways.
  

As Dr. Daniela Kaufer and her colleagues have shown, chronic stress can change how brain stem cells develop. It prompts an increase in myelin-producing cells and a decrease in the formation of new neurons. Over time, these shifts may impact memory, emotional regulation, and the brain’s ability to adapt and learn ^[4].

The Short and Long Term Effects of Stress on the Body

Learning how stress affects the body gives us more than information. It offers a way to notice what’s happening in our daily lives and respond before things escalate. When sleep becomes restless, irritability creeps in, or energy runs low for days on end, these are not random. They’re signals that the body is working overtime and may need care and support to recover.

A stress-informed approach means paying attention to those early signs and choosing moments to rest, reset, or ask for help. It also means adjusting expectations during high-pressure times rather than pushing through at full speed. When people make small adjustments early, they’re more likely to avoid more profound exhaustion later.

Stress is a natural part of life, and in many situations, it can even be helpful. But when it becomes constant, it places more demand on the body than it can safely manage. Taking care of ourselves during stressful times means noticing the early signs and responding with intention. Health, both mental and physical, depends not on avoiding stress altogether but on how we meet it when it shows up.

Caring for yourself during stressful times doesn’t have to mean significant changes. Sometimes it’s as simple as saying no when you need rest, stepping outside to catch your breath, or sharing what you’re going through with someone you trust. These actions aren’t indulgences. They are essential steps toward protecting your health, restoring balance, and staying connected to what matters most over time.

From Awareness to Action

Stress affects more than just our day-to-day mood. It touches every part of our health, from how we think to how well we sleep and heal. And while short-term stress can be helpful to us, the long-term toll it takes on our bodies and minds often goes unnoticed until we feel burned out, disconnected, or overwhelmed.

The General Adaptation Syndrome offers more than a biological explanation. It gives us a way to understand what we feel and why. When we can name the alarm, the strain of resistance, or the profound fatigue of exhaustion, we begin to see our reactions not as flaws, but as natural signals from a body that wants to recover.

This kind of awareness creates room for compassion. It reminds us that rest is not a luxury; it is necessary. Reaching out for support shows strength, not weakness. Even small steps, when taken with care, can help restore balance and build resilience.Stress may always be a part of life, but how we respond to it can change. With understanding, self-care, and steady support, it becomes possible to protect our energy and strengthen our capacity to cope. Professional guidance can also be a vital part of this process, offering tools and perspective that make long-term recovery more sustainable.