睡眠Sleep

睡眠非常重要，尤其对于学习和记忆！ 

The best way to learn is to fall asleep. 
One of the most important things your brain does when you're asleep is remind itself about things you've learned during the day. This includes practical skills, such as music and sports, as well as filing away new facts.

Experts decribe four stages, known as a sleep cycle. In a good night's sleep, most of people complete 4 or 5 cycles. 专家研究睡眠分4个阶段循环，大部分人完成4到5个循环。 

1 - Light sleep 浅度睡眠

eyes closed, brain activity slows down, muscles start to relax, but can sometimes twitch, waking a person up.  眼睛闭合，大脑活跃度下降，肌肉开始放松，但是有时候身体还会动，醒来。

2 - Main sleep 主要睡眠

body and brain at least active, a few people experience sleep walking or sleep talking during this stage

身体和大脑活动极少，一些人会梦游或者说梦话

3 - Slow-wave sleep 慢波睡眠

brain files away memories , the body can repair itself by building new cells rapidly. 大脑整理记忆，身体修复快速修复新细胞。 

4 - REM Sleep (Rapid Eyes Movement Sleep) 眼睛快速运动的睡眠

eyes move quickly and brain beats faster, breathing rate goes up, skeletal muscles don't move, brain very active.  Dream occusrs - but no one knows how it happens or what it is for. 

眼睛和大脑都在活跃状态，呼吸率升高，骨骼肌肉不动，大脑却活跃。 这时候梦境产生，但是没有人知道为什么会产生梦和它的作用。

It is easy to wake up during stage 1 and 4. but very hard during stage 2 and 3.  人们容易从第一阶段和第四阶段的睡眠中醒来，而不容易从第二和第三阶段的睡眠中醒来。