Dreams are experiences of mental activity that occur during sleep. Dreams can be understood from both a physiological as well as a psychological perspective.
Physiologically, sleep states are measured by an EEG (electroencephalograph) connected to the scalp by surface electrodes. These electrodes measure brain activity, called waves. Sleep can be divided into two distinct catagories measured by brain wave activity. These stages are REM (rapid eye movement) sleep and NREM (non-rapid eye movement) sleep.
NREM sleep is divided into four stages with the sleeper entering deeper states of sleep with progressively larger and slower brain waves. Stage 1 is the transition between wakefulness and Stage 1 sleep. Brain wave activity moves from beta, wakefulness, to alpha. Stage 2 is light sleep. The body prepares for deep sleep by moving through theta waves to delta waves. Stages 3 and 4 are deep sleep stages characterized by delta waves with Stage 4 being more intense.
In REM sleep, brain activity is hightened, resembling the awake state or Stage 1. The eyes, though closed, move back and forth, and muscle paralysis is present, perhaps as a means of keeping the dreamer safe. Heart rate and respiration increase. It is during REM states that dreams are most easily recalled. REM sleep is called "paradoxical" sleep because it incorporates both wakefulness and sleep.
In sleep experiments, subjects are awakened at different stages of sleep and asked to report dreams. While dreaming is reported in NREM sleep, by far the majority of dreams are reported during REM states. Some physiological theorists have stressed the importance of the adaptive function of dreams. Sites in the brain that are activated during dreams lead researchers to believe that the brain is rehersing events, mastering situations, or adapting to novel stimuli.
Psychologically, dream interpretation differs with different schools of thought.
Behaviorists stress the adaptive function of dreams similar to physiologists and do not look for unconscious contents or meaning outside the dream content.
For Freud and psychoanalysis, dreams are the "royal road" to the unconscious. They are the expression of repressed, conflicted, and mostly sexual and aggressive, desires and wishes. The meaning of a dream is not in the "manifest content" that the dreamer remembers but in the "latent content" that is mostly unconscious and accessed by free association. The latent content, being disturbing in nature, is censored by the primary process mechanism, and changed into symbolic form in the dream, so the dreamer will not become disturbed during sleep by fear or anxiety. Most dreams express a repressed wish, most often sexual or aggressive in nature and usually with origins in childhood. The wish is repressed because it is socially unacceptable. One example is the Oedipus complex where a boy wishes to kill his father and marry his mother. If this scenerio is not acted upon, the wish is repressed and sinks into the unconscious until such a time that it is activated by some event in the present. The then repressed and proscribed wish manifests in a symbolic form in a dream.
Analytical psychology, founded by C.G.Jung, considers dreams as organized communications from the unconscious to the conscious mind. Dreams are generated from the central organizing, yet unconscious, center of the psyche called the "Self". taken seriously, and seen as an adjunct to conscious thought, dreams clarify issues of everyday life and point out the direction to take to achieve "wholeness" or individuation, the integration of the psyche. Dreams speak through symbols and images using metaphor and the language of myth for communication. Dreams have access to the entire cultural myth of a civilization and are not limited to the dreamers knowledge or experience. This resevoir of information is called the collective unconscious. The images in the collective unconscious are called archetypes. Archetypes are universal images and motifs and are found in all myths, religions, and folklore. Such universal archetypes include the king, the hero, and the mother. Archetypes can also be of a personal nature and include images of the shadow, the anima, and the animus. Numinous dreams are the product of the collective unconscious. These are dreams whose content and images create a powerful emotion, linger in the mind, and are not easily dismissed. Numinous dreams can be communicatons about ones own life or speak to wider issues of a culture or country. Portentous dreams would fall into this catagory.
In general, dreams appear to encompass a variety of functions. It would be misleading to believe that dreams serve a single purpose.