The activation-synthesis model, proposed by J. Allan Hobson and Robert McClarley back in the 1970s, suggests that, as circuits in the brain become activated during REM sleep, areas of the limbic system including the amygdala and hippocampus (the parts of the brain involved in emotions, sensations and memories) also becomes active. The brain synthesizes and subjectively interprets this internal brain activity, and attempts to find meaning in the random signals, the results of which are what we call dreams.
Despite years of research, the neurological basis for dreams is still not entirely certain, and several theories (including the activation-synthesis theory, the long-term memory excitation theory, the continual-activation theory, and others) have been proposed for how and why dreams occur at all, none of which are entirely satisfactory or universally accepted.
Perhaps the best answer is that dreams help us regulate, analyze, explain and remember recent events in our lives, in a kind of “mental housekeeping” process. Most dreams are based in large part on everyday experiences from recent days, suggesting that at least part of their function is to help the brain sort through and make sense of recent real-life experiences.
Reliving experiences internally through dreams helps to facilitate learning and to reconsolidate long-term memories (both the procedural memories of how to do things and the declarative memories of facts and events), and also provide an opportunity to delete unwanted or junk data from memory.
The Role of Dreams
People say dream interpretation is a pseudoscience, but the truth is, we know very little about dreaming in general. Whether it’s the meaning of dreams or the purpose of dreams in general, we’re still mostly lost in the dark. But thanks to the hard work of some brilliant minds, we do have a few clues to help us understand the function of dreaming.
Emotional Processing
One way dreams help us is by resolving some of our emotional problems and inconsistencies arising from our daily activities, as well as aiding in mood regulation. Interestingly, some recent research based on dream reports has found that sleepers tend to report more negative emotions when woken during rapid eye movement (REM) sleep than during non-REM sleep, suggesting that REM sleep may produce more negative dreams.
In particular, the amygdala, which specializes in dealing with unpleasant emotions (e.g. anger, aggression, fear, etc), is very active during REM sleep, and may be involved in the process of resolving emotional issues. This may be supported by the example of clinical depression: one symptom of depression is that sufferers tend to spend a disproportionately large amount of time in REM sleep, in which case these negative emotions may be over-represented, thus perpetuating the depression (deliberate deprivation of REM sleep is often helpful in alleviating depression symptoms).
It has also been suggested that the way the brain suppresses the rational, controlled activity of the prefrontal cortex while allowing the more primitive, emotional side (associated with the amygdala, or emotion center, of the brain) to run riot may be a way of keeping us safely sane by allowing us to “go crazy” in our nightly dreams, while simultaneously working out our emotional problems.
Dreams may, therefore, function to contain and absorb negative emotional surges, allowing the dreamer to remain soundly asleep despite the inner turmoil, and to expunge these negative emotions from waking memory.
According to this theory, sometimes referred to as the selective mood regulatory theory of dreaming, the first main dream of the night is therefore likely to have the most disturbing dream content, with subsequent dreams becoming increasingly better-adjusted, in an ongoing, and largely automatic, process of emotional resolution. Recurring dreams occur where this process becomes stuck at an early stage, and the emotional issues remain unresolved.
Fight or Flight Training
A more evolutionary explanation for dreaming that has been proposed is that dreams may be a kind of mental threat simulation of potential real-life threatening events, allowing for a kind of safe “dry run”, and the testing and analysis of possible responses and mental schemas. It is also proposed by some that these schemas may also be passed on in the form of so-called “genetic memories”.
Interestingly, children tend to have many “primal” dreams and nightmares, similar to the kinds of dreams mankind may have experienced millennia ago; as we age, these atavistic dreams tend to fade and our dreams tend to become more attuned to the subject matter of modern adult life.
So, the theory holds that our dreams, and the long, detailed REM dreams in particular, may use our past experiences to anticipate possible future experiences, and allow us to test out possibilities and scenarios which might help us to deal with those experiences if they ever occur. Some of the scenarios constructed may be completely nonsensical, as indeed many dreams are, but some may prove genuinely useful in providing insights, so this may be at least a partial function of at least some dreams.
