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。

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When you sleep you get the following stages.

睡眠有以下几个阶段。

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You start off and you go down, down, down, and then you come up, then you get your first REM cycle, again, again, again, again, again, and then you wake up.

从第一阶段开始，到下面的阶段，然后又往上，这时就出现REM睡眠，然后再一次次地循环，直到你睡醒。

耶鲁公开课 - 心理学导论课程节选