In an extensive 2012 literature review, the psychologist Matthew Merced notes that, even though nobody knows for certain why we dream, advances in the technology and techniques of brain research have at least helped explain how we dream. Humans, at least, dream a lot, multiple times a night, and the brain is very active during dream periods. Dreaming must be important, even if it remains mysterious.
Many researchers think that dreams are simply the product of chemical changes in the brain during sleep. In this view, dreams are evolutionary hitchhikers: REM sleep is beneficial, and dreams tag along with it. The fact that we mostly forget our dreams lends some support to this view: Memory formation systems are mostly turned off during dreaming.
Assuming there is a purpose, natural selection suggests that dreaming must provide some sort of survival benefit. Why spend energy on involuntary movements and brain activation if nothing is being achieved? One possibility is that dreams are kind of a virtual reality world, a space where humans can safely practice coping with threats (being chased, for example, is pretty common). REM sleep and dreaming may also help process important or traumatic memories.
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