课上内容（Lesson）
a fact n. 事实；实际；真相
take a nap (30-60min.) 睡午觉；小睡一下 # deep sleep
a power nap(20 min.) 一个小盹
词汇（Key Word ）
expert 专家
regular 规律
hardly 几乎不
recommended 推荐
exactly 正确地
sunset 日落，傍晚
during 在...期间
句型（Sentences）
In modern society 在现在社会
It depends 看情况而定
during the day 在白天期间
Why do we sleep?
From birth, we spend a third of our lives asleep,but scientists still don't know exactly why.
Why do humans have problems sleeping?
In modern society, many people don't get the recommended seven or eight hours a night. We work long hours and we rarely go to bed at sunset.
Why do we sleep differently?
It depends on the timeof yeuar and also our age.Teenagers always need more sleep than adults.Lots of elderly people don't sleep longer than four or five hours,but they often take naps during the day.
语法（Grammar）
Adverbs of frequency 频率副词
1. An advbers of frequency goes after the verb to be but before the man verb.
2. An expression of frequency usually goes at the beginning or at the end of a sentence.
always 总是
usually 通常
normally / generally 通常地
often / frequently adv. 常常，时常
sometimes 有时
occasionally adv. 偶尔；间或
seldom 很少
rarely 难得
never 从不
