Adequate, restful sleep is integral to our physical and mental health. Over the last 60 years we have learned more about sleep than we have over the previous 6,000 years. Culturally and philosophically we still maintain a predominantly Descartian view of what is in fact a highly differentiated, liminal state. Sleep is an active process that with ongoing research as to its function continues to illuminate our understanding of consciousness, wakefulness, unconsciousness, dissociation and states in between. This lecture will focus on the importance of sleep to human health, describe stages of sleep and also pathologies of sleep states that put in question our binary views of a process that is fundamental to the existence of all living creatures.