Rem Sleep And Consciousness: A Similar State Of Mind

Dreaming is a mental state that occurs during sleep, and is characterised by fictive events that are organised in a story-like manner, with a range of internally generated sensory, perceptual, and emotional experiences. Dreaming is often associated with the rapid eye movement (REM) sleep stage, which is characterised by REMs, global high-frequency and low-amplitude electroencephalogram (EEG) activity, as well as increased heart rate, respiratory activity, and muscle atonia. However, dreaming can also occur during the non-rapid eye movement (NREM) sleep stage, and the two can be dissociated.

REM sleep and dreaming can be differentiated by lesions in the forebrain and brain stem. Dreaming during REM sleep is typically bizarre, hallucinatory, and emotionally intense, while dreaming during NREM sleep is more thought-like, less vivid, and less emotionally intense. Dreaming during NREM sleep is also more conceptual, more plausible, and involves greater volitional control.

The neural correlates of dreaming are still not well understood, but it is thought that dreaming arises from brain activity that is largely independent of interactions with external stimuli. Dreaming during REM sleep is associated with hyperactivity in several brain regions, including higher-order occipito-temporal visual association areas, motor regions, and limbic and paralimbic structures. Dreaming during NREM sleep is associated with decreased activity in the right inferior parietal cortex, which is involved in waking volition and self-reflection.

The discontinuity of consciousness during sleep is thought to be functional and normal, and may reflect the consolidation of memories. Dreaming may be a purer form of consciousness, which is free of the constraints imposed by the perception of, and interaction with, physical environments.

Sub-topic 1: Dreaming and consciousness during REM sleep

Dreaming and Consciousness During REM Sleep

The REM (rapid eye movement) sleep stage is characterised by relaxed muscles, quick eye movement, irregular breathing, elevated heart rate, and increased brain activity. Dreaming mostly occurs during this stage, and dreams are typically intense and vivid. During REM sleep, the brain is highly active and resembles the brain activity of a person when they are awake.

REM sleep is associated with brain development, memory consolidation, emotional processing, and dreaming. Dreaming during REM sleep is thought to be a way for the brain to simulate threatening situations and process emotions. Dreams during this stage are bizarre, with a loose and fanciful narrative, and the dreamer is often uncertain about time, space, and personal identities. The dreamer's critical self-reflection is reduced, and they typically have the subjective experience of being awake.

The brain regions that are active during REM sleep include the pontine tegmentum, thalamus, basal forebrain, amygdala, hippocampal formation, anterior cingulate cortex, medial prefrontal cortex, and circuits of the medial temporal lobe region. The prefrontal cortex, which is involved in cognitive control, metacognition, and ego functions, is deactivated during REM sleep, which may explain the lack of insight, restricted volitional capabilities, and impaired metacognition during dreaming.

The brain activity during REM sleep is similar to that of wakefulness, and individuals are most likely to report dreams after awakening from this stage. However, it is important to note that REM sleep and dreaming can be dissociated. Brain lesions can prevent REM sleep while individuals continue to dream, and lesions in the forebrain can leave REM sleep intact while dreaming ceases.

Dreaming and Consciousness During Non-REM Sleep

Non-REM (NREM) sleep is divided into three stages (N1, N2, and N3) and is characterised by a global low-frequency and high-amplitude EEG signal, slow and regular breathing and heart rate, and low blood pressure. Dreaming during NREM sleep is thought to be related to "covert REM" brain activation processes.

Reports of dreaming after NREM sleep awakenings are sparse and less elaborate than after REM sleep. Dreams during the early stages of NREM sleep are thought-like, fragmented, and related to current concerns. Dreaming during late-night NREM sleep is more hallucinatory and indistinguishable from REM sleep dreams.

Dreaming and Consciousness During Lucid Dreaming

Lucid dreaming is a rare state of sleep in which individuals are aware that they are dreaming. It is considered a part of REM sleep and typically occurs during late-night REM sleep periods. However, there is preliminary evidence that lucid dreaming can also occur during NREM sleep.

