We spend approximately one third of our lives asleep and require sleep to stay alive—chronic sleep deprivation is fatal. Sleep has been rigorously studied over the last 60 years; however, the function of sleep and dreams remains elusive. This is in part because sleep is a complex state of consciousness involving many interconnected neurochemical and anatomical systems. This lecture will review the current knowledge about what happens in a sleeping brain and the consequences of sleep deprivation. In particular, Tartar will discuss the role of sleep in memory consolidation, emotion processing, hormone regulation, and immune functioning.