13．Just as our bodies produce waste every day-which is why we need to shower and use the toilet-our brains produce harmful waste proteins.But how can our brains "shower" themselves？A 2013 study found the answer.Researchers at the US University of Rochester studied the brains of mice and discovered that they cleaned themselves while the mice slept.A kind of fluid in the brain，called cerebral spinal fluid（CSF 腦髓液），was found to increase dramatically during sleep，washing away waste proteins that had been building up between braincells in waking hours.
"This study shows that the brain has different functional states when asleep and when awake， "Maiken Nedergaard，the lead researcher，told NBC News.It also explains why we can't think clearly after a sleepless night while a good night's sleep leaves us feeling sharp and refreshed.
Now a new study，published on October 31 in Science，digs a little deeper into our brains' self-cleaning procedure.Instead of mice，this time humans were the test subjects.
Researchers at Boston University，US，monitored the brain waves of 13 healthy adults who were sleeping，using accelerated FMRI，which is capable of recording faster changes inside the brain than a regular FMRI machine.They found that every 20 seconds，blood flowed out of the brain，making room for a large amount of CSF to come in and "clean".This cycle coincided （巧合）with the rhythm of the brains slow waves -an electrical activity that happens when we're in deep sleep.
It's still unknown how these brain activities are connected.But the mere fact that they are connected is exciting enough，since it allows researchers to piece together possible new explanations for misunderstood diseases.
For example，slow-wave sleep has been proven to play a role in strengthening our memories.This may explain why people with Alzheimer's often have fewer and weaker slow brainwaves.Based on this new study，there could be one more explanation for diseased brains：They are not clean.
Nedergaard，leader of the 2013 study，is also excited about the new findings. "Maybe the most important take-home message is that sleep is a serious thing， " she told Scientific American. "You really need to sleep to keep a healthy brain because it links electrical activity to a practical housekeeping "function'. "
（1）The body waste is mentioned at the beginning of the passage to
.
A．show the similarity between body and brain
B．explain the main components of brain waste
C．introduce the topic of passage through comparison
D．remind us of the necessity of regular body cleaning
（2）What is the passage mainly about？
A．The self-cleaning function of human brains.
B．The importance of sleep at night for humans.
C．The cause and possible cures of diseased human brains.
D．The different states of human brains at daytime and night.
（3）The findings of the new study are important because they
.
A．a(chǎn)re based on more advanced technology
B．give us new insight into some brain diseases
C．confirm the connection between brain activities
D．reveal the process of brains cleaning themselves
（4）What can we learn from the passage？
A．Slow brainwaves are caused by the brain cleaning process.
B．The diseased brains break down for not cleaning themselves.
C．Sleep is the most important factor for improving our memory.
D．Slow brainwaves and brain cleaning occur with identical frequency.
