Dreams are one of the most common experience in our lives. The vivid and bizarre features of dreams have fascinated humankind throughout history. Dreaming is an ancient physiological and psychological phenomenon, and whether it fulfils an adaptive function has inspired varied theories. Some theories take a perspective that dreaming serves no adaptive function but rather a biologically epiphenomena. Some theories claim that dreaming fulfill various neurophysiological functions. However, the dream contents are viewed as random noise generated by the sleeping brain. Other theories believe that what appears in dreams is important since it serves functions which relate to psychological adjustment. More recent theories tend to relate dreaming to intellectual or cognitive functions, such as learning, memory consolidation, problem solving or simulating of threats in real life. The enigma of dreams is extensively studied with the development of science technology. Although the dream science was initiated from the REM (rapid eye movement sleep) sleep dreams, the equation ″REM=dreaming″ is an oversimplification. Dream-like mentation may occur during all stages of sleep. The controversy now is whether dreams reported from REM and NREM (non-rapid eye movement sleep) sleep are equivalent or different from each other. Three different theoretical models about the REM/NREM mentation have been proposed: (1) An one-generator model, which claims that regardless of the sleep stage in which the mention occurs, it only stems from a single set of process; (2) a two-generator model, which holds a perspective that REM and NREM mentions have differentiated generation systems; (3) a covert REM sleep model, which assumes that the NREM sleep dream is generated from the covert activity that occurs in REM sleep. Since there are experimental evidences supported both the similarities and differentiation in phenomenological aspects and the underlying brain activities of REM and NREM sleep dream, it is still controversial which model is the most optimal interpretation for the two mentation. The dream is a special state of consciousness. The continuity and discontinuity between waking and dreaming has been a controversy in dream science. The similarities and differences in both phenomenon and brain activity between dreaming and waking provide important knowledge of human consciousness, especially lucid dreaming, a hybrid state of consciousness with features of both waking and dreaming, which provides an unique chance to reveal the neural basis of consciousness. Moreover, dream studies are significant in the clinical practice in psychiatry and psychology. Although advances in neurophysiological and neuroimaging methods have offered new insight into the brain correlates of dreaming, the private nature of dreaming has made it a formidable study object. The studies of dreaming are still confronted with methodological challenges. To reveal the mystery of dreaming, integration of the designs and methods from multiple fields (such as phenomenology, neuroscience, clinical practice, etc) is necessary.
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