Why do we need sleep? In the search for answers, scientists have uncovered more thought-provoking mysteries central to what sleep is, how it evolved and the benefits that it provides. In this episode, the mathematician and science communicator Steven Strogatz speaks with Dragana Rogulja, an assistant professor of neurobiology at Harvard Medical School who recently discovered how sleep deprivation causes death in fruit flies, and Alex Keene, a neurogeneticist at Texas A and M University studying cave fish to understand sleep's evolutionary history.