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》

I'm a very light sleeper and I can hardly get any sleep at all.

我是个睡觉非常轻的人，几乎一点儿也睡不着。

《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》

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

VOA: special.2009.03.28

And then we let the Lake Placid Lodge do what it does best: another fine meal, another gibbous moon, another night to sleep by the light of the fireplace.

FORBES: Magazine Article

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.

ENGADGET: Jawbone Up review (2012): a reboot and a shot at redemption

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).

ENGADGET: Jawbone Up review (2012): a reboot and a shot at redemption