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    We humans spend about one-third of our lives asleep. This may sound like a lot of time, but it is not wasted. Sleep not only helps us stay healthy but it also helps our brains remember. Our brains need good sleep to remember what we do and learn during the other two thirds of our lives when we are awake.

    Researchers at the University of Wisconsin-Madison have found that sleep helps improve brain performance by shrinking(收缩)synapses in the brain. A synapse is the area where cells pass messages to other cells.

    Scientist Chiara Cirelli is a leader at the school's Center for Sleep and Consciousness. She told the reporter that sleep is when the human brain mixes information it has learned while awake into its general collection of knowledge. Meanwhile, the brain forgets unimportant details. This forgetting is important. It makes space for new learning and new memories.

    Cirelli said that the Centers research began with this hypothesis(假设): We sleep so that our brain can repair and refresh itself. She said the idea seems simple and reasonable. However, testing and discovering how it works has been extremely difficult.

    Synapses are only about 20-40 nanometers(纳米)wide. The team began their study by measuring the size of the synapses to look for changes in these already tiny spaces between nerve cells. Cirelli says the process(过程) is difficult because"all the actual measurements of the synapses have to be done by hand." The team had to wait until improvements in laboratory technology made it possible to see these tiny changes. A University of Wisconsin press release called the research a"huge job." Many research specialists worked for four years to photograph, rebuild and study certain areas of a mouse brain. The report also said the scientists measured 6,920 synapses.

    Cirelli says they found that our synapses shrink as our brains clean themselves during sleep. We wake up refreshed and ready to fill those synapses with new information. The research findings are the result of years of hard work at the University of Wisconsin Madison. The researchers published their findings in the journal Science.