What Causes Brain Fog? Common Triggers and Contributing Factors

What Is Brain Fog?

Brain fog is not a medical diagnosis but rather a term used to describe a collection of cognitive symptoms that affect thinking, focus, and memory. Research defines it as a subjective experience of neurological symptoms characterized by difficulties with memory, attention, and information processing.¹ People experiencing brain fog often describe feelings of mental cloudiness, lack of mental clarity, or feeling "spacey" and disconnected from their surroundings.

The experience of brain fog can vary from person to person but typically involves slower thinking, difficulty finding the right words, and trouble maintaining concentration on tasks. While brain fog itself isn't a disease, it serves as an important symptom that may indicate underlying health conditions requiring attention.

Common Symptoms of Brain Fog

Brain fog symptoms manifest in several ways that can significantly impact daily functioning:

• Difficulty focusing - Trouble maintaining attention on tasks or conversations

• Memory problems - Forgetting appointments, names, or where you placed items

• Mental fatigue - Feeling mentally exhausted even after adequate rest

• Confusion - Difficulty processing information or making decisions

• Lack of mental clarity - Thoughts feel "fuzzy" or unclear

• Slower thinking - Taking longer than usual to understand or respond

• Word-finding difficulties - Struggling to recall common words during conversation

• Feeling disconnected - A sense of being mentally "foggy" or detached

These concentration issues can range from mild and occasional to severe and persistent, depending on the underlying cause.

Sleep-Related Causes

Poor sleep quality and sleep deprivation are among the most common contributors to brain fog. During sleep, the brain consolidates memories, clears metabolic waste products, and restores cognitive function. When sleep is disrupted or insufficient, these essential processes cannot occur properly.²

Research shows that acute total sleep deprivation significantly increases cortisol levels and disrupts emotion, cognition, and inflammatory markers in the body.³ Chronic sleep deprivation diminishes cognitive performance, alters mood states, and dysregulates stress hormones that are critical for mental clarity.

Sleep disorders such as sleep apnea can cause repeated interruptions in breathing throughout the night, leading to fragmented sleep and reduced oxygen delivery to the brain. This creates a perfect environment for persistent brain fog symptoms, even when you believe you're getting adequate hours of rest.

Stress and Mental Health Causes

Chronic stress floods the body with cortisol, a hormone that, in excess, can impair brain function and contribute to cognitive difficulties. Prolonged stress exposure affects the hippocampus, a brain region critical for memory formation and retrieval.⁴

Mental health conditions including anxiety and depression are strongly associated with brain fog. These conditions can alter neurotransmitter levels—particularly dopamine and serotonin—that are essential for maintaining focus, motivation, and mental clarity. The cognitive symptoms of depression often include difficulty concentrating, slowed thinking, and impaired decision-making.

Mental exhaustion and burnout from prolonged stress can deplete cognitive resources, leaving you feeling mentally drained and unable to think clearly. The relationship between stress, mental health, and brain fog creates a cycle that can be difficult to break without addressing the root causes.

Nutritional and Lifestyle Causes

Your diet and lifestyle choices play a significant role in cognitive function. Dehydration, even mild, can impair attention, memory, and motor skills. The brain is approximately 75% water, and inadequate hydration directly affects its ability to function optimally.

Vitamin B12 deficiency is particularly problematic for cognitive health. Vitamin B12 is vital for myelin integrity, neurotransmitter synthesis, and homocysteine metabolism. Research has shown that abnormally low levels of vitamin B12 can cause significant cognitive dysfunction, and screening for this deficiency is recommended, especially in the elderly population.⁵

Other nutritional factors that may contribute to brain fog include:

• Vitamin D deficiency - Low levels have been linked to cognitive impairment

• Low blood sugar - Glucose fluctuations affect brain energy availability

• Poor diet quality - Diets lacking healthy fats and nutrients can impair brain function

• Lack of exercise - Physical inactivity reduces blood flow to the brain and decreases production of brain-derived neurotrophic factor (BDNF), which supports cognitive function

Medical Conditions That Cause Brain Fog

Several medical conditions are commonly associated with brain fog as a persistent symptom:

Autoimmune Diseases: Conditions such as lupus, multiple sclerosis, and Sjögren's syndrome can cause neuroinflammation that leads to cognitive symptoms. The immune system's attack on body tissues can affect the brain and nervous system, resulting in brain fog.

Thyroid Disorders: Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can significantly impact cognitive function. Thyroid hormones are essential for brain metabolism and neurotransmitter production.

Long COVID: Brain fog has emerged as one of the most common and debilitating symptoms of post-COVID syndrome. Research indicates that 86% of long COVID patients in the United States who were not hospitalized during their initial infection reported brain fog.⁶ Studies have identified potential mechanisms including a leaky blood-brain barrier and widespread increases in certain receptor densities linked to cognitive impairment.⁷

Chronic Fatigue Syndrome: This condition is characterized by persistent, unexplained fatigue accompanied by significant cognitive difficulties. The cognitive symptoms may result from altered cerebral blood flow and decreased ability to process information.¹

Fibromyalgia: Often called "fibro fog," the cognitive impairment associated with fibromyalgia includes memory problems, difficulty concentrating, and mental confusion.

Diabetes: Both high and low blood sugar levels can impair cognitive function, and diabetes-related vascular changes may affect brain health over time.

