Dreams are caused by some nerve impulses released by the brain when processing information and consolidating long-term memory, and are interpreted by the conscious brain as bizarre vision and hearing.
One of the functions of sleep is to transform temporary memory into long-term memory. REM sleep deals with unconscious "procedural memory" rather than conscious "declarative memory".
In the REM stage, the unconscious part of the brain is processing programmed memory, while the activity of the conscious part is reduced to a minimum because the sense is cut off. At this time, the information pulse from the memory bank will activate the conscious part and let it weave a dream through association. When another pulse came, I had another dream, and the dream suddenly changed.
Psychologists believe that dreams are the satisfaction of subconscious desires, and people can effectively suppress the subconscious when they are awake, so that those desires that violate moral customs cannot do whatever they want. However, when people fall asleep or relax, some desires will avoid the inspection of the subconscious, secretly emerge from the conscious level, and show themselves in various images. This is the formation of dreams.
Some psychologists also believe that human dreaming is a preview of how the brain handles dangerous situations in a virtual environment. Especially nightmares, human beings are trained in safety in nightmares.
From the perspective of psychoanalytic psychology, the meaning of dreams lies in the satisfaction of desires. In an unrealistic life, many endless desires cannot be satisfied immediately, or even at all. Desires that can't or can't be satisfied in real life can be psychologically satisfied in dreams and adjust psychological balance.