While We Dream: The Neuroscience of the Sleeping Brain

“We are such stuff as dreams are made on.” (William Shakespeare)

On average, we spend about 26 years sleeping in life which equates to 9,490 days or 227,760 hours. This is basically one-third of our life [1]! Indeed, sleeping is a complex process regulated by a series of cellular and molecular mechanisms, and it covers a central role in a variety of behavioral, physiological, and vital functions. These functions include developmental processes, energy restoration and conservation, regulation of the immune response, cognitive capacity and performance, attention, psychological well-being, and brain waste clearance [2]. Therefore, quality sleep is indispensable (like water and food supply)  to allow our brain and body to properly function. Essentially to make us humans and alive. In fact, the brain and the body are always active, even while sleeping, but even though the biological purpose of sleeping is still unclear, what is clear is the negative effects that a chronic lack of sleep may have on our health [3].

The neuroanatomy of sleep and sleep stages

Sleep is a complex phenomenon regulated by the mutual interaction between different brain structures. Sleep and arousal are regulated by the hypothalamus in which, a structure called the suprachiasmatic nucleus (SCN), receives information about light exposures and therefore controls our behavior and sleep cycle. Moreover, the brainstem interacts with the hypothalamus while controlling the transition from a state of being awake or asleep. Indeed, to initiate sleep, the brain stem releases chemicals (GABA) to reduce arousal activity consequently, but also it sends signals to relax muscles and rest our body limbs. Another structure called the thalamus receives sensory information and sends these to the cerebral cortex (higher-level processing of information from short to long term memory). The activity of the thalamus is mostly silent during sleep. However, during the REM  stage, it activates and sends to the cortex images, and sensory inputs to the cortex while dreaming. Between the hemispheres, the pineal gland receives inputs from the SCN to consequently increase melatonin production, promoting the process of sleep when the light is diminished. In addition, as a part of the midbrain, the basal forebrain is a structure also involved in this process and responsible for the cycle between sleep and wakefulness. Indeed, releasing adenosine in this region would promote sleep, whereas caffeine would promote wakefulness by blocking the effect of adenosine. Lastly, the amygdala, a central structure responsible for emotion regulations, is active during REM stages and probably also involved in dreams [3].

Talking about sleep, I am sure everyone heard the word REM at least once in life. Sleep stages are indeed categorized between rapid eye movement (REM) and non-REM (which occurs across three stages). Through these stages, the pattern and rhythms of brain oscillations change, and brain waves' modulations are easily observed by recording the electroencephalographic (EEG) signal. Typically, healthy individuals go through all stages and repeat them several times in a single night of sleep (between four to six times). Non-REM sleep stages (from 1 to 3) characterize the process from slowly falling asleep to deeper sleep states. Heartbeat, body temperature, muscle tension, and brain waves decrease (lower frequencies and higher amplitudes) and tend to slow progressively. In particular, going toward stages 2 and 3 is important to make you feel energized once you wake up. The REM stage is characterized by rapid eye movements that appear behind closed eyelids, and more rapid brain oscillations (higher frequencies and lower amplitudes) are also observed. This stage is also accompanied by increased breath, irregular heartbeat, and blood pressure. Usually, dreams tend to appear in this stage, in which body muscles are temporarily paralyzed to prevent body movements. This stage also tends to be shorter while aging, and in interaction with non-REM stages, memory processes also occur [3]. Overall, at the beginning of the night, individuals spend more time in non-REM stages, and progressively through the night get closer to REM sleep through the night, mostly in stage two. This process continues and repeats until the individual wakes up.

In addition, our attitude to be “early-birds” or “night-owls” might depend on our circadian rhythms and sleep-wake homeostasis that control our biological clock and track the amount of sleep needed [3].  Regarding how much sleep we would need, check out these articles from Peter Diamandis and me about why sleep is so important for our health and longevity while developing a longevity mindset.

Why do we dream?

Dreams can be bizarre, unsense, black and white, colorful, funny, and much more. Dreams manifest in various ways, and it looks like an attempt from the brain to make sense of multiple internal inputs that occur during memory consolidation. Was Alan Hobson, from Harvard University, right in proposing the “activation-synthesis” model?

