Late at night; at a late hour (near midnight), used adverbially or as a noun phrase indicating time.
夜遅くまで残業して疲れた。
I was tired from working late into the night.
夜遅くに電話をかけるのは失礼ですよ。
Calling someone late at night is rude, you know.
夜遅くの電車は本数が少ないので注意してください。
There are fewer trains late at night, so please be careful.