In the psychology subfield of oneirology, a lucid dream is a type of dream in which the dreamer realizes that they are dreaming whilst dreaming. The capacity to have lucid dreams is a trainable cognitive skill. During a lucid dream, the dreamer may gain some amount of volitional control over the dream characters, narrative, or environment, although this control of dream content is not the salient feature of lucid dreaming. An important distinction is that lucid dreaming is a distinct type of dream from other types of dreams such as prelucid dreams and vivid dreams, although prelucid dreams are a precursor to lucid dreams, and lucid dreams are often accompanied with enhanced dream vividness. Lucid dreams are also distinct state from other lucid boundary sleep states such as lucid hypnagogia or lucid hypnopompia.
A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep. Humans spend about two hours dreaming per night, and each dream lasts around 5 to 20 minutes, although the dreamer may perceive the dream as being much longer than this.
A hallucination is a perception in the absence of an external stimulus that has the qualities of a real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. Hallucination is a combination of two conscious states of brain wakefulness and REM sleep. They are distinguishable from several related phenomena, such as dreaming, which does not involve wakefulness; pseudohallucination, which does not mimic real perception, and is accurately perceived as unreal; illusion, which involves distorted or misinterpreted real perception; and mental imagery, which does not mimic real perception, and is under voluntary control. Hallucinations also differ from "delusional perceptions", in which a correctly sensed and interpreted stimulus is given some additional significance.
Rapid eye movement sleep is a unique phase of sleep in mammals and birds, characterized by random rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream vividly.
Sleep paralysis is a state, during waking up or falling asleep, in which a person is conscious but in a complete state of full-body paralysis. During an episode, one may hallucinate, which often results in fear. Episodes generally last no more than a few minutes. It can recur multiple times or occur as a single episode.
An altered state of consciousness (ASC), also called an altered state of mind or mind alteration, is any condition which is significantly different from a normal waking state. By 1892, the expression was in use in relation to hypnosis, though there is an ongoing debate as to whether hypnosis is to be identified as an ASC according to its modern definition. The next retrievable instance, by Max Mailhouse from his 1904 presentation to conference, however, is unequivocally identified as such, as it was in relation to epilepsy, and is still used today. In academia, the expression was used as early as 1966 by Arnold M. Ludwig and brought into common usage from 1969 by Charles Tart. It describes induced changes in one's mental state, almost always temporary. A synonymous phrase is "altered state of awareness".
A hypnic jerk, hypnagogic jerk, sleep start, sleep twitch, myoclonic jerk, or night start is a brief and sudden involuntary contraction of the muscles of the body which occurs when a person is beginning to fall asleep, often causing the person to jump and awaken suddenly for a moment. Hypnic jerks are one form of involuntary muscle twitches called myoclonus.
The sleep cycle is an oscillation between the slow-wave and REM (paradoxical) phases of sleep. It is sometimes called the ultradian sleep cycle, sleep–dream cycle, or REM-NREM cycle, to distinguish it from the circadian alternation between sleep and wakefulness. In humans, this cycle takes 70 to 110 minutes. Within the sleep of adults and infants there are cyclic fluctuations between quiet and active sleep. These fluctuations may persist during wakefulness as rest-activity cycles but are less easily discerned.
Non-rapid eye movement sleep (NREM), also known as quiescent sleep, is, collectively, sleep stages 1–3, previously known as stages 1–4. Rapid eye movement sleep (REM) is not included. There are distinct electroencephalographic and other characteristics seen in each stage. Unlike REM sleep, there is usually little or no eye movement during these stages. Dreaming occurs during both sleep states, and muscles are not paralyzed as in REM sleep. People who do not go through the sleeping stages properly get stuck in NREM sleep, and because muscles are not paralyzed a person may be able to sleepwalk. According to studies, the mental activity that takes place during NREM sleep is believed to be thought-like, whereas REM sleep includes hallucinatory and bizarre content. NREM sleep is characteristic of dreamer-initiated friendliness, compared to REM sleep where it is more aggressive, implying that NREM is in charge of simulating friendly interactions. The mental activity that occurs in NREM and REM sleep is a result of two different mind generators, which also explains the difference in mental activity. In addition, there is a parasympathetic dominance during NREM. The reported differences between the REM and NREM activity are believed to arise from differences in the memory stages that occur during the two types of sleep.
