Sleep is controlled by certain chemicals. They build up during the day，eventually reaching levels that make us tired. We can control the effects of these chemicals to some extent．For example，caffeine helps to keep us awake while alcohol and some medicines make us sleepy.

　　By using electrodes，scientists are able to study what goes on in people's heads while they are sleeping. They have discovered that when we first drop off everything slows down. The heart beats more slowly and our breathing becomes shallow．After about 90 minutes our eyes start to twitch and we go into what is called REM sleep．REM stands for Rapid Eye Movement，and it's a sign that we've started to dream.

　　You have dreams every night，even if you don't remember them. There are all sorts of theories about why we dream. One is that it gives the brain a chance to sort out the day's activities，filling everything away in the right place. Another is that the brain gets bored while we're asleep and organizes its own entertainment-a sort of late-night cinema！

　　A lot of people say they have to have eight hours’ sleep every night while others seem to manage on a lot less. One thing is for sure，we all need some sleep and going without it can have some very strange effects. An American DJ，who stayed awake for 200 hours to raise money for charity，thought things were bursting into flames all around him after 120 hours without sleep．He survived the strange horrible period of time but was depressed for three months afterwards．

【2017届河北省高三英语一轮复习55分钟课堂练习39 Word版含答案】相关文章：

★ 【人教版】2014届高考英语一轮复习指导考点针对练：必修5 unit3《Life in the future》

★ 湖南省汝城二中2014届高考英语一轮复习阅读训练 (65)

★ 【北京专用】2014届高考英语一轮复习方案课时作业：选修6 Unit 2 Poems

★ 2017高考四川省广安市英语阅读理解一轮系列训练：6（含解析）

★ 2016最新英语第一轮复习的方针和策略

★ 湖南省汝城二中2014届高考英语一轮复习阅读训练 (63)

★ 2017届高考英语二轮复习课时训练：23（含解析）

★ 2016年高考英语第二轮复习策略与技巧

★ 湖南省岳阳市2014高考英语一轮单项选择训练题（5）及答案

★ 河南省三门峡市2014高考英语一轮（暑假）基础训练（2）附答案