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10 Scientific Theories About Why We Dream

Dr. Thomas Swan has a PhD in psychology from the University of Otago. He has researched several psychological traits and disorders.

Sleep studies have found that most people dream for about two hours per day.

Sleep studies have found that most people dream for about two hours per day.

The Purpose of Dreams

Nearly all mammals and birds have dreams, which suggests that they serve a useful function that has been ingrained in these species by natural selection. The scientific study of dreaming, or oneirology, is partly an effort to reveal this function, and several fascinating theories have been proposed.

In humans, dreams are involuntary simulations that can last from a few seconds to twenty minutes, with around two hours of sleep dedicated to dreaming each night. Nearly all dreams occur during rapid eye movement (REM) sleep, in which the body undergoes several 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, and long before oneirology became a scientific field. In fact, dream interpretation is as old as the written word. 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, although 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 can improve procedural and spatial memory. The memory consolidation theory therefore suggests that dreams (which occur during REM sleep) are a way of organizing and storing memories of recent events within long-term memory, like taking the clutter of the day and filing it away in drawers.

Dreams, in this sense, would be the stimulation of those memories as well as the memories they are being associated with in long-term memory. However, several studies have also suggested that memory is not improved by dreaming. For example, 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 purging supposedly leaves space for relevant, useful memories to be strengthened.

Once again, the theory suggests that dreams should ultimately improve a person's ability to perform memory-based tasks, which has not been conclusively shown. Furthermore, the theory must explain why we remember dreams that appear to be nothing more than irrelevant noise.

Do dreams purge the mind of useless memories acquired during the day?

Do dreams purge the mind of useless memories acquired during the day?

3. Dreams Are Long-Term Memory Excitations

In 2003, Eugen Tamow suggested that dreams are produced by the standard operations of long-term memory 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" may appear as abstract representations of how recent events relate to older memories (i.e., like dreams). 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 novel 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, although it does explain the peculiar content of dreams and the inconclusive studies. Indeed, as long-term memory is always producing these excitations, no improvement to memory is proposed.

4. Ontogenetic Hypothesis of REM Sleep

Some studies have shown that children who experience sleep deprivation suffer from reduced brain mass, neural degradation, and subsequent behavioral disorders. This research led to the theory that dreams stimulate the brain during times of rest, preventing degradation by encouraging brain development.

The theory therefore claims that dreams serve no function in a mature brain, and we do dream less as we get older, supporting a developmental function. It also suggests that dreams are meaningless thoughts exuded by the brain to maintain its health (which may be interpreted in a narrative fashion). However, 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 they are three times more likely to trigger negative emotions than positive emotions. The most common negative emotion was anxiety, which has an evolutionary function to prepare people for threats by considering, imagining, and mentally simulating the negative outcomes of potential future events. The theory is therefore that dreams may be a form of threat rehearsal.

A related theory suggests that dreams help individuals to deal with past trauma by therapeutically integrating previously-experienced threats into the dreamer's memory in a nontraumatic way.

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 helps with creative and insightful thought by incorporating and reorganizing information in the brain. However, not all dreams are unpleasant, suggesting that the theory may be incomplete. Furthermore, dreams are often difficult to understand, reducing their preparatory value.

Are nightmares a way to prepare us for threats in the real world?

Are nightmares a way to prepare us for threats in the real world?

6. The Tonic Immobility Reflex

According to one theory, dreams are a byproduct 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 (including a kind of paralysis) mimic this reflex.

The theory suggests that dreams are a "threat rehearsal" that is meant to prepare the individual for a dangerous awakening. Indeed, we often incorporate external stimuli into our dreams (e.g., noises), which might allow for immediate action in the real world. One problem with this theory is that rapid eye movement and a higher breathing rate during REM 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. For example, experiments have shown that rats prevented from entering REM sleep will die from hypothermia.

According to this theory, dreams keep the brain active, which keeps it warm. Indeed, periods of REM sleep are spread throughout the night, with the longest periods toward the end of the night (usually the coldest time). However, some studies have shown that thermoregulation decreases during REM sleep, which can cause a drop in body temperature.

Rats that were prevented from dreaming died from hypothermia. The pictured setup is monitoring their brain waves (EEG), muscle tension (EMG), and behavior.

Rats that were prevented from dreaming died from hypothermia. The pictured setup is monitoring their brain waves (EEG), muscle tension (EMG), and behavior.

8. The Sentinel Hypothesis

In rats, rabbits, and some other mammals, REM sleep is proceeded by a short period of wakefulness. And, even though humans continue to sleep after episodes of REM, it is easier to wake from it than from normal "deep" sleep.

The "sentinel hypothesis" therefore suggests that REM sleep evolved as a way to place animals into a semi-wakeful state that might aid in the detection of 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 a random dream narrative 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 Byproduct of Sleep Paralysis

Sleep paralysis is one of the physiological changes that occurs during REM sleep. It is caused by a suppression of "neurotransmitters" in the brain that are used to detect sensory information. This suppression may be necessary to give the brain's receptors of these neurotransmitters time to regain maximum sensitivity.

During this suppression, the brain may develop a kind of feedback system in which sensory data is harvested from the memory instead of from the environment. Dreams may be the result of these internalized sensations, making them a functionless byproduct of sleep paralysis.

The central nervous system cannot work at 100% sensitivity all the time. Pictured: a rat's hippocampal neuron.

The central nervous system cannot work at 100% sensitivity all the time. Pictured: a rat's hippocampal neuron.

10. Dreams Protect the Brain From Itself

A recent study has suggested that the purpose of dreams is to maintain activity in the visual system of the brain (the visual cortex) to prevent its "takeover" by other brain systems. According to this "Defense Activation Theory" (DAT), such takeovers occur naturally and can proceed within hours. For example, blindness may lead to the same type of takeover of the visual cortex by neighboring brain systems.

This theory is similar to the ontogenetic hypothesis, in which the brain may degrade during times of rest. Rather than degrading, DAT suggests that brain areas are repurposed.

In this theory, then, dreams also serve no function other than being meaningless thoughts exuded by the brain to maintain its operations. Once again, the patterns and themes seen in dream content across many test subjects (e.g., of threats) would seem to disagree with the theory, although dreams may have more than one function.

Might Dreams Have No Purpose?

The inability of scientists to find a conclusive answer to why we dream might suggest that there isn't one to be found. However, this proposition is unlikely. 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 byproduct of something that does (e.g., sleep paralysis). The physical and psychological damage that is associated with a lack of REM sleep also support a functional explanation.

Future Research

Whatever the true purpose of dreams is, studies in the areas of neurobiology and psychology will continue to inform the field of oneirology, as well as amaze and mystify the rest of us until a prevailing theory is found.

Ultimately, discovering the function that dreams serve is a necessary step toward 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.

Further Reading

  • Mallick, B., Pandi-Perumal, S., McCarley, R., & Morrison, A. (Eds.). (2011). Rapid Eye Movement Sleep: Regulation and Function. Cambridge: Cambridge University Press.

© 2013 Thomas Swan

Comments

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!