What Is Stress? Definition, Causes, and What Psychology and Neuroscience Tell Us

Stress is one of the most commonly used words in modern life — and one of the most poorly understood. People are told to "manage" it, "reduce" it, "embrace" it, and "push through" it, often in the same week.

Before any of that can be useful, there's a prior question worth answering: what is stress, actually? Decades of psychology and neuroscience research provide specific answers.

Stress isn't simply a feeling — it's a measurable physiological and psychological response, one that evolved for good reasons, that causes real physical changes in the body, and that becomes genuinely harmful when sustained beyond what it was designed to handle.

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The Three Major Theories of Stress

Hans Selye and the General Adaptation Syndrome

The first systematic scientific study of stress was carried out by Hans Selye, a Hungarian-born endocrinologist, in the 1930s and 1940s.^[1] He described a predictable pattern of physiological responses appearing regardless of the specific stressor, naming it the General Adaptation Syndrome, unfolding in three stages.

Stage 1 (Alarm) — the body recognizes a stressor and activates fight-or-flight, releasing adrenaline and cortisol, increasing heart rate and blood pressure, and suppressing non-essential functions.

Stage 2 (Resistance) — if the stressor continues but is manageable, the body adapts, operating at elevated alertness while appearing to "cope." Stage 3 (Exhaustion) — if the stressor continues beyond capacity, resources deplete: immune function drops, sleep deteriorates, and illness risk increases.

Chronically sustained, this stage is what we recognize as burnout or collapse.

Lazarus and Folkman: the cognitive appraisal model

Three decades later, psychologists Richard Lazarus and Susan Folkman proposed a complementary framework in their 1984 book Stress, Appraisal, and Coping: stress isn't simply a response to external events but the product of a cognitive transaction between the individual and their environment.^[2]

The core insight: the same event doesn't produce the same stress in different people.

What determines whether you experience stress isn't what happens but how you evaluate it — through primary appraisal (is this relevant, and harmful, threatening, or a challenge?) and secondary appraisal (do I have the resources to manage it?).

When perceived demands exceed perceived resources, stress results. This is the theoretical foundation of cognitive behavioral approaches to stress management.

The NIOSH model: occupational stress

A third framework, specifically relevant to workplace stress measurement, identifies three domains: Work Stressors (conditions like workload, role ambiguity, and lack of control), Stress Reactions (emotional, cognitive, and physical responses), and Social Support (the degree to which others buffer the impact).

Research established that social support functions as a genuine buffer, reducing the health impact of stressors when present.^[5]

Types of Stress

Acute stress is brief, intense, and tied to a specific event — the deadline, the argument — activating fully then deactivating when it resolves. This is the form the physiological system is designed for.

Chronic stress is sustained and often without clear resolution — financial pressure, a difficult relationship, a demanding job with no end in sight. This is where the health costs accumulate, because systems designed for occasional, brief activation aren't built to run continuously.

Selye also distinguished eustress (the stress of positive challenge, associated with increased motivation and performance) from distress (the stress of overwhelm, associated with anxiety and, when chronic, health deterioration).

Not all stress needs to be eliminated — what matters is its nature, duration, and whether you have sufficient resources to meet its demands.

Stressors can also be categorized by source: physical (noise, illness, disrupted sleep), psychological (uncertainty, fear of judgment), social (conflict, isolation, rejection), and occupational (high workload, low control, job insecurity) — and these often interact.

What Stress Does to the Body and Brain

When the brain perceives a threat via the amygdala, it activates the hypothalamic-pituitary-adrenal (HPA) axis: the hypothalamus releases CRH, the pituitary releases ACTH, and the adrenal glands release cortisol, while the sympathetic nervous system releases adrenaline.

Together, these mobilize energy, sharpen attention, and prepare the body for action — well-designed for short-term threats, but the problem is what happens when it runs continuously.

Sustained high cortisol has documented effects across systems. It suppresses immune function, making chronically stressed people more susceptible to infection and slower to heal.

It raises heart rate and blood pressure, increasing cardiovascular disease risk — research suggests up to 23% of heart disease deaths could be prevented by reducing high-strain occupational stress to average levels. It damages the hippocampus, impairing memory and concentration.

It disrupts sleep, which in turn amplifies stress reactivity, creating a self-reinforcing cycle. And it disrupts glucose metabolism, promoting fat storage and raising Type 2 diabetes risk.

Neuroimaging documents specific brain changes: the amygdala becomes hyperactive, more reactive to potential threats; the prefrontal cortex, responsible for rational decision-making and emotional regulation, shows reduced activation; and the hippocampus shows structural reduction.

This explains the subjective experience of chronic stress — feeling reactive, unable to think clearly, and emotionally fragile even in ordinarily manageable situations.

