expression, ichidan verb
be clear-headed; have a sharp mind
Describes a state where one's mind is alert, sharp, and thinking clearly. Often used in the form 頭が冴えた (clear-headed state).
朝のコーヒーを飲むと頭が冴える。
Drinking morning coffee makes my mind clear.
彼は徹夜明けなのに頭が冴
Even after staying up all night, he is clear-headed.