late at night; at a late hour
Refers to the late hours of the night, often used adverbially or as a noun.
夜遅くまで勉強した。
I studied until late at night.
夜遅くに帰宅した。
I came home late at night.
Standard kanji spelling.
Kana-mixed spelling, less common but still used.
Compound of 夜 (よる, 'night') and 遅く (おそく, adverbial form of 遅い 'late').