Think of everything your brain processes in a single day: your breakfast, a stain on a book cover, a meeting at work. If you remembered all those things, your brain would reach capacity. Author and neuroscientist Penelope Lewis says sleep helps sort through the memories that are worth keeping.
Neuroscientist Dean Buonomano explains why our brains make mistakes when we try to remember long lists of information or add large numbers in our heads. Humans live "in a time and place we didn't evolve to live in," he says.