This issue dives into the study of sleep function, particularly as it relates to memory and cognition. Any clinician who sees patients with sleep disorders, or in particular any sleep medicine specialist, will find this information enlightening and invaluable, as it discusses the current state of understanding of how sleep affects humans' waking cognitive functions. These review articles describe the research that has taken place, and the lessons that can be taken away from them, so that clinicians can confidently advise their patients on the functional importance of adequate sleep, and recognize cognitive symptoms of inadequate sleep. Articles discuss such topics as animal and human research on sleep and memory, various imaging techniques to describe brain activity during sleep, and the role of dreams.