Investigating the process and the function of dreams has a long history. It started already with Plato, who analyzed dreams from a perceptual perspective "I can imagine a healthy man who lives in harmony with himself. He goes to sleep only after he has summoned up the rational element in his soul, nourishing it with fair thoughts and precepts" (Plato, 1985), followed by Aristotle who was the first trying to understand its functioning “If waking is the contrary of sleeping, and one of these two must be present to every animal: it must follow that the state of sleeping is necessary" (Aristotle, 1952), and most recently made famous by psychoanalysis and Sigmund Freud who wrote an entire book on dreams and their interpretations in the early 1900s, analyzing the symbolism behind dreams and relating them to wish fulfillment "Ideas and dreams and in psychoses have in common the characteristic of being fulfilments of wishes" (Freud, 1998) [4].

Nevertheless, even though we spend about 2 hours/night dreaming, its purpose is still unknown, and also, the neurological process underneath is unclear to science. However, it is also true that modern neuroimaging techniques have encouraged the deeper investigation of dreams' neural basis and mechanisms. It has been advanced the idea that dreams would help in processing emotions, and individuals affected by higher levels of stress and anxiety would indeed experience nightmares and frightening dreams. Berstein (2006) classifies nightmares as “sleep disorder” affecting REM sleep and very recurrent as prominent symptoms in Post Traumatic Stress Disorder (PTSD), while sleepwalking would occur during non-REM and is typically observed in children [4].

Philosophy and psychology apart, let’s talk about neuroscience. REM was first discovered in a sleep laboratory at the University of Chicago in which REM stage was detected while sleeping and also occurring while dreaming. Hence, based on that finding, several studies have been conducted to deeply understand sleep and dreams' physiology (Shafton, 1995). Dreams are not confined only to REM, but may also occur in non-REM stages. Contrary to the argument that sleep is needed to restore and energize the body, REM sleep consumes quite a lot of energy and oxygen, and this event might be related to the fact that maybe in this stage, humans process information being acquired through the day. Therefore, REM sleep looks critical and highly connected with cognitive processes (Varela, 1997); somehow a space in which humans can rethink, reimagine, reconceptualize, re-experience things and probably elaborate on this information into a higher-order level [4].

How does memory relate to our dreams?

"Sleep promotes primarily the consolidation of memory, whereas memory encoding and retrieval take place most effectively during waking" (Llewellyn, 2013). Thus, sleeping and dreaming would take part in organizing, processing, and storing information in memory. However, dreaming does not always match remembering what has been dreamed about. Most of the time, when recalling dreams, what primarily emerges is the intensity of emotions arising during the process of dreaming. Indeed, neuroimaging studies did show that the amygdala was highly activated in response to emotions while enhancing retention, but also the involvement of other structures during REM such as limbic and paralimbic areas, and the anterior cingulate cortex (ACC) would support the encoding of information through high-order emotional mechanisms (Llewellyn, 2013). Moreover, other studies investigated how different brain regions activate to promote memory consolidation while dreaming, and also how different brain wave patterns localized in certain regions would indicate the probability of remembering a dream or not (e.g., theta oscillations over the frontal regions). The mood is also influenced by REM sleep. Walker and van der Helm (2009) found that depression may prolong the state of REM and be accompanied by impaired memory consolidation [4].

In neuroscience, dreams have been associated with memory consolidation while sleeping. This event would regard the reorganization and store of memories according to emotional features and the transferring of memories from one brain region to the other. The hippocampus, a subcortical region involved in long-term memory and learning, is responsible for storing information into episodic memory (memory of events and facts) during the day. However, during the night, these memories stored in the hippocampus would be transferred toward cortical regions (e.g.,  the cerebral cortex), specialized in the higher-level processing of information, cognition, and knowledge [5,6]. Therefore, the hippocampal activity would “replay” the events experienced during the day, and this process would occur faster (or even in reverse) than what was experienced in real life. This process would activate both the visual cortex (responsible for visual experiences) and the prefrontal cortex (responsible for higher-order cognition, planning, reasoning, strategic thinking). So, while sleeping and dreaming, this memory “reply” would manifest during the REM stage [7].

Interpreting dreams fascinated humans for ages. The most usual approach is to attribute dreams with special codes and symbols, in turn, related to an individual’s motivations and beliefs. Hence, this attribution process would make sense if dreams are an attempt to consolidate and process life events through memory reply. Thus, the emotional content that arises while dreaming would certainly be explained [8]. In line with the hypothesis that the emotional content emerging while dreaming is an attempt of the brain to connect personal experiences, and as Stickgold et al. (2001) mentioned it “reflects an attempt, on the part of the brain, to identify and evaluate novel cortical associations in the light of emotions mediated by limbic structures activated during REM." [9] The imagery functions of dreams might also serve to explore hypothetical situations in some abstract way in order to refine action strategies for use in the future.

