Dr. Thomas Swan studied cognition and culture at Queen's University Belfast. He's researched a range of psychological traits and disorders.
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
kyleycrown on January 10, 2018:
I had a dream that I was in this abandoned mansion and I went up the stairs and heard this screaming. I went all the way up the stairs and saw this woman dangling from this hole in the ground. I look right at her and she was screaming and trying to tell me something, but I couldn't understand her because it was in a different language. I couldn't identify the language, but I don't think that it is a language that we know. I think that I made it up in my dream. I was stuck staring at her and I couldn't move to help her. What does this mean? Thanks!
Kristen Howe from Northeast Ohio on February 18, 2015:
This was interesting and fascinating to know, Thomas, about dreams. Voted up. Very insightful, too.
Thomas Swan (author) from New Zealand on September 10, 2013:
Thanks WiccanSage. Notable, threatening events will have that effect for sure. The brain will work hard to link negative events like that to memories and experiences that may be useful if the event were to ever happen again. The dreams may be a manifestation of these processes going on behind the scenes.
Mackenzie Sage Wright on September 09, 2013:
So interesting. I had a big scare and was in the hospital for about a couple of weeks in August. My dreams have been kind of crazy ever since. This actually explains a lot about some of my wild dreaming lately (between health concerns, financial concerns losing work, sitting around watching TV waaaayyy too much and other useless means of enteretainment). Great read, voted up.
Thomas Swan (author) from New Zealand on August 15, 2013:
I think it's probably a mixture of several. The evolutionary psychologist in me wants to say number 5, but I think that's a red herring. The most plausible in my opinion is number 3 - that our dreams are just our long-term memory working in the background, and this comes to the fore when our conscious brains switch off. Anxiety and other negative emotions are so prolific because our unconscious brains are naturally preoccupied with threats to our health and social reputation. That's why I think threat rehearsal is a red herring. The evidence for improved memory, I just don't buy yet, though I agree that dreams can aid creativity by reorganizing and interlinking new memories with old. That is essentially part of theory 3 anyway. Many of the other theories, such as the sentinel hypothesis and heat conservation are more to do with REM sleep, which I believe serves an important function. I would suggest that REM sleep "left the door open" for our long-term memory processes to infiltrate our brains in the form of dreams, but that REM sleep doesn't directly cause dreams. After all, we have some dreams in non-REM sleep; but it's less common.
Gratitude Journal on August 15, 2013:
Interesting. Out of these 9 theories, which one do you agree with Thomas?
Thomas Swan (author) from New Zealand on August 15, 2013:
Thanks wish-list-gifts, it could be because it has always been difficult to study dreams scientifically. Only with recent advances in neuroscience can we investigate their cause in more detail. Most of the theories described here come from neuroscience.
Clara Myers from USA on August 14, 2013:
This is the least answered question about dreaming. I've wondered why we developed the ability to dream and not many people talk about that so thanks for this article - very informative.
Thomas Swan (author) from New Zealand on August 14, 2013:
Thanks epbooks. This one took a lot of research so I'm glad you found it interesting. We forget 95% of our dreams, so you're lucky to remember so many. I remembered a dream last night in which I was eating cake! I woke up very hungry, suggesting my hunger was influencing the dream. It's amazing how our dreams incorporate sensations and stimuli from the real world.
Elizabeth Parker from Las Vegas, NV on August 13, 2013:
Interesting hub. I remember many of my dreams in vivid detail. Well-written article!