Lucid dreaming involves aspects of both primary and secondary consciousness, such as self-reflective thought, abstract thinking, metacognition, and agentive control. The dreamer has restored access to metacognitive abilities and memory functions, enabling them to execute their intentions. The neural correlates of lucid dreaming largely overlap with brain areas involved in self-reflective thought and volitional capabilities.

Sub-topic 2: Consciousness during REM sleep

REM sleep is the brain state most associated with dreaming. In dreaming, many aspects of primary consciousness are heightened, including a sense of first-person agency, internally generated percepts including movement in fictive space, and strong emotions; especially anxiety, elation, and anger. Conversely, many aspects of secondary consciousness are weakened in sleep: critical judgment, self-reflective awareness, awareness of awareness, orientation, and memory itself are all in abeyance.

REM sleep is characterised by rapid eye movement, global high-frequency and low-amplitude electroencephalogram (EEG) activity, as well as increased heart rate, respiratory activity, and muscle atonia (i.e. temporary muscular paralysis). The EEG signal during REM sleep shares large similarities with that of wakefulness, and positron emission tomography (PET) studies have shown that global brain metabolism tends to be very similar as well.

Several brain regions become particularly active during REM sleep. There is strong metabolic activity in higher-order occipito-temporal visual association areas, which might be responsible for the often very vivid visual dream imagery during REM sleep. Hyperactivity in motor regions such as the primary motor and premotor cortices, the cerebellum, and the basal ganglia may account for the frequently reported motor content of dreams. Furthermore, increased levels of activity have been observed in the pontine tegmentum, the thalamus, the basal forebrain, as well as in limbic and paralimbic structures (e.g. amygdaloid complexes, hippocampal formation, and anterior cingulate cortex) that are associated with emotional processing and might be responsible for the often very intense emotional aspects of REM sleep dreaming. There is also increased activity in other regions such as the medial prefrontal cortex, circuits of the medial temporal lobe region, and the posterior cingulate cortex that are implicated in memory and self-referential processing. In fact, there is striking overlap between the default mode network (i.e. the network of brain regions that are active when an individual is awake and not currently engaged in a task), which is associated with self-referential processing, and areas that become increasingly active during REM sleep. This network may play a key role in both mind-wandering and dreaming and possibly represents a shared neural substrate of the two phenomena.

However, a number of structures show decreased levels of activity during REM sleep. Among these structures is the right inferior parietal cortex, which is involved in waking volition and which contributes to a unified representation of self and self versus other perspectives. Decreased activity of the right inferior parietal cortex might allow the dreamer to participate in both first- and third-person perspectives. Moreover, there is deactivation of executive regions of the prefrontal cortex such as the dorsolateral prefrontal cortex (DLPFC) and the orbitofrontal cortex, but also in regions including the posterior cingulate gyrus, the precuneus, and the inferior parietal cortex. These areas are typically involved in cognitive control, metacognition, and ego functions (e.g. orientation in time and space, reality testing, and self-monitoring) and may underlie the lack of insight, restricted volitional capabilities, and impaired metacognition during dreaming. Hypoactivation of the prefrontal cortex may also be a contributing factor for dream amnesia.

REM sleep dreaming and waking cognition have been found to be largely comparable in many respects. Dreaming and waking cognition may differ in terms of their origin (i.e. their respective causal pathway), with the former partly representing an offline, internally generated simulation of the latter. In line with this notion, it has been suggested that dreams may be seen as a purer form of consciousness, which is free of the constraints imposed by the perception of, and interaction with, physical environments.

REM sleep dreaming shows mostly aspects of primary but not of secondary consciousness. During REM sleep, the dreamer tends to have less metacognitive activity (i.e. the processes by which individuals monitor and control their own cognitive processes), reflective thought, and volitional capabilities. The dreamer has only limited access to information about the past and anticipated future, and typically concerns him or herself exclusively with the present content of the dream narrative. However, some reports of REM dreams involve reflective thought, such as puzzlement about improbable or impossible events, contemplative alternatives in decision-making, and reflection during social interactions, as well as theory of mind processes (i.e. the ability to attribute mental states to oneself and others).

