So what is happening during those precious hours when we're asleep? Is it really a time of restoration for our brains? And is it possible that it's more than that?
What happens in our brains while we're asleep is a question neuroscientist Penelope Lewis is trying to answer.
Lewis directs the Sleep and Memory Lab at the University of Manchester in England. In her new book, The Secret World of Sleep: The Surprising Science of the Mind at Rest, she discusses how sleep makes memory stronger, provides what she terms "spring cleaning" for the brain, and plays a role in depression.