Stress, cortisol & immunity: Why being 'run down' makes you sick

Discover how chronic stress affects your immune system. Learn how cortisol, sleep, nutrition and vitamin C support resilience when you're feeling run down.

The link between psychological stress and immune function is one of the most documented relationships in physiology. The Australian Bureau of Statistics 2022 National Health Survey found that 14% of Australian adults experienced high or very high levels of psychological distress, up from 11% in 2011–12. Research over the past two decades has clarified how prolonged stress interacts with the body's natural defence systems, helping explain a connection many people intuitively recognise.

This article looks at what happens in the body during stress, how cortisol shapes the immune response, what the science says about chronic stress and illness, and the everyday habits that may support resilience during demanding periods.

What happens in the body during stress

Stress is a physiological response to anything the body perceives as a challenge. When a stressor arises, whether physical, emotional or environmental, two interconnected systems activate together.

The first is the sympathetic nervous system, which releases adrenaline within seconds. This produces familiar sensations: faster heartbeat, sharper focus, redirected blood flow. The second is the hypothalamic-pituitary-adrenal (HPA) axis, a slower-acting hormonal cascade that ends with cortisol release from the adrenal glands.

A 2025 review published in the International Journal of Molecular Sciences on chronic stress and HPA axis function describes the HPA axis, primarily via cortisol secretion as the major neuroendocrine mediator of stress responses, influencing both immune regulation and systemic homeostasis. The system is designed for short-term activation. Once a stressor passes, cortisol falls and the body returns to baseline through negative feedback loops.

Understanding cortisol and the stress response

Cortisol is a glucocorticoid hormone produced in the adrenal cortex. It serves multiple functions beyond stress, regulating metabolism, blood sugar, blood pressure and inflammation throughout the day. Cortisol follows a natural circadian rhythm, peaking in the early morning to help with wakefulness and declining steadily towards evening.

Acute versus chronic cortisol exposure

Short bursts of cortisol can briefly enhance immune readiness while chronic stress leads to dysregulation and immune suppression. Higher cortisol levels can also be linked to lower numbers of certain infection-fighting white blood cells, a sign that sustained cortisol elevation may reduce the body's immune responsiveness.

This distinction matters as cortisol itself is not the problem. The issue is sustained elevation across weeks, months or longer, when the body never quite returns to baseline.

How chronic stress may affect immune function

When cortisol stays elevated for long periods, it changes the way immune cells behave. A review in the Journal of Clinical Medicine on the immunology of stress describes some of these effects. Prolonged cortisol exposure can reduce T-cell proliferation (the immune cells that drive the adaptive response), impair B-cell antibody production, and alter the activity and movement of neutrophils (white blood cells that rush to sites of infection). Over time, this dampening effect may leave the body slower to respond when a new virus or bacteria arrives.

The everyday consequences show up in human research. A meta-analysis published in Psychosomatic Medicine found that people experiencing higher levels of psychological stress were more likely to develop an upper respiratory infection. The pattern held across studies regardless of how stress or infection was measured.

There is another piece to this picture. A review in the International Journal of Molecular Sciences describes how immune cells can become less responsive to cortisol's signals over time. One of cortisol's normal jobs is to switch off inflammation once it is no longer needed. When immune cells stop registering that signal, the body can settle into a low-grade inflammatory state, which is often observed in people under sustained stress.

The inflammation paradox

Cortisol normally helps regulate inflammation. When chronic stress causes immune cells to become resistant to cortisol's signals, two things may happen at once. Defence against new pathogens can weaken, and underlying inflammation may rise. This combination is part of why prolonged stress is associated with increased illness susceptibility and slower recovery.

Common signs of being physically and mentally run down

The phrase "run down" does not have a formal clinical definition, yet it describes a familiar pattern that emerges when stress and depletion compound over time. Common observations include:

• Persistent tiredness that sleep does not fully resolve
• Catching seasonal infections more readily or taking longer to recover
• Lower tolerance for everyday demands
• Difficulty concentrating or maintaining motivation
• Disrupted sleep patterns, including early waking or trouble falling asleep
• Changes in appetite or digestion

These signals indicate that current demands may be exceeding available recovery capacity. They are not a diagnosis, and they are worth paying attention to, particularly when several appear together and persist for weeks.

Everyday habits that support stress resilience

Stress is an unavoidable part of modern life. What is more modifiable is how the body recovers between episodes. The following habits have a well-established evidence base for supporting stress resilience and overall wellbeing.

Quality sleep

Sleep and stress have a reciprocal relationship. A meta-analysis of 24 studies published in Endocrine Journal examined acute sleep deprivation and cortisol, finding effects on cortisol levels varied across studies, reflecting the complexity of the sleep-HPA relationship. A review in Annals of Neurosciences on sleep and immune system crosstalk notes that prolonged sleep deprivation can disrupt cortisol rhythms, potentially leading to immune dysregulation and increased susceptibility to infections.

The Sleep Health Foundation recommends most adults aim for seven to nine hours per night. Consistency in bed and wake times helps regulate cortisol's natural rhythm.

Movement and physical activity

Regular moderate exercise has well-documented benefits for stress regulation, mood and immune function. The aim during stressful periods is movement that supports recovery rather than adds further physical strain. Walking, swimming, yoga and resistance training can all play a role. The Australian Physical Activity Guidelines recommend 150 to 300 minutes of moderate-intensity activity per week for adults.

Recovery and mindfulness practices

Activities that activate the parasympathetic nervous system, the "rest and digest" counterpart to the stress response, help the body return to baseline. Slow breathing, meditation, time in nature and meaningful social connection have all been shown to influence stress markers when practised regularly.

