With two appearances on CBS This Morning and Fresh Air's most popular interview of 2017, Matthew Walker has made abundantly clear that sleep is one of the most important but least understood aspects of our life. Until very recently, science had no answer to the question of why we sleep, or what good it served, or why we suffer such devastating health consequences when it is absent. Compared to the other basic drives in life—eating, drinking, and reproducing—the purpose of sleep remains more elusive.