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

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

"Your mental health is affected. You get brain fog, you're tired, you're fatigued. You can't function as a woman – and that's every day for 10 years," she said.

From BBC • Feb. 27, 2026

"And he often complained about suffering with headaches, brain fog, and anxiety, and it became quite scary for me."

From BBC • Feb. 10, 2026

She said the patches helped relieve musculoskeletal and joint pain, night sweats, and brain fog.

From The Wall Street Journal • Feb. 7, 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