When people sleep, nerve cells are extensively suppressed, but this inhibition process is incomplete. As a result, some nerves in the cerebral cortex are still in an excited state, resulting in dreams. It can be seen that dreams occur when a few cells in the cerebral cortex are active. If the activities of a few cells lose the control and regulation of the entire cerebral cortex during the awakening state, certain fragments in memory will be resurrected without restraint, and people will have strange dreams. If during sleep, the few cells in an excited state are some nerve cells in the cerebral cortex related to language or exercise, then the phenomena such as sleeping talking and sleepwalking that people usually see will occur. Dreams are the result of neural activity during human sleep, especially during rapid eye movement sleep. Dreams are also a psychological activity, the result of activities at a certain level of consciousness.

According to Freud, the human unconscious contains a large number of concepts, thoughts, desires, impulses, etc. These concepts and ideas are usually suppressed in the unconscious because of conflicts with social ethics and morality, and individuals cannot detect them. Freud compared human psychology to an iceberg. Human consciousness is the tip of the iceberg that emerges from the water, and the unconscious is the part below the water. Although the content in human consciousness cannot be realized, it can be revealed through one way or another. One of the important ways is dreams.

Dreams are the result of some nerve cell activity caused by some stimuli. Freud pointed out in his 'Interpretation of Dreams' that there are four types of stimulation for dreams: one is external sensory stimulation; the other is internal (subjective) sensory stimulation; the third is internal (physical) physical stimulation; and the fourth is stimulation of pure spiritual origin. Judging from current research results, the factors responsible for the formation of dreams are extremely complex, caused by multiple factors.

(Editor in charge：Dreams and Health)