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State between wakefulness and sleep (articles)
Hypnagogia is the experience of the transitional state from wakefulness to sleep: the hypnagogic state of consciousness, during the onset of sleep. Mental phenomena that occur during this "threshold consciousness" phase include lucid thought, lucid dreaming, hallucinations, and sleep paralysis.

Sometimes the word hypnagogia is used in a restricted sense to refer to the onset of sleep, and contrasted with hypnopompia, Frederic Myers's term for waking up. However, hypnagogia is also regularly employed in a more general sense that covers both falling asleep and waking up, and Havelock Ellis questioned the need for separate terms. Indeed, it is not always possible in practice to assign a particular episode of any given phenomenon to one or the other, given that the same kinds of experience occur in both, and that people may drift in and out of sleep. In this article hypnagogia will be used in the broader sense, unless otherwise stated or implied.
Other terms for hypnagogia, in one or both senses, that have been proposed include "presomnal" or "anthypnic sensations", "visions of half-sleep", "oneirogogic images" and "phantasmata", "the borderland of sleep", "praedormitium", "borderland state", "half-dream state", "pre-dream condition", "sleep onset dreams", "dreamlets", and "wakefulness-sleep transition" (WST).
Threshold consciousness (commonly called "half-asleep" or "half-awake", or "mind awake body asleep") describes the same mental state of someone who is moving towards sleep or wakefulness, but has not yet completed the transition. Such transitions are usually brief, but can be extended by sleep disturbance or deliberate induction, for example during meditation.

Early references to hypnagogia are to be found in the writings of Aristotle, Iamblichus, Cardano, Simon Forman and Swedenborg. Romanticism brought a renewed interest in the subjective experience of the edges of sleep. In more recent centuries, many authors have referred to the state; Edgar Allan Poe, for example, wrote of the "fancies" he experienced "only when I am on the brink of sleep, with the consciousness that I am so."
Serious scientific inquiry began in the 19th century with Johannes Peter Müller, Jules Baillarger and Alfred Maury, and continued into the 20th century with Leroy.
Charles Dickens' Oliver Twist, contains an elaborate description of the hypnagogic state in Chapter XXXIV.
The advent of electroencephalography (EEG) has supplemented the introspective methods of these early researchers with physiological data. The search for neural correlates for hypnagogic imagery began with Davis et al. in the 1930s, and continues with increasing sophistication. While the dominance of the behaviorist paradigm led to a decline in research, especially in the English speaking world, the later twentieth century has seen a revival, with investigations of hypnagogia and related altered states of consciousness playing an important role in the emerging multidisciplinary study of consciousness. Nevertheless, much remains to be understood about the experience and its corresponding neurology, and the topic has been somewhat neglected in comparison with sleep and dreams; hypnagogia has been described as a "well-trodden and yet unmapped territory".
The word hypnagogia entered the popular psychology literature through Dr. Andreas Mavromatis in his 1983 thesis, while hypnagogic and hypnopompic were coined by others in the 1800s and noted by Havelock Ellis. The term hypnagogic was originally coined by Alfred Maury to name the state of consciousness during the onset of sleep. Hypnopompic was coined by Frederic Myers soon afterwards to denote the onset of wakefulness. The term hypnagogia is used by Dr. Mavromatis to identify the study of the sleep-transitional consciousness states in general, and he employs hypnogogic (toward sleep) or hypnapompic (from sleep) for the purpose of identifying the specific experiences under study.
Important reviews of the scientific literature have been made by Leaning, Schacter, Richardson and Mavromatis.

Transition to and from sleep may be attended by a wide variety of sensory experiences. These can occur in any modality, individually or combined, and range from the vague and barely perceptible to vivid hallucinations.

Among the more commonly reported, and more thoroughly researched, sensory features of hypnagogia are phosphenes which can manifest as seemingly random speckles, lines or geometrical patterns, including form constants, or as figurative (representational) images. They may be monochromatic or richly colored, still or moving, flat or three-dimensional (offering an impression of perspective). Imagery representing movement through tunnels of light is also reported. Individual images are typically fleeting and given to very rapid changes. They are said to differ from dreams proper in that hypnagogic imagery is usually static and lacking in narrative content, although others understand the state rather as a gradual transition from hypnagogia to fragmentary dreams, i.e., from simple Eigenlicht to whole imagined scenes. Descriptions of exceptionally vivid and elaborate hypnagogic visuals can be found in the work of Marie-Jean-Léon, Marquis d'Hervey de Saint Denys.

