Why do humans sleep?  What is sleep’s evolutionary basis? And what is really going on while we sleep?  This week, we broadcast a conversation with cognitive neuroscientist Matthew Walker, talking to Indre Viskontas, originally recorded in 2015.  Walker is an expert in sleep science, and his research reveals that every tissue in the body and every process within the brain is enhanced as we sleep – and impaired when we’re not sleeping enough.  His research also examines the effects of stress, medications, and alcohol on sleep, and the ways we can improve our sleeping habits.