Sleep is a fundamental part of our lives. But we actually know very little about what our brain is doing while we're dreaming. Why do we need sleep? And what happens when we don't get enough sleep? Neuroscientist Ysbrand van der Werf dreams of solving the mystery of sleep. In his sleep laboratory he takes a peek into the sleeping brain of his subjects. And to understand the differences between good and bad sleepers, he collects information about the sleeping habits of the Dutch through slaapregister.nl.