Tetris effect
People who have spent a long time at some repetitive activity before sleep, in particular one that is new to them, may find that it dominates their imagery as they grow drowsy, a tendency dubbed the Tetris effect. This effect has even been observed in amnesiacs who otherwise have no memory of the original activity. When the activity involves moving objects, as in the video game Tetris, the corresponding hypnagogic images tend to be perceived as moving. The Tetris effect is not confined to visual imagery, but can manifest in other modalities. For example, Robert Stickgold recounts having experienced the touch of rocks while falling asleep after mountain climbing. This can also occur to people who have travelled on a small boat in rough seas, or have been swimming through waves, shortly before going to bed, and they feel the waves as they drift to sleep, or people who have spent the day skiing who continue to "feel snow" under their feet, also people who have spent considerable time jumping on a trampoline will find that they can feel the up-and-down motion before they go to sleep. Many chess players report the phenomenon of seeing the chess board and pieces during this state. New employees working stressful and demanding jobs often report doing work-related tasks in this period before sleep.

Hypnagogic hallucinations are often auditory or have an auditory component. Like the visuals, hypnagogic sounds vary in intensity from faint impressions to loud noises, like knocking and crash and bangs (exploding head syndrome). People may imagine their own name called, crumpling bags, white noise, or a doorbell ringing. Snatches of imagined speech are common. While typically nonsensical and fragmented, these speech events can occasionally strike the individual as apt comments on — or summations of — their thoughts at the time. They often contain word play, neologisms and made-up names. Hypnagogic speech may manifest as the subject's own "inner voice", or as the voices of others: familiar people or strangers. More rarely, poetry or music is heard.

Sleep paralysis
Humming, roaring, hissing, rushing, zapping, and buzzing noises are frequent in conjunction with sleep paralysis. This happens when the REM atonia sets in sooner than usual, before the person is fully asleep, or persists longer than usual, after the person has (in other respects) fully awoken. Sleep paralysis is reportedly very frequent among narcoleptics. It occurs frequently in about 6% of the rest of the population, and occurs occasionally in 60%. In surveys from Canada, China, England, Japan and Nigeria, 20% to 60% of individuals reported having experienced sleep paralysis at least once in their lifetime. The paralysis itself is frequently accompanied by additional phenomena. Typical examples include a feeling of being crushed or suffocated, electric "tingles" or "vibrations", imagined speech and other noises, the imagined presence of a visible or invisible entity, and sometimes intense emotion: fear or euphoria and orgasmic feelings. Sleep paralysis has been proposed as an explanation for at least some alien abduction experiences, the Night Hag and shadow people hauntings.

Other sensations
Gustatory, olfactory and thermal sensations in hypnagogia have all been reported, as well as tactile sensations (including those kinds classed as paresthesia or formication). Sometimes there is synesthesia; many people report seeing a flash of light or some other visual image in response to a real sound. Proprioceptive effects may be noticed, with numbness and changes in perceived body size and proportions, feelings of floating or bobbing, and out-of-body experiences. Perhaps the most common experience of this kind is the falling sensation, and associated hypnic jerk, encountered by many people, at least occasionally, while drifting off to sleep.

