She drifted into a light sleep.

她迷迷糊糊地睡得不沉。

《牛津词典》

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》

The first 30 minutes is a light sleep that helps improve motor performance.

头30分钟是浅睡，帮助改善运动功能。

youdao

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

You see, researchers now know that blue-tinted light suppresses production of melatonin a hormone that makes it easier for us to go to sleep at night while red light does not (I wrote about the topic in a bit more detail here).

FORBES: The Best Product of 2012: The Philips Hue LED Lighting System