Written By: Jayden Zhang

(Disclaimer: much of this article will be a compilation of evidence from the webpages linked, for more exact information, as well as sources, check the links at the bottom.)

I - Dreams

Dreams are weird, there’s no doubting it. They usually fall in a sort of uncanny valley, where the events that transpire feel realistic enough to be believable in the moment, but are much too fragmented when looking back. Thus, dreams may have people who are recognizable, or places which are familiar, but the exact details do not match. People might have different personalities, shifted voices, or even different appearances. To find out why this is, we have to look at the root cause of dreams.

First of all, there are multiple theories for how dreams are created. The Activation-Synthesis model argues that dreams are a consequence of random neural firing in the brain. The Information-Processing theory argues that dreams are a way for the brain to consolidate information and events that have transpired. Either way, they both result in what can be described as a hallucinatory state, where imaginary scenarios are both created and experienced through signals in the brain.

Another piece of the puzzle is the existence of a default brain network. The default brain network is a system in the brain that consists of the Medial Prefrontal Cortex (mPFC), which helps you interpret information and understand other people, the Posterior Cingulate Cortex (PCC), which helps integrate memory and experience with thought, the Inferior Parietal Lobule (IPL), which connects spatial awareness and action, and the Hippocampus, which helps create memories. These parts work together as a system which serves as the key actor in dreams, both while awake and asleep. This system is also the reason why dreams seem to have a story-like structure. Inherently, though, dreams have no organization. They are simply an input of unrelated video and audio cues. However, when these random sensory inputs are sent into the default brain network, the system treats the inputs as if it expects a story. It strings together the disjointed cues and creates a plot-focused structure. This is the reason why dreams seem coherent, even if they feel unreal or uncanny. However, the final piece of the puzzle lies in a specific structure in the default system.

The default brain network is also present when people are awake. It is the key to daydreaming, as well as self-reflection and mental health. However, there is a difference between daytime and nighttime dreams. During the night, when the brain enters REM sleep (the cycle in which dreams are most prevalent), the mPFC, as well as other prefrontal cortex structures, quiet down and gradually decrease in activity. The impact of this is that dreams will be much more emotional, as the prefrontal cortex, which is responsible for rational decision-making and behavior control, is overshadowed by limbic system structures such as the amygdala (responsible for emotion) and the hippocampus.

Thus, what we see is that dreams, while resembling familiar events and taking the shape of coherent stories, are usually far beyond what is acceptable as reality.

II - Nightmares

The increased emotional regard also results in to another type of dream, nightmares. Colloquially, nightmares are any dream which seem frightening, or otherwise unsettling. However, the actual scientific definition varies a bit. Some scholars argue that nightmares are only dreams which induce a sense of fear, while others argue that nightmares can cause an influx of any negative emotion, such as anger or sadness. Depending on what source is researched, nightmares can also be limited solely to dreams that cause the person experiencing it to wake up.

The exact cause of nightmares is entirely unknown, although there are many clues and risk factors nightmares are related to. For the most part, nightmares are a relatively normal occurrence. Stress and strong emotions that are present during the waking hours can be probable causes for nightmares. Sleep habits and psychotherapeutic drugs also play a large role.

Nevertheless, the largest contributor to nightmares is none other than mental disorders, specifically Post-Traumatic Stress Disorder. Nightmares are a key symptom of PTSD, with over 90% of PTSD patients suffering from nightmares. These nightmares can occur up to 40-50 years after the traumatic event and can even be used as a predictor for further PTSD symptoms. PTSD-induced nightmares also may be more coherent, and more prevalent than typical nightmares.

Finally, out of all factors related to sleep, nightmares are the only one with a statistically significant relationship with higher suicide rates. People who have higher frequencies of nightmares also tend to have more suicidal ideations. However, it is unclear whether this correlationis the result of casual relationshi with nightmares or due to confounding factors such as PTSD.

In the end, although there are numerous variables correlated with nightmares, it is still unclear how many are a result of a cause and effect relationship. Scientists simply know too little about the biological causes. Furthermore, nightmares also seem to appear less often in a lab setting while the subject is monitored by polysomnographic recordings, making them harder to assess.

III - Forgetting

Did you have a dream last night? Some may answer yes, others may answer no. However, there is only one correct answer to this question. Everyone dreams, every night. Typically, an average person will have 4-6 dreams per night. The question now though is why? Why do we not remember the dreams we have? It’s a complex answer, but in its simplest form, it’s because we forget. The reason we forget dreams so often is up to many factors, both cognitive and biologically.

Cognitively, a reason we forget dreams is simply because our brain deems them unimportant. Throughout the lucid dreaming community, a common way to remember more dreams is to keep a dream journal. By recognizing the patterns in dreams, as well as actively reflecting on dreams, people are able to essentially trick the brain into valuing them. Some scientists also suggest that how people wake up affects dream recovery. Jolting awake could cause dreams to vanish, while waking up naturally and slowly allows the body to consolidate what it has seen. The timing that one wakes up also has an effect, with awakenings during REM sleep leading to higher rates of memory due to heightened vividness in the dream.

Biologically, though, there is another reasonable explanation for the gap in memory. It comes down to the neuropeptide Hypothalamic MCH. This neuropeptide is responsible for many cognitive processes, including monitoring the sleep cycle, but most importantly, it can influence memory. Studies with mice have shown that a presence of Hypothalamic MCH can cause lapses in memory. Mice that had higher rates of Hypothalamic MCH struggled to differentiate between new and familiar objects.

Coincidentally, Hypothalamic MCH is also most present in the REM cycle of sleep, where dreams are also more prevalent. Thus, it seems that the brain directly works to forget the dreams that it has.

III - Conclusion

Ultimately, scientists simply do not know enough about dreams. While many of the factors in dreaming are relatively set in stone, the ultimate question is “why”. Why do dreams exist, and what purpose do they serve? Until more breakthroughs are made, the answer to that question will continue to elude us, staying as hidden as a dream fading from memory.

Sources and Links:

Pace-Schott, Edward F. Dreaming as a Story-Telling Instinct - PMC - Nih, pmc.ncbi.nlm.nih.gov/articles/PMC3613603/

Hasler, Brant, and Anne Germain. Correlates and Treatments of Nightmares in Adults - PMC, pmc.ncbi.nlm.nih.gov/articles/PMC2806673/.

Thomas, Christopher G. The Brain May Actively Forget during Dream Sleep | National Institutes of Health (NIH), www.nih.gov/news-events/news-releases/brain-may-actively-forget-during-dream-sleep.

An overview of the current understanding of dreams and nightmares

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