There is evidence that certain medical conditions (normally only neurological conditions) can impact dreams. For instance, people with synesthesia have never reported entirely black-and-white dreaming, and often have a difficult time imagining the idea of dreaming in only black and white.

Therapy for recurring nightmares (often associated with posttraumatic stress disorder) can include imagining alternative scenarios that could begin at each step of the dream.

Dreams and psychosis

A number of thinkers have commented on the similarities between the phenomenology of dreams and that of psychosis. Features common to the two states include thought disorder, flattened or inappropriate affect (emotion), and hallucination. Among philosophers, Kant, for example, wrote that ‘the lunatic is a wakeful dreamer’. Schopenhauer said: ‘A dream is a short-lasting psychosis, and a psychosis is a long-lasting dream.’ In the field of psychoanalysis, Freud wrote: ‘A dream then, is a psychosis’, and Jung: ‘Let the dreamer walk about and act like one awakened and we have the clinical picture of dementia praecox.’

McCreery has sought to explain these similarities by reference to the fact, documented by Oswald, that sleep can supervene as a reaction to extreme stress and hyper-arousal. McCreery adduces evidence that psychotics are people with a tendency to hyper-arousal, and suggests that this renders them prone to what Oswald calls ‘microsleeps’ during waking life. He points in particular to the paradoxical finding of Stevens and Darbyshire that patients suffering from catatonia can be roused from their seeming stupor by the administration of sedatives rather than stimulants.

Griffin and Tyrrell go so far as to say that "schizophrenia is waking reality processed through the dreaming brain.

[From Wikipedia, the free encyclopedia]