结果发现，受到辐射的那组研究对象进入深度睡眠第一阶段所用的时间较长，而且深度睡眠的时间也较短。

The first group took longer to enter the first of the deeper stages of sleep and spent less time in the deepest one.

youdao

在第一阶段，人的睡眠很浅。

Stage one sleep is very light.

youdao

这被称为第一阶段睡眠。

This is called stage 1 sleep.

youdao

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