She had drifted into a light sleep.

她已经慢慢进入了浅睡眠。

《柯林斯英汉双解大词典》

She drifted into a light sleep.

她迷迷糊糊地睡得不沉。

《牛津词典》

You program the clock with the latest time at which you want to be wakened, and it then duly wakes you during the last light sleep phase before that.

你给时钟设定了你想要醒来的最晚时间，然后它会在之前的浅睡眠阶段按时叫醒你。

youdao

So much light in the sky would not let his daughter sleep.

VOA: special.2009.03.28

It then waits till you fall asleep then starts the timer, waiting for you to get roughly 26.5 minutes of shuteye, before rousing you during a light sleep cycle.

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Instead of making a loud noise at a predetermined time, the Smart Alarm gently vibrates when you're in a light sleep, somewhere around the time you'd like to wake up (up to 30 minutes before you've actually set it).

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Light and quality of sleep are well-known to be linked, as anyone who has ever pulled a night shift will attest.

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