Every animal that has been studied carefully seems to sleep in some way. This is strange, because sleeping is dangerous. An animal that is cannot look for food, for its young, or an coming near. If sleep brought no at all, one would that animals which slept less would better and that sleep would slowly . Instead, it has remained for millions of years, and even creatures with very simple brains show periods of that look much like it. This suggests that sleep does something important.
One thing that happens during sleep is the sorting of memories. During the day the takes in far more information than it can keep. While a person sleeps, patterns of from the day are repeated, and the experiences that matter are stored more firmly, while others fade away. In experiments, people who learned a list of words and then slept remembered more of the list than people who stayed for the same number of hours. Scans taken while volunteers slept showed the same cells firing in the same as they had during the itself.
Sleep also appears to clean the . Like the of the body, the produces as it works, but it does not have the same system for carrying that away. In the early 2010s, researchers reported that the spaces between cells grow wider during sleep, allowing fluid to through more easily and wash the out. Some of the substances removed in this way are the same ones found in large amounts in the brains of patients with diseases.
these findings, many people sleep less than they once did. light, night work, and screens have all pushed bedtimes later, and in some societies sleeping little is treated as a sign of a serious worker. Researchers that this is mistaken. Still, they also caution against simple rules. The often repeated that everyone needs eight hours is not supported by , since the amount required differs from person to person.
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Q1
The fact that animals have continued to sleep suggests that
Q2
What did the experiment with a list of words show?
Q3
What did researchers report in the early 2010s?
Q4
According to the passage, one reason people sleep less today is that