Nutrition basics

A diet rich in vegetables, fruit, whole grains, lean protein, oily fish and legumes provides the micronutrients involved in immune function and the stress response. Periods of high stress sometimes coincide with reaching for refined carbohydrates, caffeine and ultra-processed foods, which can leave the body short on the nutrients it draws on most heavily during demanding times.

The role of key nutrients during stressful periods

Several micronutrients play specific roles in the body's stress response and immune function. Three with strong evidence and practical relevance are vitamin C, magnesium and the B-group vitamins.

Vitamin C

Vitamin C has a well-researched role in immune function. A review in Nutrients on vitamin C and immune function describes how vitamin C contributes to immune defence by supporting cellular functions of both the innate and adaptive immune system, accumulates in phagocytic cells such as neutrophils, and supports epithelial barrier function against pathogens.

When it comes to colds specifically, a meta-analysis published in BMC Public Health examined 15 comparisons from 10 randomised trials of vitamin C at doses of at least 1 g per day. The analysis found that vitamin C significantly decreased the severity of the common cold by 15% compared with placebo, with the effect more pronounced on severe symptoms.

The Australian NHMRC Nutrient Reference Value for vitamin C sets the recommended dietary intake at 45 mg per day for most adults, with higher amounts recommended during pregnancy and lactation. Food intake varies day to day, and dietary intake does not always align with the higher doses studied in clinical trials.

Blackmores Bio C 1000 is a high-strength formulation containing 1000 mg of vitamin C across three forms (ascorbic acid, calcium ascorbate and sodium ascorbate), along with citrus bioflavonoids, hesperidin, rutin and extracts from acerola (Malpighia glabra) and rosehip (Rosa canina). The combination reflects a formulation focused on antioxidant activity and immune system support, and the higher per-tablet dose aligns with the dose range examined in cold-related clinical trials.

Magnesium

Magnesium contributes to over 300 enzymatic reactions in the body, including those involved in muscle function, nervous system regulation and energy metabolism. A review in Nutrients on magnesium status and stress describes a two-way relationship: stress can increase magnesium loss, and low magnesium status can, in turn, heighten the body's stress response. Adequate intake from green leafy vegetables, nuts, seeds, legumes and whole grains contributes to maintaining levels during demanding periods.

B-group vitamins

B-group vitamins are involved in energy metabolism and nervous system function. They are water-soluble, meaning the body does not store them in large amounts, and intake is needed regularly through food or supplementation. Whole grains, eggs, leafy greens, legumes and lean meat are dietary sources.

Building a sustainable routine during stressful periods

Sustainable habits tend to outperform dramatic interventions. The aim during a stressful period is not to overhaul daily life, but to maintain a few foundational habits with consistency.

A practical starting point:

• Anchor sleep and wake times within a 30-minute window most days

• Build at least one daily activity that activates rest and recovery (walking outside, breathing exercises, time without screens)
• Include vegetables, protein and whole grains across most meals
• Identify and reduce one stressor where you have agency, even modestly
• Be realistic about energy reserves rather than pushing through indefinitely

Small, consistent steps tend to outlast short-lived intensity.

When to talk to your healthcare professional

Most people experience stretches of feeling run down that resolve with rest, nutrition and time. Some signs suggest a conversation with a GP or other healthcare professional may be helpful:

• Persistent fatigue lasting more than four weeks despite adequate sleep
• Frequent infections that disrupt daily life
• Significant changes in mood, appetite or sleep patterns
• Symptoms that interfere with work, relationships or daily activities

Healthcare professionals can assess for underlying contributors including thyroid changes, nutrient deficiencies and mental health concerns that may benefit from targeted support.

Frequently asked questions

Can stress weaken the immune system?

The evidence indicates a relationship between prolonged stress and reduced immune function. The Psychosomatic Medicine meta-analysis of 27 prospective studies found a significant association between psychological stress and increased susceptibility to upper respiratory infection. Acute stress affects the body differently from chronic stress, with chronic exposure more clearly linked to immune dysregulation.

Can stress make me sick?

Sustained cortisol elevation may change how immune cells behave, including reducing T-cell proliferation, impairing antibody production, and causing immune cells to become less responsive to cortisol's signals over time. The combined effect tends to weaken defence against new pathogens while raising baseline inflammation.

How can I support my immunity during a stressful period?

Foundational habits offer the most reliable support: consistent sleep, regular moderate movement, a nutrient-rich diet, and activities that activate the parasympathetic nervous system. Adequate intake of vitamin C, magnesium and B-group vitamins from food may contribute to immune function and the stress response. Supplementation may fill gaps where dietary intake is insufficient.

Is being 'run down' a medical condition?

No. The term describes a common pattern of fatigue, frequent illness and reduced resilience, but it is not a clinical diagnosis. Persistent symptoms warrant assessment by a healthcare professional to identify underlying causes.

Key takeaways

Stress is a physiological response coordinated by the sympathetic nervous system and HPA axis. Short-term cortisol elevations can briefly enhance immune readiness, and chronic elevation is associated with immune dysregulation, slower recovery from illness and higher susceptibility to common infections.

Australian data indicate psychological distress has risen over the past decade, making the science of stress and immunity directly relevant to everyday wellbeing. The most reliable support comes from foundational habits: sleep, movement, recovery, nutrition and connection. Adequate intake of vitamin C, magnesium and B-group vitamins may contribute to immune function and the stress response.

For persistent symptoms or significant changes in well-being, a healthcare professional can help identify underlying contributors and the right course of support.

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Disclaimer: This article provides general information only and is not intended to diagnose, treat, cure, or prevent any disease. The information presented is not a substitute for professional medical advice. If you have concerns about your health, please consult your GP or healthcare provider for personalised assessment and recommendations. Supplements should not replace a balanced diet.