i-adjective
up late at night; staying up late
Describes the state of being awake deep into the night; often used in the phrase 夜が深い or 夜も更けて.
夜深い時刻なので、そろそろ寝ましょう。
It is a late-night hour, so let's go to bed soon.
彼は夜深くまで勉強している。
He studies until late at night.