Sleep，asmysteriousasitisvitalforourwellbeing，ispartofaperson'sdailyphysiologicalactivity．Overthedecades，researchershaveproposedseveralmechanismsthroughwhichsleephelpsus（41）___，butwestilldon'tfully（42）__

　　Sleep，as mysterious as it is vital for our wellbeing，is part of a person's daily physiological activity．Over the decades，researchers have proposed several mechanisms through which sleep helps us（41）___，but we still don't fully（42）___the big picture．Now，two recently published studies come up with an interesting（43）___：we sleep to forget some of the things we learn during the day．

　　We store memories in networks in our brains．Whenever we learn something new，we（44）___new connections between neurons（神经元），called synapses（突触）．In 2003，two biologists proposed something very（45）___：during the day，we learn so much and develop so many synapses that things（46）___get ourselves confused．

　　For starters，they showed that neurons can prune out（修剪）some synapses，at least in the lab．But they（47）___the same things happens every day，naturally，in our brains-probably during（48）___．

　　So they set up a painstaking experiment，in which an assistant scientist collected 6，920synapses from （49）___，both awake and sleeping．Then，they（50）___the shape and size of all these synapses，learning that the synapses in sleeping mice were 18percent smaller than in awake ones．

　　After this，they designed a（51）___test for mice．They placed the animals in a room where they would get a mild electrical shock if they walked over one particular（52）___of the floor．They injected（注射）some of them with a substance that had been proved to prevent the pruning of new synapses．The mice that experienced this were（53）___likely to forget about the section and after a good night's sleep，they（54）___to walk as usual，while mice that slept normally remembered better．

　　Then，they found that the pruning didn't strike every neuron．Some 20% were unchanged，likely well-established memories that shouldn't be tampered with（被窜改）．In other words，we sleep to（55）___-but in a smart way．

　　41．A．releaseB．removeC．refreshD．relieve42