Sub-topic 3: Brain activity during REM sleep

REM sleep is characterised by rapid eye movement, global high-frequency and low-amplitude electroencephalogram (EEG) activity, as well as increased heart rate, respiratory activity, and muscle atonia (i.e. temporary muscular paralysis). During REM sleep, the brain is both off-line and chemically differentiated compared with the waking brain.

REM sleep is associated with the activation of the pons, thalamus, limbic areas, and temporo-occipital cortices, and the deactivation of prefrontal areas, in line with theories of REM sleep generation and dreaming properties.

The thalamus, a central structure for the generation of spindles, is involved in the generation of sleep spindles. These are synchronised by the depolarising phase of the slow oscillation through cortico-thalamocortical loops.

The slow oscillation is a cortically generated rhythm, made up of two phases: a prolonged depolarisation ("up" state) associated with brisk neuronal firing and a prolonged hyperpolarisation ("down" state) when neurons are silent. The synaptic reflection of the cortical slow oscillation in thalamic reticular neurons creates conditions for the generation of spindles and explains the grouping of spindles by the depolarising phase of the slow oscillation.

The neural correlates of REM sleep dreaming have been investigated using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). These studies have shown a decrease in brain activity during NREM sleep and a sustained level of brain function during REM sleep when compared to wakefulness, in addition to specifically segregated patterns of regional neural activity for each sleep stage.

REM sleep has been associated with the activation of the pons, thalamus, limbic areas, and temporo-occipital cortices, and the deactivation of prefrontal areas, in line with theories of REM sleep generation and dreaming properties.

REM sleep is also the sleep stage during which dreams are prominent. The functional brain mapping during REM sleep might therefore also be interpreted in light of dreaming properties.

During REM sleep, there is strong metabolic activity in higher-order occipito-temporal visual association areas, which might be responsible for the often very vivid visual dream imagery during REM sleep. Hyperactivity in motor regions such as the primary motor and premotor cortices, the cerebellum, and the basal ganglia may account for the frequently reported motor content of dreams. Furthermore, increased levels of activity have been observed in the pontine tegmentum, the thalamus, the basal forebrain, as well as in limbic and paralimbic structures (e.g. amygdaloid complexes, hippocampal formation, and anterior cingulate cortex) which are associated with emotional processing and might be responsible for the often very intense emotional aspects of REM sleep dreaming. There is also increased activity in other regions such as the medial prefrontal cortex, circuits of the medial temporal lobe region, and the posterior cingulate cortex which are implicated in memory and self-referential processing.

There is also striking overlap between the default mode network (i.e. the network of brain regions that are active when an individual is awake and not currently engaged in a task), which is associated with self-referential processing, and areas that become increasingly active during REM sleep. This network may play a key role in both mind-wandering and dreaming and possibly represents a shared neural substrate of the two phenomena.

During REM sleep, there is deactivation of executive regions of the prefrontal cortex such as the dorsolateral prefrontal cortex (DLPFC) and the orbitofrontal cortex, but also in regions including the posterior cingulate gyrus, the precuneus, and the inferior parietal cortex. These areas are typically involved in cognitive control, metacognition, and ego functions (e.g. orientation in time and space, reality testing, and self-monitoring) and may underlie the lack of insight, restricted volitional capabilities, and impaired metacognition during dreaming.

Sub-topic 4: Dreaming and consciousness during NREM sleep

Dreaming during the different stages of sleep varies in terms of phenomenological characteristics and consciousness. Dreaming during REM sleep typically follows loose, fanciful, and often very bizarre narratives; relates to current concerns; reflects interests, personality, and mood; draws on long-term memory; and involves social interactions. The dreamer is often uncertain about time, space, and personal identities and typically has the subjective experience of being awake. Reports of dreaming tend to be most elaborate and bizarre after waking up from the last period of REM sleep.

After the sleep-onset, NREM dreams are typically more thought-like, fragmentary, and related to current concerns, unlike the vivid, hallucinatory, and mainly visual content of REM dreams. After awakenings from sleep stage N3 early during the night, reports tend to be short, thought-like, less vivid, less visual, less motorically animated, less emotional, and less emotionally pleasant than REM reports, while being more conceptual, more plausible, more concerned with current issues, and typically involve greater volitional control. Late night NREM sleep reports are usually longer and more hallucinatory, often indistinguishable from REM sleep reports.

