To be wide awake or alert (unable to sleep); often used when caffeine, excitement, or tension keeps someone awake.
コーヒーを飲んだら夜中まで目が冴えてしまった。
After drinking coffee I stayed awake until late at night.
試験前で緊張して目が冴えて眠れない。
I'm too nervous before the exam and can't sleep.
深夜の明るい画面を見ると目が冴えることがある。
Looking at a bright screen late at night can make you wide awake.