A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep. Humans spend about two hours dreaming per night, and each dream lasts around 5 to 20 minutes, although the dreamer may perceive the dream as being much longer than this.The content and function of dreams have been topics of scientific, philosophical and religious interest throughout recorded history. Dream interpretation, practiced by the Babylonians in the third millennium BCE and even earlier by the ancient Sumerians, figures prominently in religious texts in several traditions, and has played a lead role in psychotherapy. The scientific study of dreams is called oneirology. Most modern dream study focuses on the neurophysiology of dreams and on proposing and testing hypotheses regarding dream function. It is not known where in the brain dreams originate, if there is a single origin for dreams or if multiple regions of the brain are involved, or what the purpose of dreaming is for the body or mind. The human dream experience and what to make of it has undergone sizable shifts over the course of history. Long ago, according to writings from Mesopotamia and Ancient Egypt, dreams dictated post-dream behaviors to an extent sharply reduced in later millennia. These ancient writings about dreams highlight visitation dreams, where a dream figure, usually a deity or a prominent forebear, commands the dreamer to take specific actions and may predict future events.
Dreams have also featured in fantasy and speculative fiction since the 19th century. One of the best-known dream worlds is Wonderland from Lewis Carrolls Alices Adventures in Wonderland, as well as Looking-Glass Land from its sequel, Through the Looking-Glass. Unlike many dream worlds, Carrolls logic is like that of actual dreams, with transitions and flexible causality. Other fictional dream worlds include the Dreamlands of H. P. Lovecrafts Dream Cycle and The Neverending Storys world of Fantastica, which includes places like the Desert of Lost Dreams, the Sea of Possibilities and the Swamps of Sadness. Dreamworlds, shared hallucinations and other alternate realities feature in a number of works by Philip K. Dick, such as The Three Stigmata of Palmer Eldritch and Ubik. Similar themes were explored by Jorge Luis Borges, for instance in The Circular Ruins. Modern popular culture often conceives of dreams, as did Freud, as expressions of the dreamers deepest fears and desires. In speculative fiction, the line between dreams and reality may be blurred even more in service to the story. Dreams may be psychically invaded or manipulated (Dreamscape, 1984; the Nightmare on Elm Street films, 1984–2010; Inception, 2010) or even come literally true (as in The Lathe of Heaven, 1971). Lucidity
Lucid dreaming is the conscious perception of ones state while dreaming. In this state the dreamer may often have some degree of control over their own actions within the dream or even the characters and the environment of the dream.