Dream is in the process of sleep in a normal physiological and psychological phenomenon, is a kind of unconscious control special imagination.

Freud according to his theory of psychoanalysis, the dream as an unconscious process, is the expression of the repressed desire and conflict.
Pavlov's theory of higher nervous activity is that sleep is a dispersion of the cerebral cortex inhibition process, while the dream is in such conditions, the influence of various stimulus from outside the body and exciting activities.Modern physiology research suggests that sleep can be divided into fast wave and slow wave sleep two phase state, began to sleep, first into slow-wave sleep, into fast wave after 1 ~ 2 hours sleep, about half an hour into slow-wave sleep again.
The entire process of sleep, two kinds of sleep again and again about 4 ~ 5 times.Dream most appear in the stage of fast wave sleep.
It is generally believed that dreams are basically people at the time of awakening of thoughts, feelings, needs and desires to continue, it about the content of thought and awakening.