问题：

[单选,材料题] The subject of this passage is ________．

根据以下内容，回答下面的题目。
Researchers have found that REM (rapid eye movement) sleep is important to human beings．This type of sleep generally occurs four or five times during one night of sleep lasting five minutes to forty minutes for each occurrence．The deeper a person’s sleep becomes, the longer the periods of rapid eye movement．
There are physical charges in the body to show that a person has changed from NREM (non-rapid eye movement) to REM sleep．Breathing becomes faster, the heart rate increases, and, as the name implies, the eyes begin to move quickly．
Accompanying these physical changes in the body is a very important characteristic of REM sleep．It is during REM sleep that a person dreams．
According to the passage, how often does REM sleep occur in one night?

A . why people sleep
B . the human need for REM sleep
C . the characteristic of REM sleep
D . physical changes in the human body