A false awakening is a vivid and convincing dream about awakening from sleep, while the dreamer in reality continues to sleep. After a false awakening, subjects often dream they are performing their daily morning routine such as showering or eating breakfast. False awakenings, mainly those in which one dreams that they have awoken from a sleep that featured dreams, take on aspects of a double dream or a dream within a dream. A classic example is the double false awakening of the protagonist in Gogol's Portrait (1835).
Hypnagogia is the transitional state from wakefulness to sleep, also defined as the waning state of consciousness during the onset of sleep. Its opposite state is described as hypnopompia – the transitional state from sleep into wakefulness. Mental phenomena that may occur during this "threshold consciousness" phase include hypnagogic hallucinations, lucid dreaming, and sleep paralysis.
In the field of psychology, the subfield of oneirology is the scientific study of dreams. Current research seeks correlations between dreaming and current knowledge about the functions of the brain, as well as an understanding of how the brain works during dreaming as pertains to memory formation and mental disorders. The study of oneirology can be distinguished from dream interpretation in that the aim is to quantitatively study the process of dreams instead of analyzing the meaning behind them.
Closed-eye hallucinations and closed-eye visualizations (CEV) are hallucinations that occur when one's eyes are closed or when one is in a darkened room. They should not be confused with phosphenes, perceived light and shapes when pressure is applied to the eye's retina, or some other non-visual external cause stimulates the eye. Some people report CEV under the influence of psychedelics; these are reportedly of a different nature than the "open-eye" hallucinations of the same compounds. Similar hallucinations that occur due to loss of vision are called "visual release hallucinations".
Parasomnias are a category of sleep disorders that involve abnormal movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, sleeping, between sleep stages, or during arousal from sleep. Parasomnias are dissociated sleep states which are partial arousals during the transitions between wakefulness, NREM sleep, and REM sleep, and their combinations.
Anomalous experiences, such as so-called benign hallucinations, may occur in a person in a state of good mental and physical health, even in the apparent absence of a transient trigger factor such as fatigue, intoxication or sensory deprivation.
Narcolepsy is a chronic neurological disorder that involves a decreased ability to regulate sleep–wake cycles. Symptoms often include periods of excessive daytime sleepiness and brief involuntary sleep episodes. Narcolepsy paired with cataplexy is evidenced to be an autoimmune disorder. These experiences of cataplexy can be brought on by strong emotions. Less commonly, there may be vivid hallucinations or an inability to move while falling asleep or waking up. People with narcolepsy tend to sleep about the same number of hours per day as people without it, but the quality of sleep tends to be lessened.
Scholarly interest in the process and functions of dreaming has been present since Sigmund Freud's interpretations in the 1900s. The neurology of dreaming has remained misunderstood until recent distinctions, however. The information available via modern techniques of brain imaging has provided new bases for the study of the dreaming brain. The bounds that such technology has afforded has created an understanding of dreaming that seems ever-changing; even now questions still remain as to the function and content of dreams.
Secondary consciousness is an individual's accessibility to their history and plans. The ability allows its possessors to go beyond the limits of the remembered present of primary consciousness. Primary consciousness can be defined as simple awareness that includes perception and emotion. As such, it is ascribed to most animals. By contrast, secondary consciousness depends on and includes such features as self-reflective awareness, abstract thinking, volition and metacognition. The term was coined by Gerald Edelman.
The activation-synthesis hypothesis, proposed by Harvard University psychiatrists John Allan Hobson and Robert McCarley, is a neurobiological theory of dreams first published in the American Journal of Psychiatry in December 1977. The differences in neuronal activity of the brainstem during waking and REM sleep were observed, and the hypothesis proposes that dreams result from brain activation during REM sleep. Since then, the hypothesis has undergone an evolution as technology and experimental equipment has become more precise. Currently, a three-dimensional model called AIM Model, described below, is used to determine the different states of the brain over the course of the day and night. The AIM Model introduces a new hypothesis that primary consciousness is an important building block on which secondary consciousness is constructed.
The neuroscience of sleep is the study of the neuroscientific and physiological basis of the nature of sleep and its functions. Traditionally, sleep has been studied as part of psychology and medicine. The study of sleep from a neuroscience perspective grew to prominence with advances in technology and the proliferation of neuroscience research from the second half of the twentieth century.
