"The brain uses sleep to wash away the waste toxins built up during a hard day's thinking, researchers have shown. The US team believe the "waste removal system" is one of the fundamental reasons for sleep. Their study, in the journal Science, showed brain cells shrink during sleep to open up the gaps between neurons and allow fluid to wash the brain clean. They also suggest that failing to clear away some toxic proteins may play a role in brain disorders." (Quoted from article)