Cognitive and affective phenomena
Thought processes on the edge of sleep tend to differ radically from those of ordinary wakefulness. Hypnagogia may involve a "loosening of ego boundaries ... openness, sensitivity, internalization-subjectification of the physical and mental environment (empathy) and diffuse-absorbed attention." Hypnagogic cognition, in comparison with that of normal, alert wakefulness, is characterized by heightened suggestibility, illogic and a fluid association of ideas. Subjects are more receptive in the hypnagogic state to suggestion from an experimenter than at other times, and readily incorporate external stimuli into hypnagogic trains of thought and subsequent dreams. This receptivity has a physiological parallel; EEG readings show elevated responsiveness to sound around the onset of sleep.
Herbert Silberer described a process he called autosymbolism, whereby hypnagogic hallucinations seem to represent, without repression or censorship, whatever one is thinking at the time, turning abstract ideas into a concrete image, which may be perceived as an apt and succinct representation thereof.
The hypnagogic state can provide insight into a problem, the best-known example being August Kekulé’s realization that the structure of benzene was a closed ring while half-asleep in front of a fire and seeing molecules forming into snakes, one of which grabbed its tail in its mouth. Many other artists, writers, scientists and inventors — including Beethoven, Richard Wagner, Walter Scott, Salvador Dalí, Thomas Edison, Nikola Tesla and Isaac Newton — have credited hypnagogia and related states with enhancing their creativity. A 2001 study by Harvard psychologist Deirdre Barrett found that, while problems can also be solved in full-blown dreams from later stages of sleep, hypnagogia was especially likely to solve problems which benefit from hallucinatory images being critically examined while still before the eyes.
A feature that hypnagogia shares with other stages of sleep is amnesia. But this is a selective forgetfulness, affecting the hippocampal memory system, which is responsible for episodic or autobiographical memory, rather than the neocortical memory system, responsible for semantic memory. It has been suggested that hypnagogia and REM sleep help in the consolidation of semantic memory, but the evidence for this has been disputed. For example, suppression of REM sleep due to antidepressants and lesions to the brainstem has not been found to produce detrimental effects on cognition.
Hypnagogic phenomena may be interpreted as visions, prophecies, premonitions, apparitions and inspiration (artistic or divine), depending on the experiencers' beliefs and those of their culture.

Physiological studies have tended to concentrate on hypnagogia in the strict sense of spontaneous sleep onset experiences. Such experiences are associated especially with stage 1 of NREM sleep, but may also occur with pre-sleep alpha waves. Davis et al. found short flashes of dreamlike imagery at the onset of sleep to correlate with drop-offs in alpha EEG activity. Hori et al. regard sleep onset hypnagogia as a state distinct from both wakefulness and sleep with unique electrophysiological, behavioral and subjective characteristics, while Germaine et al. have demonstrated a resemblance between the EEG power spectra of spontaneously occurring hypnagogic images, on the one hand, and those of both REM sleep and relaxed wakefulness, on the other.
To identify more precisely the nature of the EEG state which accompanies imagery in the transition from wakefulness to sleep, Hori et al. proposed a scheme of 9 EEG stages defined by varying proportions of alpha (stages 1–3), suppressed waves of less than 20μV (stage 4), theta ripples (stage 5), proportions of sawtooth waves (stages 6–7), and presence of spindles (stages 8–9). Germaine and Nielsen found spontaneous hypnagogic imagery to occur mainly during Hori sleep onset stages 4 (EEG flattening) and 5 (theta ripples).
The "covert-rapid-eye-movement" hypothesis proposes that hidden elements of REM sleep emerge during the wakefulness-sleep transition stage. Support for this comes from Bódicz et al., who note a greater similarity between WST (wakefulness-sleep transition) EEG and REM sleep EEG than between the former and stage 2 sleep.
Respiratory pattern changes have also been noted in the hypnagogic state, in addition to a lowered rate of frontalis muscle activity.

Daydreaming and waking reveries
Microsleep (short episodes of immediate sleep onset) may intrude into wakefulness at any time in the wakefulness-sleep cycle, due to sleep deprivation and other conditions, resulting in impaired cognition and even amnesia.
Gurstelle and Oliveira distinguish a state which they call daytime parahypnagogia (DPH), the spontaneous intrusion of a flash image or dreamlike thought or insight into one's waking consciousness. DPH is typically encountered when one is "tired, bored, suffering from attention fatigue, and/or engaged in a passive activity." The exact nature of the waking dream may be forgotten even though the individual remembers having had such an experience. Gustelle and Oliveira define DPH as "dissociative, trance-like, [...] but, unlike a daydream, [...] not self-directed"—however, daydreams and waking reveries are often characterised as "passive", "effortless", and "spontaneous", while hypnagogia itself can sometimes be influenced by a form of autosuggestion, or "passive concentration", so these sorts of episode may in fact constitute a continuum between directed fantasy and the more spontaneous varieties of hypnagogia. Others have emphasized the connections between fantasy, daydreaming, dreams and hypnosis.
In his book, Zen and the Brain, James H. Austin cites speculation that regular meditation develops a specialized skill of "freezing the hypnagogic process at later and later stages" of the onset of sleep, initially in the alpha wave stage and later in theta.

