How are brain fog and fatigue related?

Reviewed by Yoshinori Abe, MD

Internal Medicine

Brain fog and fatigue frequently occur together and often share the same underlying triggers. Brain fog refers to mental sluggishness, difficulty concentrating, or confusion, while fatigue describes persistent physical or mental exhaustion that doesn't improve with rest.

Common causes of both symptoms include:

Poor sleep quality or sleep disorders like insomnia and sleep apnea
Nutritional deficiencies, particularly in vitamin B12, iron, or vitamin D
Mental health conditions such as depression, anxiety, and chronic stress
Infections, including long COVID, mononucleosis, and Lyme disease
Hormonal imbalances, such as thyroid disorders or perimenopause
Chronic conditions like fibromyalgia, autoimmune diseases, or diabetes

Because these symptoms overlap with so many possible conditions, self-diagnosis can be difficult—and delaying answers only prolongs the discomfort. The fastest way to narrow down potential causes is to review your specific symptoms systematically. Take a free, instant, online symptom check to better understand what may be driving your brain fog and fatigue, and get clear guidance on your next steps.

Reviewed for medical accuracy: 07/15/2026

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Explanation

Brain fog is when your mind feels fuzzy, and it has trouble thinking clearly. Fatigue is when you feel so tired that even simple tasks seem hard to do. Research shows that many of the same factors can cause both brain fog and fatigue. For example, not getting enough sleep, eating a poor diet, or feeling sad or stressed can contribute to both. Some studies have found that problems with your gut or changes in your body's balance after infections such as COVID-19 can also lead to these symptoms. Both brain fog and fatigue may be signs that your body is dealing with stress, inflammation, or changes in your brain. This means that when someone experiences fatigue, they could also feel mentally slow or unfocused because the underlying causes affect both the body and the brain. If you're experiencing these symptoms and want to better understand what might be causing them, you can take a free symptom checker test to get personalized insights. Doctors and researchers believe that understanding all these factors is important to help people feel better.

(References)

McWhirter L, Smyth H, Hoeritzauer I, Couturier A, Stone J, Carson AJ. What is brain fog? J Neurol Neurosurg Psychiatry. 2023 Apr;94(4):321-325. doi: 10.1136/jnnp-2022-329683. Epub 2022 Dec 6. PMID: 36600580.
Altinsoy C, Dikmen D. How Are Brain Fog Symptoms Related to Diet, Sleep, Mood and Gastrointestinal Health? A Cross-Sectional Study. Medicina (Kaunas). 2025 Feb 15;61(2):344. doi: 10.3390/medicina61020344. PMID: 40005460; PMCID: PMC11857395.
Ortelli P, Ferrazzoli D, Sebastianelli L, Maestri R, Dezi S, Spampinato D, Saltuari L, Alibardi A, Kofler M, Quartarone A, Koch G, Oliviero A, Versace V. Fatigue and "brain fog" in the aftermath of mild COVID-19: A neuropsychological and TMS study. J Neurol Sci. 2021 Oct;429:119854. doi: 10.1016/j.jns.2021.119854. Epub 2021 Oct 8. PMCID: PMC8498602.

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