What Causes Stress

Work-related stressors include high workload and time pressure, low job control, role ambiguity or conflict, poor relationships with supervisors or colleagues, job insecurity, and lack of recognition.

Karasek's Job Demand-Control Model identified the combination of high demands and low control as the most stressogenic work situation, producing the highest rates of stress-related illness.^[4]

Life event stressors also matter significantly.

Holmes and Rahe's Social Readjustment Rating Scale identified 43 life events with quantified stress loads, from bereavement to minor legal violations, and found that accumulating enough life change in a year was associated with an 80% probability of serious illness within the following year.^[3]

Everyday stressors matter too — research on minor daily hassles found their cumulative effect may predict health outcomes as well as or better than major life events, because they're more frequent and less likely to produce the acute stress response that enables resolution.

Who Is Most Affected?

Individual factors matter: Neuroticism (emotional instability) predicts greater stress reactivity, and low optimism, poor self-efficacy, and insecure attachment are associated with higher reactivity and poorer coping.

Social factors matter too — isolation significantly amplifies stress effects, while strong support networks show lower reactivity and better recovery.

Occupational factors play a role: high-demand, low-control, low-support occupations show the highest rates of work-related stress and burnout.

And life circumstances — financial insecurity, housing instability, caregiving responsibilities, discrimination — create chronic background stressors that reduce the resources available for handling additional demands.

The Warning Signs of Problematic Stress

Stress becomes a health concern when it's chronic, severe, or producing functional impairment. Physical signs include persistent fatigue, frequent illness, headaches, digestive problems, muscle tension, and sleep disturbance.

Emotional signs include persistent anxiety or irritability, low mood, and feeling overwhelmed. Cognitive signs include difficulty concentrating, memory problems, and intrusive worrying. Behavioral signs include social withdrawal, reduced productivity, and increased substance use.

None of these alone indicates problematic stress — they exist on continua and can have other causes. But patterns of these symptoms, particularly linked to ongoing stressors, deserve attention.

What Actually Helps

Cognitive reframing — changing how you appraise stressors, consistent with Lazarus and Folkman's model — has the strongest evidence base, with CBT techniques showing significant, durable effects.

Mindfulness-based interventions show consistent, moderate effects on perceived stress,^[8] and physical activity is one of the most robust tools, with direct effects on cortisol regulation, sleep, and mood.

Social support provides both direct wellbeing benefits and the buffering effect documented in research, and when stressors are amenable to change, addressing them directly tends to produce more durable relief than managing the emotional response alone.

For a comprehensive guide, see: How to Manage Stress

Understanding the theory is one thing. Seeing your own stress profile — across work stressors, body reactions, and social support — is where this becomes personally useful.

Frequently Asked Questions

What is the difference between stress and anxiety?

Stress is typically a response to an external stressor — something specific demanding more than you feel equipped to provide. Anxiety is a more internal experience — apprehension that may not be linked to a specific current threat.

They often co-occur, but stress typically resolves when the stressor resolves, while anxiety may persist even when the stressor is gone.

Is stress always bad?

No. Acute, moderate stress — what Selye called eustress — is associated with improved performance and engagement. The Yerkes-Dodson law describes an inverted-U relationship: too little stimulation produces poor performance, too much produces anxiety, and moderate arousal optimizes performance.

What's harmful is chronic, unrelenting stress without adequate recovery.

Can stress be measured?

Yes, through multiple methods. Self-report measures capture the subjective experience. Physiological measures — cortisol in saliva or blood, heart rate variability, inflammatory markers — capture the biological response. Different methods capture different aspects of the experience.

What is burnout and how does it relate to stress?

Burnout is a syndrome resulting from chronic, unresolved work stress, defined by Christina Maslach as comprising emotional exhaustion, depersonalization, and reduced personal accomplishment.^[6] It's the end stage of prolonged exposure to demands exceeding available resources, with documented associations with serious physical health consequences.^[7]

How do I know if my stress level is a concern?

Stress becomes a health concern when it's persistent rather than episodic, interfering with daily functioning, accompanied by physical symptoms that don't resolve with rest, or associated with harmful coping strategies. The CommStyle Stress Check provides a structured starting point.

What is the fight-or-flight response?

The acute physiological activation triggered by perceived threat — the first stage of Selye's General Adaptation Syndrome. It involves rapid release of adrenaline and cortisol, increased heart rate, and suppression of non-essential functions.

Designed for physical danger, it's activated by psychological threats too — which is why a difficult conversation can produce the same response as a physical threat.

Understanding the theory is one thing. Seeing your own stress profile — across work stressors, body reactions, and social support — is where this becomes personally useful.