In the middle of the night; at night or during late-night hours. Can be used as a noun or adverb ('夜中に').
夜中に犬が吠えて目が覚めた。
A dog barked in the middle of the night and woke me up.
試験前で夜中まで勉強した。
I studied until late at night before the exam.
彼はよく夜中に散歩に出かける。
He often goes out for a walk in the middle of the night.