The most common altered state of consciousness is sleep. During sleep, we may dream. In addition to ordinary dreams, we could have lucid dreams, in which we know we are dreaming, and precognitive dreams, in which we appear to dream of events before they occur. Sleep has historically served as a prototype for other altered states of consciousness and, in particular, hypnosis. Hypnosis is a state of apparently heightened suggestibility. It appears to be due to a compliant attitude toward hypnotic suggestions, the capacity for vivid fantasy, or the dissociation of elements of the psyche, depending on the individual. The state of being hypnotized has often been regarded as a trance: a state in which a person behaves as though aware of what she or he is doing, but in which the person is actually acting involuntarily, with decreased responsiveness to surrounding events. Trance can occur in contexts other than hypnosis, such as possession, in which a person appears to have come under the influence of an unseen spirit, or mediumship, in which there is apparent communication with the dead or other ostensibly nonphysical entities.