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to sleep (lying down)
to go to bed; to lie in bed
to lie down
to lie flat (e.g. of hair)
to lie idle (of funds, stock, etc.)
to sleep (with someone, i.e. have intercourse)
to ferment (of soy sauce, miso, etc.)
昨日は疲れて早く寝た。
I was tired yesterday and went to bed early.
明日は朝早いから、今夜は早く寝るよ。
I have to be up early tomorrow, so I'll go to bed early tonight.
昨日は十一時に寝ました。
I went to bed at eleven yesterday.