But what is really happening inside the body when we drift off?

但是，當我們漸漸入睡時，身體裡究竟發生什麼事？

And what's the impact if we don't get enough sleep?

此外，要是我們睡眠不足，對我們又有什麼影響？

Sleep is regulated by your circadian rhythm, or body clock, located in the brain.

睡眠由腦內的晝夜節律或生理時鐘所管理。

The body clock responds to light cues,

生理時鐘對光信號有所反應，

ramping up production of the hormone melatonin at night and switching it off when it senses light.

在夜晚會加速賀爾蒙－褪黑激素的分泌，並在感應到光線時關閉。

There are four stages of sleep that the body experiences in cycles throughout the night.

身體整個晚上會經歷四個階段的睡眠週期。

On a good night, we cycle through these stages four or five times.

在正常的夜晚，這些階段我們會循環四至五次。

Stages one and two are light sleep.

第一和第二階段為淺眠期。

This is a transition from being awake to falling asleep.

這是從意識清醒到入睡的過渡期。

Heart rate and breathing begin to slow,

心率和呼吸開始減慢，

body temperature falls, and muscles may twitch.

體溫下降，而且肌肉可能會抽搐。

Stage three is sometimes referred to as delta sleep

第三階段有時也稱為三角洲睡眠，

because of the slow delta brainwaves that are released during this stage.

因為在此階段會釋放緩慢的三角腦波。

This is the first stage of deep sleep,

這是深度睡眠的第一階段，

where our cells produce the most growth hormone to service bones and muscles, allowing the body to repair itself.

我們細胞會產生最大量的生長激素提供給骨頭和肌肉，讓身體進行自我修復。

Stage four is where we begin to dream.

第四階段我們會開始做夢。

The body creates chemicals that render it temporarily paralyzed so that we do not act out our dreams.

身體會分泌化學物質，使身體暫時癱瘓，這樣我們就無法做出夢裡的動作。

In this stage, the brain is extremely active and our eyes, although closed, dart back and forth as if we were awake.

在此階段，大腦會非常活躍，雖然我們的眼睛閉著，但會來回快速移動，彷彿我們醒著一樣。

Humans roughly spend 1/3 of their lives asleep.

人類生命中約有三分之一的時間在睡眠上。

Modern lifestyles, stress, and the proliferation of technology mean that people are sleeping far less today than they were a century ago.

現代生活方式、壓力、以及科技的快速進步，代表現代人的睡眠時間遠少於一個世紀前的人類。

Sleeping less than seven hours per day is associated with an increased risk of developing chronic conditions, which could reduce life expectancy.

每天的睡眠時間少於 7 小時，與增加罹患慢性疾病的風險有關，進而可能降低預期壽命。

So for a healthier, longer life, get some shuteye.

因此，為了要活得更健康、更長壽，就得多睡一點。