Investigative methodology
Self-observation (spontaneous or systematic) was the primary tool of the early researchers. Since the late 20th century, this has been joined by questionnaire surveys and experimental studies. All three methods have their disadvantages as well as points to recommend them.
Naturally, amnesia contributes to the difficulty of studying hypnagogia, as does the typically fleeting nature of hypnagogic experiences. These problems have been tackled by experimenters in a number of ways, including voluntary or induced interruptions, sleep manipulation, the use of techniques to "hover on the edge of sleep" thereby extending the duration of the hypnagogic state, and training in the art of introspection to heighten the subject's powers of observation and attention.
Techniques for extending hypnagogia range from informal ones (e.g., the subject holds up one of their arms as they go to sleep, so as to be awakened when it falls), to the use of biofeedback devices to induce a "theta" state, characterized by relaxation and theta EEG activity. The theta state is produced naturally the most when we are dreaming.
Another method is to induce a state said to be subjectively similar to sleep onset in a Ganzfeld setting, a form of sensory deprivation. But the assumption of identity between the two states may be unfounded. The average EEG spectrum in Ganzfeld is more similar to that of the relaxed waking state than to that of sleep onset. Wackerman et al. conclude that "the Ganzfeld imagery, although subjectively very similar to that at sleep onset, should not be labeled as 'hypnagogic'. Perhaps a broader category of 'hypnagoid experience' should be considered, covering true hypnagogic imagery as well as subjectively similar imagery produced in other states."
Sleep onset is the transition from wakefulness into sleep. Sleep onset usually transmits into non-rapid eye movement sleep (NREM sleep) but under certain circumstances (e.g. narcolepsy) it is possible to transit from wakefulness directly into rapid eye movement sleep (REM sleep).

During the 1920s an obscure disorder that caused encephalitis and attacked the part of the brain that regulates sleep influenced Europe and North America. Although the virus that caused this disorder was never identified, the psychiatrist and neurologist Constantin von Economo decided to study this disease and identified a key component in the sleep-wake regulation. He identified the pathways that regulated wakefulness and sleep onset by studying the parts of the brain that were affected by the disease and the consequences it had on the circadian rhythm. He stated that the pathways that regulated sleep onset are located between the brain stem and the basal forebrain. His discoveries were not appreciated until the last two decades of the 20th century when the pathways of sleep onset were found to reside in the exact place that Constantin von Economo stated.

Physiological overview
Sleep electrophysiological measurements can be made by attaching electrodes to the scalp to measure the electroencephalogram (EEG) and to the chin to monitor muscle activity, recorded as the electromyogram (EMG). Electrodes attached around the eyes monitor eye movements, recorded as the electro-oculogram (EOG).
Von Economo, in his studies, noticed that lesions in the connection between the midbrain and the diencephalon caused prolonged sleepiness and therefore proposed the idea of an ascending arousal system. During the past few decades major ascending pathways have been discovered with located neurons and respective neurotransmitters. This pathway divides into two branches: one that ascends to the thalamus and activates the thalamus relay neurons, and another one that activates neurons in the lateral part of the hypothalamus and the basal forebrain, and throughout the cerebral cortex. The cell group involved in the first pathway is an acetylcholine-producing cell group called pedunculopontine and laterodorsal tegmental nucleus(PPT/LDT). These neurons play a crucial role in bridging information in between the thalamus and the cerebral cortex. These neurons have high activation during wakefulness and during REM sleep and a low activation during NREM sleep. The second branch originates from monoaminorgenic neurons. These neurons are located in the locus coeruleus, dorsal and median raphe nuclei, ventral periaqueductal grey matter, and tuberomammillary nucleus. Each group produces a different neurotransmitter. The neurons in the locus coeruleus produce noradrenaline, as fore the neurons in the dorsal and median raphe nuclei, ventral periaqueductal grey matter, and tuberomammillary nucleus produce serotonin, dopamine and histamine respectively. They then project onto the hypothalamic peptidergic neurons, which contain melanin-concentrated hormones or orexin, and basal forebrain neurons which contain GABA and acetylcholine. These neurons then project onto the cerebral cortex. It has also been discovered that lesions to this part of the brain cause prolonged sleep or may produce coma.
The sleep cycle is normally defined in stages. When an individual first begins to sleep, stage 1 is entered, marked by the presence of some theta activity, which indicates that the firing of neurons in the neocortex is becoming more synchronized, as well as alpha wave activity (smooth electrical activity of 8–12 Hz recorded from the brain, generally associated with a state of relaxation). This stage is a transition between sleep and wakefulness. An individual's eyelids will from time to time slowly open and close and their eyes will roll upward and downward. Before one reaches sound sleep, stage 2 is entered. The EEG during this phase is normally irregular, but contains periods of theta activity, sleep spindles, and K complexes. Sleep spindles are short bursts of waves of 12–14 Hz that occur between two and five times a minute during states 1-4 of sleep. They appear to play a role in memory consolidation, and increased number of sleep spindles are correlated with increased scores on tests of intelligence. K complexes are sudden, sharp waveforms, which, unlike sleep spindles, are usually found only during stage 2 of sleep. They spontaneously occur at the rate of approximately one per minute, but often can be triggered by unexpected noises. It has been found that K complexes consist of isolated periods of inhibition. They appear to be the precursor of delta waves, also known as slow wave sleep, which appear during the deepest levels of sleep. Both these stages are classified as non-REM sleep.

