WASHINGTON, Oct. 17 (Xinhua) -- Scientists said Thursday they have found what could be sleep's basic purpose: it clears the brain of toxic metabolic byproducts. The findings, published in the U.S. journal Science, revealed that the brain's unique method of waste removal, dubbed the glymphatic system, is highly active during sleep, clearing away toxins responsible for Alzheimer's disease and other neurological disorders. Researchers from the University of Rochester, New York, also found that during sleep the brain's cells reduce in size, allowing waste to be removed more effectively. "This study shows that the brain has different functional states when asleep...