Dreams function to protect our brain

Learning a new skill or even changing habits is known to modify the brain's structure and how the brain is functionally organized. In neuroscience, this event is called neuroplasticity and in fact, relates to the ability of the brain of being flexible and rewire itself in response to changes. It is incredible to think that a system like a brain, containing 86 billion neurons and 0.2 quadrillion connections can continuously rewire itself through life. It was also common to think that different brain regions were specialized to carry on particular functions. However, advances in research proved that the brain could reorganize and re-allocate specific functions to other areas. For example, the visual cortex, specialized to processing visual information, can be reassigned to perform other tasks (e.g., processing sounds), and this phenomenon happens in case of brain lesions or dysfunctioning (e.g., patients who lost sight and are blind). Continuing this example, how do brain flexibility and brain reorganization relate to dreaming?

Eagleman and Vaughn (2020) [10] suggested that the function of dreams would be to prevent other senses from taking over the brain’s visual cortex while being deactivated during sleeping. Hence, dreams might counterbalance the brain's functioning against its high flexibility. According to the “defensive activation theory,” dreams would maintain the activity in the visual cortex to contrast the takeover by other senses. Hence, dreams are perceived mostly in a visual format because vision is the only disadvantaged sense during night and darkness.  As previously discussed, dreams mostly occur during REM sleep, where specialized neurons activate the visual cortex and therefore generate that visual experience typical of dreams (e.g., eyes closed).

Moreover, the defensive activation theory supports the idea that the amount of sleep spent during REM also decreases due to the decrease in brain flexibility throughout the lifespan. Indeed, newborns spend half of their sleep in the REM stage, and differently, the amount of time spent in REM is just 18% in older individuals. This fact would indicate a clear relationship between brain flexibility and REM sleep. Thus, becoming REM sleep progressively less necessary, the brain is also less flexible in parallel. The authors extended their investigation on animal models to test the defensive activation theory’s prediction that the more flexible an animal’s brain, the more REM sleep it should have to defend its visual system during sleep. Indeed, they found that animal species with higher brain flexibilities spend longer time during REM.

Furthermore, this dream circuitry is essential and even found in blind individuals. Nevertheless, blind individuals (who were born blind or become blind later on in life) do not experience visual imagery while dreaming. Still, they rely on other senses (e.g., feelings or hearing things). This is explained by the fact that other senses took over the function of the impaired visual cortex. Blind and sighted individuals have dreams, and they activate the same region of the brain. However, the difference lies in the type of sensory experience they implement to process (and experience) information while dreaming (e.g., hearing things or seeing things). In addition, a difference can also be made between individuals who were born blind and those who became blind after the age of 7. Indeed, the latter may experience more visual content in their dreams, and back to the defensive activation theory, older brains are less flexible in reorganizing. Therefore non-visual senses may not reach the visual cortex easily [10].

Altered consciousness while dreaming

Another interesting topic that fascinated science was the investigation of the interaction between consciousness and dream sleep, therefore exploring the neural correlates of dreams. A review by Mutz and Javadi (2017) [11] aimed to explore dreams during REM and non-REM stages, followed by an overview of lucid dreams. The discussion continues by investigating how dreams vary according to the phenomenological aspects characteristic of different sleep stages, and whether individuals are conscious during their sleep cycle. The review also discusses how while sleeping, neurofunctional changes would cause the way dreams are generated. Based on the review, the main findings were:

1. In contrast to the amount of research on dreams occurring during REM sleep, more studies should also focus on investigating the mechanisms of dreams in non-REM, thus comparing the different phases of non-REM with wakefulness, REM, and lucid dreams.
2. To better understand the relation between brain activity and behavior, non-invasive brain stimulation techniques such as transcranial alternating current stimulation (tACS) and transcranial magnetic stimulation (TMS), should be implemented to make causal inferences. Indeed, tACS was recently implemented to induce lucid dreams, and thus useful when exploring the neural basis of dreams (Noreika et al. 2010; Stumbrys et al. 2013; Voss et al. 2014).
3. Instead of relying on subjective reports, objective assessments of dreams are feasible. Against individual reports, in which the memory can be biased and altered (and easily forgotten), brain imaging techniques may obtain objective measures of dreams (e.g., functional near-infrared spectroscopy and machine learning algorithms)
4. To better understand what consciousness is, the investigation of coma and vegetative states may help to gain insight into the field of consciousness (and dreams).
5. In the clinical setting, lucid dreams can be trained as a therapeutic method against nightmares. However, more research is needed in this context due to the complexity of lucid dreams, their manifestation, and neurofunctional mechanism (Spoormaker et al. 2003; Spoormaker and Van Den 2006).
6. Understanding consciousness and how the underlying mechanisms in healthy individuals would empower clinical research in the context of sleep disorders, enabling potential treatments to ameliorate dysfunctional processes typically observed in patients (Dodet et al., 2015).

