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to get late; to advance; to wear on
夜が深けるにつれて気温が下がった。
As the night deepened, the temperature dropped.
秋が深けると山の紅葉が見頃を迎える。
As autumn deepens, the mountain foliage reaches its peak.
夜が深けてから静かに家を出た。
I quietly left the house after night had fallen deeply.