To establish the true meaning of the dream, Freud believed it was necessary to distinguish between the dream's manifest content - the actual symbols of the dream - and its latent content, the hidden desires that are too disturbing to be expressed consciously.
An alternative view, the activation-synthesis hypothesis of Hobson and McCarley proposes that dreaming is a consequence of random activity in the brain. During REM sleep, for reasons that are not particularly clear, cells in the hindbrain tend to spontaneously activate the higher centers of the brain.
Dreams are most likely to have vivid imagery during REM sleep, whereas images are vaguer and more fleeting during NREM sleep. Also, you tend to dream every time you are in REM sleep. Therefore, if you sleep for 8 hours and undergo five sleep cycles, you may have five dreams. Dreams may compress time the way a movie does by skipping over hours or days to a future time, but the actual action tends to take place in "real time."
Information-processing theory offers an alternative, more cognitive, view of dreaming. According to this view, dreams are the mind's attempt to sort out and organize the day's experiences and to fix them in memory. Consistent with this perspective, studies have revealed that interrupting REM sleep - and so interrupting dreams - impedes a person's ability to remember material that he or she has learned just before going to sleep.
Dreams filled with intense anxiety are called nightmares. The feeling of terror can be so great that the dreamer awakens from the dream, often crying out. Nightmares generally evoke feelings of helplessness or powerlessness, usually in situations of great danger.
People are engaged in lucid dreaming when they are aware that they are in the midst of a dream. The most unusual aspect of these states is that lucid dreamers can consciously alter the content of their dreams. This characteristic has led researchers to begin to explore the possibilities of utilizing lucid dreaming to treat nightmares and for other therapeutic purposes.
The emotional features in dreams would be related to the activation of amygdalar complexes, orbitofrontal cortex, and anterior cingulate cortex. Secondly, the relative hypoactivation of the prefrontal cortex would explain the alterations in logical reasoning, working memory, episodic memory, and executive functions which characterize dream reports obtained from REM sleep awakenings. Therefore, the origin of the dream emotionality would derive from the limbic activation, while the REMS frontal deactivation may cause the appearance of dream-associated executive impairment.
By all evidence, outrageously bad dreams are a universal human experience. Sometimes the dreams are scary enough to jolt the slumberer awake, in which case they meet the formal definition of nightmares — bad dreams that wake you up.
A big reason bad dreams offer insight into the architecture of dreams generally is that, as a host of studies have shown, most of our dreams are bad. Whether research subjects keep dream journals at home or sleep in research labs and are periodically awoken out of rapid eye movement, or REM, sleep — the stage most often associated with dreaming — the results are the same: about three-quarters of the emotions described are negative.
Regional EEG results suggest that the electrophysiological mechanisms involved in encoding and recall of episodic memories across wakefulness and sleep are the same. Brain lesions and functional neuroanatomy of healthy subjects identify two cortical systems crucial for dream generation, which are also involved in waking mental imagery and visuospatial abilities (i.e., the temporo-parieto-occipital junction) and in waking encoding and retrieval of episodic memories (i.e. the ventromesial regions of prefrontal cortex).