What are the potential causes of brain fog?

Reviewed by Yoshinori Abe, MD

Internal Medicine

Brain fog is a common symptom—not a medical diagnosis—characterized by confusion, forgetfulness, poor focus, and mental fatigue. It can be triggered by a wide range of factors, including chronic stress, physical or mental exhaustion, poor sleep, hormonal changes, nutritional deficiencies, infections (such as COVID-19 or the flu), and underlying conditions like anxiety, depression, thyroid disorders, or autoimmune disease.

Because brain fog has so many possible causes, pinpointing the reason behind yours is the essential first step toward feeling clear-headed again. Rather than guessing, take a free, instant, AI-powered symptom check to explore what may be behind your symptoms and get guidance on the right next steps—no signup required.

Reviewed for medical accuracy: 07/15/2026

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Explanation

Brain fog is when your thoughts feel slow or fuzzy. It can be caused by different things. One cause is infections like COVID-19. When someone has COVID-19, this may result in inflammation of the brain, which can slow thinking. In some people, even after the virus is gone, the brain might still feel foggy.

Another cause is sleep problems. Disorders that make you very sleepy, or make your sleep less refreshing, can lead to foggy brain during the day. Also, when you are very tired or stressed, your brain might not work as well, which makes it hard to remember things or concentrate. In some cases, feelings of low mood or anxiety can complicate these other issues.

In short, brain fog might come from the body's response to a virus, poor sleep, being overly tired, or feeling down in mood. If you're experiencing brain fog and want to better understand what might be causing your symptoms, try our free AI-powered symptom checker to get personalized insights. All these factors can affect how clearly you think, making it hard to focus and remember things.

(References)

Aghajani Mir M. Brain Fog: a Narrative Review of the Most Common Mysterious Cognitive Disorder in COVID-19. Mol Neurobiol. 2024 Dec;61(12):9915-9926. doi: 10.1007/s12035-023-03715-y. Epub 2023 Oct 24. PMID: 37874482.
Rosenberg R, Thorpy MJ, Doghramji K, Morse AM. Brain fog in central disorders of hypersomnolence: a review. J Clin Sleep Med. 2024 Apr 1;20(4):643-651. doi: 10.5664/jcsm.11014. PMID: 38217475; PMCID: PMC10985301.
Delgado-Alonso C, Díez-Cirarda M, Pagán J, Pérez-Izquierdo C, Oliver-Mas S, Fernández-Romero L, Martínez-Petit Á, Valles-Salgado M, Gil-Moreno MJ, Yus M, Matías-Guiu J, Ayala JL, Matias-Guiu JA. Unraveling brain fog in post-COVID syndrome: Relationship between subjective cognitive complaints and cognitive function, fatigue, and neuropsychiatric symptoms. Eur J Neurol. 2025 Jan;32(1):e16084. doi: 10.1111/ene.16084. Epub 2023 Oct 5. PMID: 37797297; PMCID: PMC11618112.

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