睡眠占了我们人生近三分之一的时间，是我们人体一项极为重要的生命活动。我们每天需要通过睡觉来调节和修整身心。但你知道吗？晚上睡着之后，我们身体的器官还会不停歇地进行各种活动。
Every night, almost everyone on the planet enters into a state of unconsciousness and paralysis.
But what is really happening inside the body when we drift off?
And what's the impact if we don't get enough sleep?
Sleep is regulated by your circadian rhythm, or body clock, located in the brain.
The body clock responds to light cues, ramping up production of the hormone melatonin at night and switching it off when it senses light.
There are four stages of sleep that the body experiences in cycles throughout the night.
On a good night, we cycle through these stages four or five times.
Stages one and two are light sleep.
This is a transition from being awake to falling asleep.
Heart rate and breathing begin to slow, body temperature falls, and muscles may twitch.