Neural circuit
Sleep onset is finally induced by sleep-promoting neurons, located in the ventrolateral preoptic nucleus (VLPO). The sleep-promoting neurons are believed to project GABA type A and galanin, two known inhibitory neurotransmitters, to arousal-promoting neurons, such as histaminergic, serotonergic, orexinergic, noradrenergic, and cholinergic neurons (neurons mentioned above). Levels of acetylcholine, norepinepherine, serotonin, and histamine decrease with the onset of sleep, for they are all wakefulness promoting neurotransmitters. Therefore, it is believed that the activation of sleep-promoting neurons causes the inhibition of arousal-promoting neurons, which leads to sleep. Evidence has shown that during the sleep-wake cycle, sleep-promoting neurons and the arousal-promoting neurons have reciprocal discharges, and that during NREM sleep, GABA receptors increase in the arousal-promoting neurons. This had led some to believe that the increase of GABA receptors in the arousal-promoting neurons is another pathway of inducing sleep.
Specifically, Adenosine is known as the sleep promoting nucleoside neuromodulator. Astrocytes maintain a small stock of nutrients in the form of glycogen. In times of increased brain activity, such as during daytime, this glycogen is converted into fuel for neurons; thus, prolonged wakefulness causes a decrease in the level of glycogen in the brain. A fall in the level of glycogen causes an increase in the level of extracellular adenosine, which has an inhibitory effect in neural activity. This accumulation of adenosine serves as a sleep-promoting substance. Additionally, the production of the neurotransmitter acetylcholine increases in the hippocampus as sleep continues, and also increases in the cortex until REM sleep is reached.
The majority of sleep neurons are located in the ventrolateral preoptic area (vlPOA). These sleep neurons are silent until an individual shows a transition from waking to sleep. The sleep neurons in the preoptic area receive inhibitory inputs from some of the same regions they inhibit, including the tubermammillary nucleus, raphe nuclei, and locus coeruleus. Thus, they are inhibited by histamine, serotonin, and norepinepherine. This mutual inhibition may provide the basis for establishing periods of sleep and waking. A reciprocal inhibition also characterizes an electronic circuit known as the flip-flop. A flip-flop can assume one of two states, usually referred to as on or off. Thus, either the sleep neurons are active and inhibit the wakefulness neurons, or the wakefulness neurons are active and inhibit the sleep neurons, Because these regions are mutually inhibitory, it is impossible for neurons in both sets of regions to be active at the same time. This flip-flop, switching from one state to another quickly, can be unstable.
Yoga nidra (Sanskrit: योग निद्रा) or yogic sleep is a state of consciousness between waking and sleeping, like the "going-to-sleep" stage. It is a state in which the body is completely relaxed, and the practitioner becomes systematically and increasingly aware of the inner world by following a set of verbal instructions. This state of consciousness (yoga nidra) is different from meditation in which concentration on a single focus is required. In yoga nidra the practitioner remains in a state of light pratyahara with four of his senses internalised, that is, withdrawn, and only the hearing still connects to the instructions. The yogic goal of both paths, deep relaxation (yoga nidra) and meditation are the same, a state called samadhi.
Yoga nidra is among the deepest possible states of relaxation while still maintaining full consciousness. In lucid dreaming, one is only, or mainly, cognizant of the dream environment, and has little or no awareness of one's actual environment.
The practice of yoga relaxation has been found to reduce tension and anxiety. The autonomic symptoms of high anxiety such as headache, giddiness, chest pain, palpitations, sweating and abdominal pain respond well. It has been used to help soldiers from war cope with posttraumatic stress disorder (PTSD).
Yoga nidra refers to the conscious awareness of the deep sleep state, referred to as prajna in Mandukya Upanishad.