Memory and Learning
Another very prevalent theory on the purpose of dreams is that dreaming helps with memory consolidation and problem-solving. Dreams actually appear to refine and improve memories, making them more useful for the future, so that the subsequent performance of learned tasks is improved. This has been conclusively demonstrated experimentally, for example in tests where rats are set to learn tasks like navigating mazes.
Related: Sleep and learning
The brain is particularly active during dream-heavy REM sleep, so it seems logical that REM dreams play an active role in keeping the brain and nervous system in good working order in some way. However, it is becoming increasingly apparent that non-REM dreams may be just as important in facilitating learning as REM dreams.
Experiments on rats indicate that, during non-REM sleep, neuronal activity largely mirrors the neuronal activity of tasks performed during the previous day, although abbreviated, compressed or sped up. On the other hand, in the much more detailed dreams of REM sleep (typically five times longer than non-REM dreams), brain wave activity appears almost identical to that during the original physical experiences, as though the tasks are being played out in full, but internally.
Making Sense of Randomness
There is, though, a significant school of thought that believes that dreams do not actually serve any natural purpose and have no significant influence on our waking actions at all, but are just a random by-product of REM sleep. The peculiar nature of dreams is therefore attributable to certain parts of the brain trying to piece together a story out of what is essentially random information presented to it by the release of acetylcholine from the brainstem during REM sleep.
This theory might also go some way to explaining why dream recall is so difficult, especially if the brain were to consider them non-meaningful information or “noise” that we should just forget. This idea, that dreams may be instrumental in eliminating unwanted or unnecessary information and synaptic connections, is sometimes referred to as “active unlearning”.
Of course, it can also be counter-argued that the way the forebrain chooses to interpret these random signals into a scenario or a story is in itself significant, and can tell us something about ourselves, in much the same way as our interpretation of a Rorschach ink-blot can.
The History of Dream Research Theories
Even though we can’t say for sure what the exact role of our dreams is, scientists throughout the ages have come up with many theories.
Freud and the Unconscious
The vast majority of Sigmund Freud’s ideas have been debunked since his popularity in the early 1900s, but in the process of debunking Freud, we have learned a lot, so it seems important to mention him nonetheless. Freud’s understanding of dreams was based on the unconscious, a part of the mind that Freud suggested housed our most base desires. According to Freud’s book on the study of dreams, The Interpretation of Dreams, these desires could not be expressed in real life, so instead they appeared in our dreams.
The Activation-Synthesis Model
Long-Term Memory Excitation Theory
Eugen Tarnow developed his long-term memory excitation theory in 2003, arguing that dreams are just the brain’s internal excitations of long-term memories. This theory holds that such excitations are going on all the time, even during our waking hours, but the “reality checking” of the brain’s executive function (which takes place in the prefrontal cortex, and which is known to be actively suppressed during sleep) usually excludes them from our notice.
In some ways, this is consistent with many of Freud’s psychoanalytical ideas about dreams, with Freud’s unconscious replaced by our long-term memory.
Continual-Activation Theory of Dreams
In 2004, Jie Zhang developed the continual-activation theory of dreams, which argues that dreams are part of the brain’s consolidation of working memories into long-term memories (declarative memories during non-REM sleep, and procedural memories during REM sleep).
Zhang believes that both the conscious and non-conscious subsidiary systems of working memory need to be continually activated in order to maintain proper brain functioning and, when the level of activation of either subsidiary system falls below a given threshold, the brain automatically triggers the generation of a data stream from its memory stores. Dreaming is therefore merely an incidental result of the brain’s need for continual activation.
What Happens When We Dream?
Whatever the underlying reasons for why dreams happen, the physiological and neurological mechanisms underpinning the act of dreaming are gradually beginning to become clearer as modern research progresses.
For a long time, researchers believed that dreams took place during the REM part of the sleep cycle, but in recent years, we’ve learned that we can dream in many stages of sleep, and some researchers even believe all of our dreams happen in a couple of microseconds as we wake up.
