brain fog

American  

[breyn fog, fawg] / ˈbreɪn ˌfɒg, ˌfɔg /

Also brain-fog,

noun

1. a mental state marked by difficulty remembering, concentrating, or thinking clearly, often caused by exhaustion, stress, or illness: For new parents, fatigue and brain fog seem inevitable.

   He had so much brain fog that a single thought felt impossible.

   For new parents, fatigue and brain fog seem inevitable.

   Research has found that long COVID can cause brain fog and memory loss.

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Etymology

Origin of brain fog

First recorded in 1850–55; brain ( def. ) + fog ¹ ( def. )

Example Sentences

Long Covid, according to the NHS, is when symptoms of the virus, such as fatigue, brain fog, and pain, last longer than 12 weeks.

From BBC • Mar. 23, 2026

That’s on top of the injections Leite gives herself regularly: NAD+, which she says makes her feel energized and alleviates her brain fog, and glutathione, which is marketed for antioxidant and immune support.

From The Wall Street Journal • Mar. 18, 2026

Tessa Gonzalez, a 51-year-old from Stockton, Calif., said her primary care provider told her symptoms of brain fog and inconsistent sleep were normal for perimenopause.

From The Wall Street Journal • Mar. 1, 2026

Within this group, 86% of U.S. patients reported experiencing brain fog.

From Science Daily • Jan. 28, 2026

There were fever and unconsciousness and delirium, and through each phase Dr. Cornish, late of the Foreign Legion, brought his patient with studious care—through all, that is, save the brain fog.

From The Key to Yesterday by Buck, Charles Neville