Probably the best answer is our body is worn out during the day and sleep is necessary to put it back into shape.

最佳的答案应该是,一天下来，我们的身体已经疲惫了,有必要进行睡眠，以恢复状态。

耶鲁公开课 - 心理学导论课程节选

You start off with a transition period when you're falling asleep. We call that stage I.

一开始是过渡期,你开始进入睡眠状态，这是第一阶段。

耶鲁公开课 - 心理学导论课程节选

First,we'll put you to sleep when we do all this procedure.

首先会让你进入睡眠状态。

耶鲁公开课 - 死亡课程节选