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all night; the whole night
「夜中」 has other readings with different meanings.
試験前は夜中ずっと遅くまで勉強していた。
I was studying all night.
夜中に隣の部屋で騒いでいる人がいて迷惑だ。
Someone was making noise all night.
夜中まで残業して働いてとても疲れた。
I got tired from working all night.