Wide awake with 目が冴える

The expression 目(め)が冴(さ)える literally translates to “the eyes clear” or “the eyes become sharp”, but it is used figuratively to describe a state of mental clarity, alertness, or sharpness. It often conveys the sense of feeling awake, focused, or having heightened awareness, typically in the context of being more awake than usual, like after a good rest, a cup of coffee, or when one is extremely alert and energized.

Dictionary Definition

神経(しんけい)が高(たか)ぶって、眠(ねむ)れなくなる。また、眠気(ねむけ)がなくなる。

Unable to sleep due to heightened nerves. Also, to become no longer sleepy.

Goo Dictionary

Examples

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Unable to fall asleep after a horror movie with 目が冴える

Komura and his mother watch a late-night horror movie. After his mother heads off to bed, Komura texts with Mie from class, who had also watched the movie with her father. The two switch to a phone call, and after a bit the subject of bedtimes comes up. Normally Mie can’t keep awake as late as Komura does.

小村(こむら):

「あ　じゃあもう眠(ねむ)い…？」

“Ah, then right now you're sleepy?”

三重(みえ):

「あ　今日(きょう)は…　なんか…　目(め)が冴(さ)えちゃって…」

“Ah, today I'm somewhat unable to fall asleep.”