Introduction:
Sleep-related hallucinations also called waking dreams, cause a person to awake from sleep. For centuries, both philosophers and scientists have been intrigued by the various perceptual experiences that occur during sleep, such as hypnagogic (a state before falling asleep) and hypnopompic (state lying between full waking and sleep) hallucinations at the edges of sleep, as well as dreams and parasomnias. These phenomena have been closely examined for their similarities to hallucinations experienced during wakefulness. Alfred Maury, a French researcher, highlighted the connection between the forms and causes of dreams and hallucinations. John Hughlings Jackson, an English neurologist, suggested that strong sensory discharges might be a common mechanism behind both dreams and hallucinations.
Some clinical disorders can also cause sleep-related hallucinations.
What Are Hallucinations?
Hallucinations are subjective perceptual experiences characterized by three main criteria:
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They involve perceptions in various sensory modalities (like auditory or visual).
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They are not triggered by external stimuli.
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They usually occur involuntarily, though this last criterion is not essential in all definitions.
There is no standard pattern for hallucinations; they can manifest differently across sensory modalities, and there is a blurry line between sensory perceptions and misperceptions, suggesting a continuum of subjective experiences.
Auditory and visual hallucinations are the most common studies, with variations depending on the specific population. For instance, people with schizophrenia spectrum disorders often experience auditory hallucinations (‘voices’) that may be seen as meaningful and external.
In Parkinson’s disease, visual hallucinations are more prevalent and usually involve complex visual scenes. In individuals with eye diseases like Charles Bonnet Syndrome (CBS), simple or detailed visual hallucinations can occur but are typically recognized as not real and not distressing.
What Are the Phenomena of Sleep-Related Hallucinations?
Hypnagogia: This term refers to brief perceptual experiences as a person transitions from wakefulness to sleep or from sleep to wakefulness. These experiences are involuntary, and spontaneous, and can involve visual, auditory, and tactile sensations. They are common in the general population, occurring in up to 70 percent of people. Visual phenomena are often kaleidoscopic and may include geometric patterns, while auditory sensations like voices or sounds are less common. Somatic experiences like bodily distortions can also occur. Hypnopompic hallucinations often continue dream sequences upon waking.
Dreams: Dreams occur during sleep and involve a continuous stream of perceptions, thoughts, and emotions. They encompass visual, auditory, and somatic or tactile sensations, with rich and dynamic visual imagery. Dreams can have nonsensical or bizarre content and the dreamer usually accepts these elements. Dreams may evoke strong emotions and often follow narrative structures, sometimes mixing real-life events with fantastical elements. Dreams are experienced in retrospect upon waking, primarily during REM (rapid eye movement) sleep, but occasionally during non-REM sleep.
Other Sleep-Related Phenomena: The incubus phenomenon refers to hallucinatory experiences during sleep paralysis, often accompanied by feelings of a person or entity on the chest, which can be intensely frightening. These experiences are typically accepted as vividly real, sometimes leading to interpretations as occult or metaphysical events. Parasomnias are sleep disorders characterized by undesirable behaviors with hallucinatory-like experiences, occurring during different sleep stages. Narcolepsy, a type of parasomnia, can involve sudden sleep episodes with muscle weakness and vivid hypnagogic images.
The Gradual Descent Hypothesis: This hypothesis suggests that various sleep-related perceptual phenomena share overlapping mechanisms, supported by neuroanatomical and neurophysiological evidence. Mental events during sleep gradually evolve from waking thoughts to hypnagogic hallucinations, dreams, and hypnopompic hallucinations, with changes in neural activation and information flow.
Similarities and Differences: Hypnagogia and dreams meet the core criteria of hallucinations. They share similarities with hallucinations in terms of vivid visual and auditory percepts. However, they differ in several ways:
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Hallucinations are typically unconnected to reality, while sleep-related perceptions occur as if in the external world.
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Hallucinations are often associated with self-reflective thoughts, whereas sleep-related perceptions have little impact on daytime activities.
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Dreams are frequently forgotten upon waking, unlike hallucinations, which are usually remembered.
What Is the Function of the Brain in Sleep-Related Hallucinations?
Sleep-Related Perceptions and Hallucinations:
1. Similarities in brain system and functions:
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Both sleep-related perceptions and hallucinations involve the thalamocortical circuit, implicated in perceptual release mechanisms.
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Increased activation in primary and secondary sensory cortices is seen in hallucinations and perceptions.
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Both involve thalamus activation and a focus on internal experiences.
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The limbic activity contributes to the emotional aspects of both phenomena.
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Both can involve spontaneous sensory discharges due to differentiations.
2. Differences in brain functions and systems:
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Both experience compromised signal propagation to the prefrontal cortex, but the extent of this differs.
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In REM sleep, there’s a complete suspension of connectivity between higher-order and sensory areas, while in hallucinations, connectivity between these regions is typically retained.
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Activation of the planum temporale during hallucinations, related to external sensory signals, is absent during sleep-related perceptions.
Neurotransmitters:
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Acetylcholine (Ach) plays a key role in both, but the balance of neurotransmitters differs.
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Hallucination models often involve ‘low’ ACh levels, while REM sleep has ‘high’ ACh levels.
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Serotonin, glutamate, and dopamine are implicated in both, with differences in their levels.
REM Dream Stage and REM Intrusion Hypotheses:
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The resemblance between dreams and hallucinations has led to theories suggesting common underpinnings.
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However, REM dreams do not fully capture the complexity of hallucinations, activate fewer networks, and have different neurotransmitter balances.
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The idea of REM dreams intrusions into wakefulness lacks conclusive evidence.
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Sleep disorders and hallucinations follow separate courses, raising questions about their common mechanisms.
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Theoretical models of hallucinations often focus on compensatory mechanisms for sensory disruptions, which differ from sleep-related perceptions.
What Is the Role of Sleep and Consciousness in Sleep-Related Hallucinations?
The concept of clear consciousness has played a significant role in defining hallucinations. However, this requirement becomes complex due to the modern understanding that consciousness encompasses various mental states, including sensory discrimination, perceptual awareness, focused attention, introspection, and voluntary actions, often overlapping.
During sleep, many of these mental functions remain active. In REM sleep, specific brain regions like the pontine nuclei and cholinergic neurons maintain high activity levels. Even in deep NREM sleep, there are patterns of neural activation in brainstem structures that resemble brief moments of wakefulness. The brain continues to process internal mental events, including sensory discrimination.
Nonetheless, sleep differs from wakefulness in several ways:
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Reduced awareness of external stimuli.
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Suspension of higher–level cognitive functions.
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A lack of meaningful interaction with the outside world.
Conclusion:
Sleep-related hallucinations are unique mental experiences that occur during various sleep stages. They share similarities with waking hallucinations but are shaped by the distinctive characteristics of sleep. Neurotransmitters, brain networks, and altered consciousness play key roles in these phenomena, which offer insights into the complexity of human consciousness during slumber. These hallucinations are not mere dream intrusions into wakefulness but represent distinct phenomena with their own underlying mechanisms.
