10 Theories About Why We Dream
The Purpose of Dreams
Nearly all mammals and birds have dreams, which suggests they serve an evolutionary function. In humans, these involuntary simulations can last from a few seconds to 20 minutes, with around 2 hours of sleep dedicated to dreaming each night. Nearly all dreams occur during rapid eye movement (REM) sleep, in which the body undergoes a number of physiological changes including increased brain activity, heart rate, and breathing rate.
The hidden purpose of dreams has been a source of intrigue and speculation for at least five millennia. For as long as humankind have been able to record their experiences in writing, dream interpretation has been a topic of interest. Ancient cultures such as the Sumerians, Egyptians, and Greeks often regarded dreams as prophetic messages from their deities.
In the 19th century, Sigmund Freud surmised that dreams are a gateway into our deepest desires and fantasies, though his undesirable methodology led to the theory being discredited. Today, our understanding of the function of dreams is limited to around 10 theories, each of which is supported by some degree of scientific evidence.
1. Memory Consolidation
Some studies have shown that REM sleep serves to improve procedural and spatial memory. This theory therefore suggests that dreams organize and store short-term memories of recent events within long-term memory. However, there is contradictory evidence from a number of experiments to suggest that memory is not improved by dreaming. Indeed, an individual with a brain lesion that inhibited REM sleep had no detectable memory degradation.
2. Dreams `Unlearn' Useless Memories
Some theorists have proposed that dreams function to `unlearn' useless memories or `noise' that has been acquired during the day. This leaves space for relevant, useful memories to be strengthened. Once again, the theory suggests that dreams should ultimately improve one's ability to perform memory-based tasks. Furthermore, the theory must explain why we remember dreams that appear to be nothing more than irrelevant noise.
3. Dreams Are Long-Term Memory Excitations
In 2003, Eugen Tamow suggested dreams are produced by the operation of our long-term memories during a period of unconsciousness. When our conscious minds switch off during sleep, the ever-present signals produced by our long-term memories are able to leak through into the rest of the brain.
These signals or `excitations' will be abstract representations of how recent events relate to older memories. Our unfamiliarity with these sensations may explain the surreal content of dreams, their vague relation to recent events, and the appearance of images from the distant past.
This fascinating theory therefore proposes that dreams are always present, but they only seep through during the night when our ability to suppress them weakens. More supporting evidence is required, though it does explain the peculiar content of dreams, and the inconclusive experiments. Indeed, as long-term memory is operating in the background, regardless of whether we are conscious or not, no improvement to memory is proposed.
4. Ontogenetic Hypothesis of REM Sleep
Studies have shown that children who experience sleep deprivation are likely to suffer from reduced brain mass, neural degradation, and subsequent behavioral disorders. As a result, dreams are proposed to stimulate the brain during times of rest; encouraging brain development and preventing cell death. Indeed, we dream less as we get older, indicating a developmental function.
The theory claims that dreams serve no function in the mature brain. It also suggests that dreams are meaningless thoughts exuded by the working brain, which are subsequently interpreted in a narrative fashion. Thus, the patterns and themes seen in dream content across many test subjects (see next section) would seem to disagree with the theory.
5. Threat Rehearsal Theory
Extensive investigation into the content of dreams has revealed that we are three times more likely to experience negative emotions while dreaming than positive emotions. The most prolific emotion is anxiety, which has an evolutionary function to prepare individuals to deal with threats by considering negative outcomes of potential future events. Thus, anxiety lends itself to simulation, and the content of dreams may be a manifestation of this paranoia.
In order to simulate threatening events that are useful to the individual, the brain needs to be creative, and studies have indeed shown that sleep aids creative and insightful thought by incorporating and reorganizing information in the brain. However, not all dreams are unpleasant, suggesting the theory may be incomplete. Furthermore, dreams are often difficult to understand, reducing their preparatory value.
6. The Tonic Immobility Reflex
According to a recent theory, dreams are a by-product of the body paralyzing itself as a defense mechanism during sleep. The tonic immobility reflex, or `playing dead', is used by many mammals and reptiles as a last line of defense against predators. The physiological changes that occur during REM sleep (such as paralysis) mimic this reflex.
The theory suggests that dreams are a `threat rehearsal' designed to prepare the individual for a dangerous awakening. Indeed, we often incorporate external stimuli into our dreams (e.g. noises), allowing for their immediate use in the real-world. One issue with this theory is the rapid eye movement that gives REM sleep its name. This and an increased breathing rate would show a predator that one is very much alive!
7. Dreams Prevent Heat Loss
Dreams and REM sleep may be required for basic physiological functions such as warming the brain and lubricating the eye. Experiments have shown that rats prevented from entering REM sleep will die from hypothermia. Thus, it's feasible that dreams serve to keep the brain active, which in turn keeps it warm. Indeed, periods of REM sleep are spread throughout the night, with the longest periods towards the end of the night (usually the coldest time). However, studies have shown that thermoregulation decreases during REM sleep, with an overall drop in body temperature.
8. The Sentinel Hypothesis
In rats, rabbits, and some other mammals, REM sleep is proceeded by a short period of wakefulness. Even though humans continue to sleep after episodes of REM, it's easier to wake from it than from normal `deep' sleep. This suggests that REM sleep evolved as a way to place animals into a semi-wakeful state in order to scan the environment for threats. For example, external stimuli such as noises and smells are often incorporated into dreams, signifying some level of contact with the environment.
For this theory, the purpose of dreams would be to interpret and incorporate external stimuli into possible narratives that may then trigger a warning signal. In the absence of external stimuli, recently experienced stimuli (such as the events of the previous day) might be used instead.
9. A By-product of Sleep Paralysis
Sleep paralysis is one of the physiological changes that occurs during REM sleep. It's caused by a suppression of various neurotransmitters in the brain. This shutdown may be needed to give the brain's receptors of these chemicals time to regain maximum sensitivity. While these receptors are suppressed, the brain may develop a kind of feedback system in which sensory data is harvested from the memory. Dreams may be the result of these internalized sensations, making them a functionless by-product of sleep paralysis.
10. Dreams Have No Purpose
Perhaps dreams have never served a purpose. Our inability to find an answer might suggest there isn't one to find. While this may be a desirable conclusion for the non-scientist to draw, it's an unlikely one. Evolution is characterized by the development of biological traits that function to overcome specific problems within our environment. Even if dreams have no direct function, they should at least be a by-product of something that does. Indeed, the psychological damage associated with a lack of REM sleep supports this reasoning.
Whatever the purpose of dreams, studies in the areas of neurobiology and psychology will continue to amaze and mystify us until a prevailing theory is found. Ultimately, discovering the function that dreams serve is a necessary step towards more attractive advances in oneirology. For example, the prospect of stimulating, controlling, and recording the content of dreams is a tantalizing potential avenue for future research.
© 2013 Thomas Swan