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

Scientists have explored this theory as a possible explanation for symptoms such as fatigue, brain fog, anxiety, and depression.

From Science Daily • May 28, 2026

Women are now pursuing treatment for symptoms like hot flashes, brain fog, insomnia and dry, itchy skin.

From MarketWatch • May 12, 2026

Tang said she experienced side effects including sleep paralysis, fatigue and brain fog, which left her "exhausted" and affected her school attendance.

From BBC • Mar. 26, 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

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