Medications That Can Cause Brain Fog

Certain medications can affect cognitive function as a side effect. If you notice brain fog symptoms after starting a new medication, it's important to discuss this with your healthcare provider rather than stopping the medication on your own.

Common medication classes that may contribute to brain fog include:

• Antihistamines - Particularly older first-generation antihistamines that cross the blood-brain barrier

• Sleep aids - Both prescription and over-the-counter sleep medications

• Anxiety medications - Benzodiazepines and some other anti-anxiety drugs

• Pain medications - Opioids and certain other pain relievers

• Chemotherapy drugs - "Chemo brain" is a well-recognized phenomenon affecting memory and concentration

Your healthcare provider can help determine if medication adjustments might improve your cognitive symptoms while still effectively managing your underlying condition.

Hormonal Causes

Hormonal fluctuations and imbalances can significantly impact cognitive function, particularly in women.

Menopause and Perimenopause: Up to 60% of midlife women report difficulties with memory, attention, and verbal fluency during perimenopause.⁸ Research validates these cognitive complaints, with objective testing confirming declines in verbal memory, working memory, and executive function. These changes correlate with fluctuations in estradiol, which directly relates to changes in memory performance and brain circuitry reorganization.⁹

Pregnancy: "Pregnancy brain" or "baby brain" refers to memory problems and difficulty concentrating that many women experience during pregnancy and postpartum. Hormonal changes, sleep disruption, and the demands of caring for a newborn all contribute.

Thyroid Imbalances: Thyroid hormones regulate metabolism throughout the body, including in the brain. Both too much and too little thyroid hormone can impair cognitive function.

The hormonal influence on cognition highlights the complex interplay between endocrine function and brain health.

When to See a Doctor About Brain Fog

While occasional mental fogginess is common, certain situations warrant medical evaluation:

• Brain fog persists for several weeks without improvement

• Symptoms progressively worsen over time

• Cognitive difficulties significantly interfere with work, school, or daily activities

• Brain fog is accompanied by other concerning symptoms such as severe headaches, vision changes, weakness, or difficulty speaking

• You experience sudden onset of severe cognitive impairment

• Memory problems are noticeably worsening

A thorough evaluation may involve blood tests to check for nutritional deficiencies, thyroid function, blood sugar levels, and inflammatory markers. Your healthcare provider may also assess for sleep disorders, medication side effects, and underlying medical conditions. In some cases, cognitive testing or brain imaging may be recommended to rule out more serious neurological conditions.

Conclusion

Brain fog can result from numerous causes ranging from modifiable lifestyle factors like sleep deprivation and poor nutrition to underlying medical conditions including autoimmune diseases, hormonal imbalances, and post-viral syndromes. Understanding the potential triggers of your cognitive symptoms is essential for finding effective solutions.

If you're experiencing persistent brain fog, working with a healthcare provider to identify the underlying cause is the most effective approach. Many causes of brain fog are treatable, and addressing the root issue can lead to significant improvement in mental clarity and cognitive function.

References

1. Ross AJ, et al. Brain Fog: A Bit of Clarity Regarding Etiology, Prognosis, and Treatment. Journal of Investigative Medicine. 2021. https://pubmed.ncbi.nlm.nih.gov/34714198/

2. Moreira CS, et al. Influence of Sleep Deprivation and Circadian Misalignment on Cortisol, Inflammatory Markers, and Cytokine Balance. Brain, Behavior, and Immunity. 2015. https://pmc.ncbi.nlm.nih.gov/articles/PMC5401766/

3. Ali M, et al. Acute sleep deprivation disrupts emotion, cognition, inflammation, and cortisol in young healthy adults. Frontiers in Behavioral Neuroscience. 2022. https://www.frontiersin.org/journals/behavioral-neuroscience/articles/10.3389/fnbeh.2022.945661/full

4. Lupien SJ, et al. Effects of stress hormones on the brain and cognition: Evidence from normal to pathological aging. Psychoneuroendocrinology. 2017. https://pmc.ncbi.nlm.nih.gov/articles/PMC5619133/

5. Ankar A, Kumar A. Vitamin B12 Deficiency and cognitive impairment: A comprehensive review of neurological impact. International Journal of Molecular Sciences. 2025. https://www.sciencedirect.com/science/article/pii/S266645932500040X

6. Center for Infectious Disease Research and Policy (CIDRAP). Brain fog, depression more common in long COVID in US than in lower-income countries, study suggests. University of Minnesota. 2024. https://www.cidrap.umn.edu/covid-19/brain-fog-depression-more-common-long-covid-us-lower-income-countries-study-suggests

7. Center for Infectious Disease Research and Policy (CIDRAP). Researchers identify mechanism behind brain fog in long COVID. University of Minnesota. 2025. https://www.cidrap.umn.edu/covid-19/researchers-identify-mechanism-behind-brain-fog-long-covid

8. Weber MT, et al. Cognition and mental health in menopause: A review. Maturitas. 2021. https://www.sciencedirect.com/science/article/abs/pii/S1521693421001644

9. Maki PM, Thurston RC. Menopause and cognitive impairment: A narrative review of current knowledge. Climacteric. 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8394691/

Medical Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for diagnosis and treatment recommendations. The information presented here should not be used as a substitute for professional medical advice, diagnosis, or treatment. If you have concerns about your health, please seek immediate medical attention.