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middle of the night; dead of night
「夜中」 has other readings with different meanings.
夜中に突然目が覚めてしまって眠れない。
I woke up in the middle of the night.
締め切りが近くて夜中まで仕事をしていた。
I was working until late at night.
夜中の三時に急に電話がかかってきた。
A call came in the middle of the night.