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Sleep scientists have long studied why humans spend roughly a third of their lives unconscious, a state that seems —puzzlingpuzzledpuzzlepuzzlement given the risks it poses to a vulnerable, immobile body. Recent research suggests sleep allows the brain to —clearcleancleanseclarify metabolic waste products that accumulate during waking hours. Chronic sleep deprivation, by contrast, has been —linkedlinkinglinklinkage to impaired memory and a weakened immune response.