History and background
The concept of yoga nidra is very ancient in Indian traditions such as Hinduism and Buddhism. Krishna is often associated with yoga nidra in the epic Mahabharata. Similarly, many yogis and rishis are supposed to have experienced yoga nidra throughout their life.
In modern times, yoga nidra was experienced by Satyananda Saraswati when he was living with his guru Sivananda Saraswati in Rishikesh. He began studying the tantric scriptures and, after practice, constructed a system of relaxation, which he began popularizing in the mid-20th century. He explained yoga nidra as a state of mind between wakefulness and sleep that opened deep phases of the mind, suggesting a connection with the ancient tantric practice called nyasa, whereby Sanskrit mantras are mentally placed within specific body parts, while meditating on each part (of the bodymind). The form of practice taught by Satyananda includes eight stages (internalisation, sankalpa, rotation of consciousness, breath awareness, manifestation of opposites, creative visualization, sankalpa and externalisation).
Satyananda used this technique, along with suggestion, on the child who was to become his successor, Niranjanananda Saraswati, from the age of four. He claims to have taught him several languages by this method.
Anandmurti Gurumaa defines yoga nidra as a state of conscious deep sleep. One appears to be sleeping but the unconscious mind is functioning at a deeper level: it is sleep with a trace of deep awareness. In normal sleep we lose track of our self but in yoga nidra, while consciousness of the world is dim and relaxation is deep, there remains an inward lucidity and experiences may be absorbed to be recalled later. Since yoga nidra involves an aimless and effortless relaxation it is often held to be best practised with an experienced yoga teacher who verbally delivers instructions.
Anandmurti Gurumaa taught two techniques based on creative visualization. Yoga nidra as Yoga of Clear Light is proposed as a spiritual path (sadhana) in its own right, held to prepare and refine a seeker (sadhaka) spiritually, emotionally, mentally and physically for consciousness and awareness. The yogi may work through the consequences of deeds (karma), cleansing the store consciousness and purifying the unconscious mind. The state may lead to realisation (samādhi) and being-awareness-bliss (satchitananda). The yogi is held to be in communion with the divine. A tantrika engaged in this sadhana may become aware of past or future lives (refer bhumi) or experience the astral planes.

