Every animal sleeps,but the reason for this has remained foggy.When lab rats are not allowed to sleep,they die within a month. 71.________

　　One idea is that sleep helps us strengthen new memories. 72.________ We

　　know that,while awake,fresh memories are recorded by reinforeing (加强)connections between brain cells,but the

　　memory processes that take place while we sleep

　　have been unclear.

　　Support

　　is

　　growing for a theory that sleep evolved so that connections between neurons(神经元)in the brain can be weakened overnight,making room for fresh memories to from the next day.

　　73.________ Now we have the most direct evidence yet that he is right. 74.________ The synapses in the

　　mice taken at the end of a period of sleep were 18 per cent smaller than those taken before sleep,showing that the connections between neurons weaken

　　while sleeping.

　　If

　　Tononi`s theory is right,it would explain why,when we miss a night`s,we find it harder the next day to concentrate and learn new information-our brains may have smaller room for new experences.

　　Their research also suggests how we may build lasting memories over time even though the synapscs become thinner.The team discovered that some synapses seem to be protected and stayed the same size. 75.________ “You keep what matters,”Tononi says.

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