brain fog
A state of mental cloudiness, difficulty concentrating, or forgetfulness. Often used in health contexts. The loanword ブレインフォグ is also common.
See also: ブレインフォグ
Lately, my brain fog has been so bad that I can't concentrate on work.
Some people experience brain fog as a symptom of menopause.
Calque from English 'brain fog', composed of 脳 (brain) + の (possessive particle) + 霧 (fog).