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to be still half asleep; to be half awake; to be not yet quite awake
彼は寝惚けたまま、砂糖と間違えて塩をコーヒーに入れた。
Still half asleep, he mistakenly put salt in his coffee instead of sugar.
寝惚けて、違う教室に入った。
Still groggy, I walked into the wrong classroom.
息子は朝によく寝惚ける。
My son is often groggy in the mornings.