Everyone knows sleep. But what is sleep, really? The more one delves into this everyday, or rather nightly, state, the more uncertain one becomes and realizes how much of normal sleep remains shrouded in mystery. Sleep is an incredibly fascinating and mysterious part of our existence. Renowned sleep researcher Jürgen Zulley explains various facets of sleep, such as sleep duration, restorative sleep, beauty sleep, the influence of the moon, and the relationship between diet and sleep, among many others, in an understandable and entertaining manner, all based on scientific findings.