Dreams are fantasies, usually visual, experienced during sleep and in certain other situations. About 25% of an adult's sleeping time is characterized by rapid eye movements (REM) and brain waves that, registered on an electroencephalograph, resemble those of a person awake (EEG). This REM-EEG state occurs in a number of short periods during sleep, each lasting a number of minutes, the first coming some 90 minutes after sleep starts an the remainder occurring at intervals of roughly 90 minutes. It would appear that it is during these periods that dreams take place, since people woken during a REM-EEG period will report and recall visual dreams in 80% of cases; people woken at other times report dreams only about 40% of the time, and of far less visual vividness. Observation of similar states in animals suggests that at least all mammals experience dreams. Dreams can also occur, though in a limited way, while falling asleep (hypnogogic) or waking (hypnopompic); the origin of these is not known.
Dream interpretation seems as old as recorded history. Until the mid-19th century dreams were regarded as supernatural, often prophetic; their possible prophetic nature has been examined in this century by, among others J. W. Dunne. According to Sigmund Freud, dreams have a latent content (the fulfillment of an individual's particular unconscious wish) which is converted by dreamwork into manifest content (the dream as experienced). In these terms, interpretation reverses the dreamwork process.