Dreams that occur during REM sleep show mostly aspects of primary but not of secondary consciousness. During REM sleep, the dreamer tends to have less metacognitive activity, reflective thought, and volitional capabilities. The dreamer has only limited access to information about the past and anticipated future, and typically concerns him or herself exclusively with the present content of the dream narrative. However, some reports of REM dreams involve reflective thought, such as puzzlement about improbable or impossible events, contemplative alternatives in decision-making, and reflection during social interactions, as well as theory of mind processes.

Reports of conscious experience across NREM sleep phases vary to a great extent. Although the frequency of reporting dreams after NREM awakenings is sparse and generally less elaborate than after periods of REM sleep, which might be due to the brain’s inability to encode memories of the dreams, the very existence of NREM dream reports provides evidence for the idea that consciousness does not fully cease during NREM sleep.

Lucid dreaming is a hybrid state of consciousness with features of both waking and dreaming. It has been suggested that lucid dreaming may occur during periods of NREM sleep. Lucid dreaming has a special status compared with non-lucid REM and NREM dreaming because it is a skill that needs to be trained and occurs only rarely in untrained individuals. Dream lucidity can be achieved through metacognitive training, developing autosuggestions, external sensory stimulation, and through frequently contemplating about one’s own state of consciousness.

Sub-topic 5: Brain activity during NREM sleep

NREM sleep is divided into three stages: N1, N2, and N3, with N3 being the deepest. NREM sleep is considered the restful or quiet sleep phase. During NREM sleep, various bodily functions slow down or stop altogether, allowing reparative and restorative processes to take place.

N1

N1 is the transitional stage between wakefulness and sleep, typically lasting one to seven minutes. During this stage, heartbeat, eye movements, brain waves, and breathing activity begin to slow down, and motor movements may diminish.

N2

N2 is still considered light sleep, during which heartbeat, breathing, muscle activity, and eye movements continue to slow. The body temperature also decreases. The brain exhibits unique types of brain activity during this stage, including sleep spindles and K-complexes, which are thought to play a role in memory consolidation and blocking out external stimuli.

N3

N3 is also known as deep sleep, and it is the stage during which the body repairs tissues, muscles, and bones, and regulates glucose metabolism, immune system functioning, hormone release, and memory. The brain waves during this stage are mostly delta waves, with slow oscillations also present. People are less responsive to external stimuli during deep sleep, and if awakened, they may experience a period of grogginess or "sleep inertia".

Brain Activity during NREM Sleep

During NREM sleep, the brain exhibits decreased activity, especially in the thalamus and the frontal cortex. Some regions of the brain, such as the hippocampus, may spontaneously reactivate, particularly after intensive learning. Brain activity during NREM sleep is associated with circumstances of decreased vigilance and reduced alertness.

NREM Sleep and Consciousness

NREM sleep has been linked to both physical recovery and memory consolidation. Abnormalities in NREM sleep have been proposed to play a role in various conditions, including schizophrenia, epilepsy, Alzheimer's, Parkinson's, and autism spectrum disorders.

NREM sleep may impact the cardiovascular system, with blood pressure dropping during slow-wave sleep. This decrease in blood pressure is thought to have a protective effect against heart disease. However, individuals with conditions such as sleep apnea, chronic insomnia, or high blood pressure may not experience this dip in blood pressure, increasing their risk of heart problems.

NREM sleep is also associated with memory consolidation, with the brain optimising mental pathways for future learning. Specific patterns during NREM sleep are linked to improved working memory, verbal fluency, motor learning, and word retrieval.

NREM sleep is thought to play a role in both declarative and procedural memory. Declarative memory is the ability to recall information, while procedural memory involves learning new tasks. It is theorised that sleep spindles help strengthen neural connections related to recent memories, while slow-wave sleep tidies up the pathways, making them ready for use the next day.

NREM sleep is a vital phase of the sleep cycle, characterised by decreased brain activity and slower bodily functions, allowing for physical restoration and memory consolidation. The different stages of NREM sleep exhibit unique brain activity patterns and serve specific functions in maintaining overall health and well-being.

Frequently asked questions