In conclusion, even though we are still at the beginning in the understanding of the neural basis of dreams, the investigation of dreams and their neural basis would enhance the study of consciousness [11].  To gain more insight into the neural correlates of dream sleep and altered states of consciousness, you can access the review here, and how cognition and consciousness are inherently linked here.

How technologies can track and manipulate your dreams

Whether digital technologies can track sleep routine while measuring sleep stages, amount of sleep, heart rate, blood pressure, and so on, nobody thought that even dreams might be detected or even manipulated through technologies.

MIT scientists have found how to manipulate dreams with Dormio, an app combined with a sleep-tracking device, showing that it is possible to insert certain topics into a person’s dreams with some bizarre outcomes. Indeed, specializing in wearables and interfaces to boost cognitive skills, the MIT researchers at the Media Lab’s Fluid used a technique called target dream incubation (TDI). According to previous studies that demonstrated that lucid dreams are special states of awareness in which dreamers can consciously shape the dream's content, TDI applied to early sleep stages (hypnagogia) showed comparable outcomes.

Furthermore, a dream-reading machine was also recently developed. Japanese scientists have also discovered a way to record dreams by using neuroimaging techniques. Thinking of dreams as a series of images and thoughts that manifest while sleeping, these series of events can be recorded with magnetic resonance imaging (MRI). Therefore, to decode those images that correspond to dreams, data were integrated into an algorithm capable of reconstructing the dream. Indeed, the brain would release specific hormones while dreaming and sleeping. Accordingly, these hormones are also related to what has been dreamt, and potentially different hormones would be associated with different dream contents. To read dreams, two neuroimaging techniques were implemented. An MRI was implemented to measure an individual’s brain activity while sleeping and dreaming in the first phase. This was followed by EEG recording, in which the different stages of sleep and also the occurrence of dreams could have been identified. The awake individual was asked to recall the dream in a second phase.  For each individual, this process was repeated almost 200 times. Collected and classified data were successively analyzed, finding that certain recurrent patterns (e.g., objects recalled from the dream) were correlated with specific brain activities measured with MRI. Searching those images most similar to the dreamed objects recalled by the person and these data entered into an algorithm would thus enable a dream-reading machine capable that can be further modeled and improved based on learning.

From phenomenology to neurophysiology: the new frontiers in dream research

For several centuries, dreams are a mysterious phenomenon due to the capacity of the brain to disengage from the external environment and re-create an internal world of conscious experiences. Therefore, understanding the neural basis of conscious experiences while dreaming sleeping is the goal of cognitive neuroscience, aimed at investigating dream phenomenology relating this event to mental imagery and perception. Besides, advances in this field of science were made possible through neuroimaging, studies in patients, brain lesions, and neurophysiological processes [12].

Neuroscience enlarged new frontiers in the field of dreams and sleep stages, exploring what ancient philosophers and more recently Freud started already to question. Neuroscience tries to investigate those processes underlying sleep and mood and how these may relate to memory, perception, and imagination. In parallel, the investigation of dreams and their neural basis might also shed light on the neurobiology of consciousness. Nevertheless, despite advances in this field of research, more have yet to come and be explored [4].

Prediction of the future or a neurobiological and adaptive behavior? Dreams are the “unsolved mystery” that caught the attention of scientists, philosophers, and psychologists, and itself the human being for ages. What do dreams mean? And why do we dream? These are the two fundamental questions that today neuroscience is trying to answer. For now, we can say that dream sleep is real, and its function would be to prevent other senses from taking over brain regions (such as the visual cortex) that are temporarily unused. Thus, the function of dreams is to counterbalance the brain flexibility underlying neuroplastic mechanisms [10].

“In dreams, we enter a world that’s entirely our own. Let them swim in the deepest ocean or glide over the highest cloud.” (J.K. Rowling)

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