"Blessed are the sleepy ones," writes Nietzsche, "for they shall soon drop off." Sleep is an extraordinarily, albeit profoundly odd, phenomenon, yet we seem to accept prolonged nightly blackouts without question. Still, sleep has played a major role in philosophical thought, with the likes of Aristotle, Locke, and Leibniz putting forth theories about just what exactly sleep and dreams are. So what is the purpose of sleeping and dreaming? How can we distinguish wakefulness from sleep, as Descartes wondered? Do we experience dreams consciously? And do we sleep to live, or live to sleep? Ken and guest co-host Jorah Danenberg stay up with Deirdre Barrett from the Harvard Medical School, co-author of The Encyclopedia of Sleep and Dreams.