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1. The most vivid and "realistic" dreaming occurs in rapid eye movement (REM) sleep, which is concentrated more in the second half of the night. While some dream activity does occur during deep (or slow wave) sleep as well, it is much less formed and structured, and recall of the content of those dreams is much more limited.
2. In REM sleep, there is loss of almost all muscle tone, except for the diaphragm and eye muscles. This explains why it is very common for people to describe feeling paralyzed and unable to move while dreaming of a stressful situation (such as being chased).
3. That being said, there is a rare condition known as REM behavior disorder in which people violently act out their dreams, often to the point of causing injure to themselves and/or their bed partners. This is mostly seen in older men, and can be a harbinger of degenerative neurological disorders, such as Parkinson's disease.
4. Certain medications can make dreams even more vivid. Beta blockers, a class of medications often prescribed for certain types of heart disease, are known to do this. Some people with narcolepsy who were prescribed amphetamines have reported hallucinations so extreme that they were initially given the diagnosis of schizophrenia, until it was realized what the underlying cause of the hallucinations was and they were seen to disappear quickly upon discontinuation of the medication.
5. Dream as the guardian of sleep (1): unpleasant or otherwise disruptive (to sleep) external stimuli are often sublimated and incorporated into a dream in order to "explain it away" and allow the brain to continue sleeping without having to respond. For example, a sleep deprived individual may hear the ringing of the alarm clock and transform that into the bell of a fire engine.
6. Dream as the guardian of sleep (2): False awakening is a phenomenon wherein a person has a dream within a dream, dreaming that he has awakened, but in fact continues directly into a second dream, avoiding the need to awaken. An example of this can be found in a child who wets his bed at night, who becomes aware of his increasing full bladder, and dreams of being awoken by his parent and taken to the bathroom, and who allows himself to begin urinating while standing over the toilet in his dream, though he remains very much in his bed, as he sadly discovers when he feels the warm wetness slowly soaking his pajamas and sheets.
7. Dream as the guardian of sleep (3): Freud held that a purpose of dreams was to allow the psyche to take un-acted upon impulses and desires and fulfill them away from the conscious mind, so as to resolve them and be free of the anxiety that even recognizing their existence generates, which in turn can disrupt or prevent sleep. Examples of these are socially unacceptable sexual desires (a man sexually attracted to his wife's sister can resolve this impulse in the setting of a dream, according to Freud, without having to perseverate upon it and suffer the ongoing frustration of not fulfilling it, the anxiety that even acknowledging this desire can trigger, and the social recriminations of acting upon it).
8. Another theory about dreaming holds that dreams embody the memory consolidation processes, learning and unlearning which occur during the different stages of sleep, especially slow wave and REM sleep. As new information is integrated into memory, the strengthening of synaptic connections and the effacement of others leads to the generation of imagery which manifests as dreams.
9. Specific, stereotypical sleep patterns seen in people with mood disorders are distinctly different from those seen in people without mood disorders. There is a shorter amount of time between sleep onset and REM onset, and the initial REM periods are longer in people with depression and/or manic episodes. How these are connected is not understood.
10. Some people are able to "take charge" of their dreams, and manipulate them in certain directions through a technique known as lucid dreaming. Use has been made of this to effectively reduce nightmares and to treat depression and self mutilation.
11. Many cultures attribute prophetic significance to dreams (an example of this can be found in the story of Joseph in the book of Genesis). Others are more skeptical. Aristotle wrote a treatise on dreams 2400 years ago in which he stated that "most so-called prophetic dreams should be classified as coincidences" ("On Prophesying by Dreams". Aristotle, translated by J. I. Beare, http://classics.mit.edu/Aristotle/prophesying.html).
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Dennis Rosen, M.D.
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