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late at night; small hours of the morning
夜ふけまで読書にふけって寝不足になった。
I was absorbed in reading until late at night and became sleep-deprived.
夜ふけの街は静まり返って人影もなかった。
The late-night streets were silent and deserted.
夜ふけに急な電話が鳴って飛び起きた。
A sudden phone call in the dead of night woke me with a start.