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While we sleep, our bodies rest from the events of the day and recharge in order to face the next round of challenges it will face during its waking life. During sleep, our brains produce a fractured, often nonsensical amalgamation of random events and people, otherwise known as dreams. These dreams often provoke powerful reactions of fear or pleasure, as, for all their improbability, they follow reality in such a way as to trick the dreamer into reacting to this fantasy world as if it actually existed. However, although dreaming undeniably is a large and memorable part of one's nightly sleep cycle, scientists have yet to define for certain the biological function of dreams. Some believe dreams are a remnant of our Neanderthal past, when our ancestors used dreams as a sort of training ground for developing appropriate reactions in the life or death struggles they faced every day. Others thinks dreams simply stem from random impulses which produce images from one's daily life with no particular significance. Others believe that dreams serve to "clean out" the emotional stress accumulated during the day. Whatever the hypothesis, it is difficult to prove for certain the purpose of dreams.
Researchers have identified four distinct stages to the sleep cycle (1). Of these, the phase known as Rapid Eye Movement (REM) is most closely associated with dreaming. The REM phase, characterized by rapid heart rate, distinct brain waves, and an increased amount of electrical activity in the brain, produces the most vivid and memorable dreams (2). Initially, scientists believed that dreams only occurred during REM sleep. While studies have identified dreams during other phases of the sleep cycle, the most powerful dreams are still associated with REM sleep(1). Previously, scientists and psychologists believed that dreams were simply a byproduct of the functions of REM sleep, but the discovery of the possibility of dreams occurring during non-REM stages of the sleep cycle undermines the validity of this theory. This has led many in the scientific community to develop new and often farfetched theories of the biological function of dreams.
The formal study of dreams first began with psychoanalysts like Sigmund Freud, whose dream theory of 1900 served as an influential and widely accepted conception of why humans dream. Freud believed that dreams reflected the baser impulses of the human subconscious, impulses which could not be acted upon in society. His observations were based on subjects whose disturbing dreams haunted them even while they were awake (2). However, while dreams certainly can reflect the subject undertaking actions which they would never have the opportunity to do in real life, Freud's theory seems to imply that only the seriously troubled dream. Research has shown that all humans dream, whether or not they remember their dreams the next day.
In the 1960s and 70s, researchers at the Harvard Laboratory of Neurophysiology focused on observing the biological causes of REM sleep in order to better understand why humans dream. They discovered that REM sleep is induced by the release of the brain chemical acetylcholine. The release of this chemical stimulates nerve impulses which recreate random bits of one's internal information in a sequence which may not conform to logic. J. Allan Hobson and Robert McCarley, the primary researchers at the Harvard laboratory, named this new theory the activation-synthesis hypothesis. From this hypothesis, Hobson developed an idea of dreaming not as an arena in which to explore hidden urges, but as an opportunity for mental "housekeeping". He also believed that dreams could serve to solidify emotional ties to memories (2).
Since scientists like Hobson established a biological basis for why humans dream, other researchers have developed their own theories regarding the purpose of dreams, while undermining others' hypotheses. Hobson's concept of dreams as an opportunity for mental reorganization has been criticized as research has shown that very little of the day's events recurs in that night's dreams (4). Rather, dreams tend to deal with larger issues of conflict and emotions, which has led others to develop a concept of dreams as stimulated by a threat simulation mechanism, a remnant of the days when humans faced life or death struggles on a daily basis. This theory also takes into consideration the recurring dreams of war veterans and trauma victims, as in these cases the brain attempts to present the dreamer with the former conflict again and again in order to prepare them to deal more effectively with such a catastrophe in case they are ever in such a position again (3). Many agree with this theory in part, as they recognize the problem solving aspect of dreams, but may not believe in the existence of a threat simulation mechanism (5). Still others believe dreams have no biological function at all, only a cultural significance assigned by human attempts to make sense of dreams (4).
Humans may never discover the actual biological function of dreams. While many theories of dreams as an opportunity to reorganize one's thoughts and to solve problems sound feasible, it is difficult to prove anything conclusively, due to the relative youth of neurobiology and the shadowy nature of dreams themselves. As our general understanding of the brain develops, scientists may be better able to understand why we dream.
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