Regardless of when exactly we dream, scientists are fairly certain where in the brain dreaming takes place. One is the spot where the occipital, temporal and parietal lobes of the cerebral cortex meet and the other is the part of the frontal cortex where dopamine is the dominant neurotransmitter. Interestingly, these areas are not related to REM sleep at all. Still, whenever researchers suppress these areas of the brain, or study people who have experienced damage in these areas, dreaming is reduced or prevented entirely.
When it comes to dreaming that occurs during the REM stage of sleep, our limbic system gets involved as well. The limbic system includes the hippocampus and amygdala, and together they are in charge of processing emotions and motivation, among other things. On the other hand, the prefrontal cortex which is involved in working memory, attention, logical reasoning and self-control, is relatively inactive during REM sleep.
Finally, it’s probably no surprise to you that your primary visual cortex is not involved in your dreams, since your eyes are closed all night. But that doesn’t mean your visual system is totally shut off at night. Actually, research has found that the extrastriate visual areas of the the occipital cortex (the area of the brain involved in sight) is highly active during REM sleep, which is probably why we can have such vivid dreams with realistic visuals.
What Influences Our Dreams?
You may have noticed that your dreams aren’t the same every night, and sometimes what you do during the day can have an impact on what kind of dreams you have at night. Again, we don’t have a perfect understanding of what influences our dreams (or how it does it) but we can make some well-educated guesses based on decades of research.
Medical Conditions
There are a variety of medical conditions that can affect how we dream. For instance, conditions that affect dopamine production or require medications to suppress or boost dopamine may experience differences in dreams based on their dopamine supply. People with Parkinson’s or ADHD, two conditions that involve dopamine abnormalities, may have more dreams, fewer dreams, or their dreams may be more or less vivid.
Mental health conditions can also have a significant impact on dreaming as well. As we mentioned before, people with depression tend to stay in REM sleep longer, which can reinforce depressive symptoms. Research also shows that people with all kinds of mood disorders, from depression to bipolar, experience more vivid dreams and more frequent nightmares.
Food and Drink Before Bed
We all know it’s a bad idea to drink caffeine at night because it can keep us from falling asleep, but did you know caffeine can impact your dreams too? Technically caffeine hasn’t been shown to change the content of your dreams, like whether it’s a good dream or a bad dream, but late-night caffeine does make you more likely to remember your dreams in the morning.
Alcohol, on the other hand, might help you fall asleep, but it can actually prevent you from sleeping as deeply. Since REM sleep is not a very deep sleep, alcohol may increase the amount of dreams you have, but some research shows that they’re more likely to be bad dreams.
Another factor is when you eat before bed. It doesn’t so much matter what you eat as long as it isn’t too spicy or fried, since those foods tend to keep us up longer. A big meal right before bed can raise your body temperature and metabolism, which can lead to increased REM sleep and possibly more dreaming.
Daily Activities
Your daily activities can have a big impact on your dreams as well. If you are relatively active throughout the day and you exercise in the morning or afternoon, you are more likely to fall asleep quickly, which gives you more time for more sleep cycles, which means more REM sleep, which means more dreams (maybe).
Your lifestyle might also feature in the content of your dreams. For instance, research shows that 80% of college students dream about university-related things. Whether this is part of memory consolidation or threat simulation (if school makes you anxious), we aren’t sure, but it’s clear that lifestyle can impact our dreams.
Why Do We Have Nightmares?
As is the case with dreaming in general, we don’t have a solid grasp on what exactly causes nightmares. Nightmares are dreams with vivid and disturbing content that you remember very well when you first wake up, though the memory may fade as the day wears on.
Some research shows that mental illnesses such as depression, anxiety and post-traumatic stress disorder (PTSD) can increase your chances of having nightmares, but normal daily stress can also cause an increase in nightmares. If you notice that you’re having more nightmares, it may be helpful to do a soothing meditation or some relaxing yoga before bed.
Summary: Why Do We Dream?
So why do we dream? Well, we can’t say for sure, but there are a lot of theories out there. Odds are none of them are completely right or wrong, but instead, they probably work with each other to explain our nighttime adventures. Future research may link them together to form one cohesive theory of dreaming, but until then, we’ll just have to keep piecing them together as best as we can.
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