But invariably the students say they can't remember three days later what they studied, Stickgold said. Those findings fit well with Krueger's theory, that sleep developed to enable nerves to process information. Then, it seems, nature also began to use sleep for other purposes, such as memory, readjusting hormones, repairing the body, fighting infections and other functions. It's like the lungs. They evolved to exchange oxygen and carbon dioxide, but later they also were put to use to produce voice.
Nature likes to reuse things that work.