brain fog

American  

[brayn 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

See Examples For:

brain fog

Common brain-related symptoms include fatigue, brain fog, memory problems, and low mood.

From Science Daily ● Sep. 22, 2026

“Perimenopausal brain fog was a real thing, though like nearly every other perimenopausal symptom it was tricky to distinguish from about a hundred other medical conditions,” Sloane tells us.

From Los Angeles Times ● Sep. 8, 2026

“My clothes were just falling off,” said the 23-year-old from Sarasota, Fla. “I wasn’t bloated, I didn’t have brain fog, I wasn’t constantly fatigued.”

From The Wall Street Journal ● Aug. 8, 2026

Charlie Paddock, 27, from south London, also said she "nearly fainted twice and the hot flushes were unreal", while Jess Allingham, 26, said she was "absolutely exhausted, more than normal", and had "total brain fog".

From BBC ● Jul. 8, 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 Charles Neville Buck