昨日(きのう)は早(はや)く寝(ね)ました。

This phrase talks about a past action related to sleep. It indicates that one went to bed earlier than usual on the previous day, often implying feeling tired or wanting to rest more.

昨日(きのう)の睡眠(すいみん)について、早(はや)く寝(ね)たことを伝(つた)える表現(ひょうげん)です。