mental sharpness; clarity of mind; bright intelligence
Refers to a state where one's thinking is clear, quick, and effective. Often used when describing good mental condition, especially after rest or when focused.
My mind is sharper in the morning.
Adequate sleep restores mental clarity.
Compound of 頭 (あたま, 'head, mind') + の (possessive particle) + 冴え (さえ, 'sharpness, clarity'). The exact historical derivation is uncertain, but the expression is a straightforward combination of these elements.