ABSTRACT
Dreaming is the most commonly occurring altered state of consciousness (ASC). In ASCs the way consciousness functions and the kind of experiences it contains is substantially different from a baseline state that is considered the standard or the normal state of waking consciousness. The history of dream research parallels the history of consciousness science: a promising start during the introspectionist era, then the Dark Ages in the grips of behaviorism and psychoanalysis, followed by two strictly separate research programs – the cognitive and the neuroscientific. The cognitive approach regarded dreaming as normal mental activity similar to waking consciousness. In lucid dreaming the realization that "this is a dream" does take place, but lucidity is a relatively rare phenomenon. The internal images may also become mixed with externally generated perception, as in hypnagogic and hypnopompic hallucinations, or they may guide external behavior, as in sleepwalking and in REM sleep behavior disorder or dreamwalking.