Scientific evaluation
Experimental evidence of the existence of a fourth state of unified, transcendental consciousness, which lies in the yoga nidra state at the transition between sensory and sleep consciousness, was first recorded at the Menninger Foundation in Kansas, United States in 1971. Under the direction of Dr. Elmer Green, researchers used an electroencephalograph to record the brainwave activity of an Indian yogi, Swami Rama, while he progressively relaxed his entire physical, mental and emotional structure through the practice of yoga nidra. What they recorded was a revelation to the scientific community. The swami demonstrated the capacity to enter the various states of consciousness at will, as evidenced by remarkable changes in the electrical activity of his brain. Upon relaxing himself in the laboratory, he first entered the yoga nidra state, producing 70% alpha wave discharge for a predetermined 5 minute period, simply by imagining an empty blue sky with occasional drifting clouds.
Next, Swami Rama entered a state of dreaming sleep which was accompanied by slower theta waves for 75% of the subsequent 5 minute test period. This state, which he later described as being "noisy and unpleasant", was attained by "stilling the conscious mind and bringing forth the subconscious". In this state he had the internal experience of desires, ambitions, memories and past images in archetypal form rising sequentially from the subconscious and unconscious with a rush, each archetype occupying his whole awareness.
Finally, the swami entered the state of (usually unconscious) deep sleep, as verified by the emergence of the characteristic pattern of slow rhythm delta waves. However, he remained perfectly aware throughout the entire experimental period. He later recalled the various events which had occurred in the laboratory during the experiment, including all the questions that one of the scientists had asked him during the period of deep delta wave sleep, while his body lay snoring quietly.
Such remarkable mastery over the fluctuating patterns of consciousness had not previously been demonstrated under strict laboratory conditions. The capacity to remain consciously aware while producing delta waves and experiencing deep sleep is one of the indications of the third state (prajna) out of the total of four states of consciousness described in the Mandukya Upanishad. This is the ultimate state of yoga nidra in which there are no dreams, but only the deep sleep state with retained consciousness/awareness. The result is a single, semi-enlightened state of consciousness and a perfectly integrated and relaxed personality.
In 2006, Kamakhya Kumar was awarded a PhD by Dr. A. P. J. Abdul Kalam (former President of India) for his work "Psycho-physiological Changes as Related to Yoga Nidra". He observed six months of effects of yoga nidra on some physiological, hematological and some psychological parameters on the practitioners and he found a significant change on above mentioned parameters. One of the pieces of research published, was entitled "A study on the impact on stress and anxiety through yoga nidra" Indian Journal of Traditional Knowledge, Vol. 7 No 3 (Published through NISCAIR).
Indian clinical psychologist Sachin Kumar Dwivedi (2009) found in his research that yoga nidra decreases levels of anxiety. S. Dwivedi, S. Awasthi and B.B. Pandey (2011) found in "Yoga Nidra increased the α-EEG on α-EEG biofeedback", that it is an open secret that yoga nidra is a type of deep meditation. M. Nikhra and S.K. Dwivedi (2010) found in a study "Yoga Nidra Reduces the Level of Stress".
Dream Yoga or Milam (Standard Tibetan: rmi-lam or nyilam; Sanskrit: स्वप्नदर्शन, svapnadarśana)—the Yoga of the Dream State—is a suite of advanced tantric sadhana of the entwined Mantrayana lineages of Dzogchen (Nyingmapa, Ngagpa, Mahasiddha, Kagyu and Bönpo). Dream Yoga are tantric processes and techniques within the trance Bardos of Dream and Sleep (Standard Tibetan: mi-lam bardo) Six Yogas of Naropa. In the tradition of the tantra, Dream Yoga method is usually passed on by a qualified teacher to his/her students after necessary initiation. Various Tibetan lamas are unanimous that it is more of a passing of an enlightened experience rather than any textual information.
In a footnote on 'Zhitro' (Tibetan: zhi khro) Namdak & Dixey, et al. (2002: p. 124) identify that the 'dream body' and the 'bardo body' is the 'vision body' (Tibetan: yid lus):
In the bardo one has...the yilu (yid lus), the vision body (yid, consciousness; lus, body). It is the same as the body of dreams, the mind body."
Luminosity or clear light (Tibetan od gsal, Sanskrit prabhāsvara), in Vajrayana, Tibetan Buddhism and Bon, refers to the nature of mind experienced in deep sleep and death.

The practitioner trains to consciously enter the deep sleep state. If one has the ability to remain conscious during deep sleep, one will be able to recognize the luminosity of death and gain Buddhahood. This is called the meeting of mother and child luminosities, resulting in the state of thuktam at death.
Йога-нидра (санскр. योगनिद्रा, yoganidrā IAST) — психотехника, разработанная Свами Сатьянанда Сарасвати. В средине XX века на основе исследований физиологии мозга и древних йогических практик Сатьянанда разработал систему йога-нидры. Эта техника связана с последовательной проработкой частей нашего тела в мотокортексе и сенсорной области мозга, что вызывает глубокое медитативное состояние. Эту практику называют йогой сна, или йогическим сном.
Практика йоги-нидры способствует снижению напряженности и беспокойства. Вегетативные симптомы высокой тревожности, такие как головная боль, головокружение, боль в груди, учащённое сердцебиение, повышенная потливость, боли в животе хорошо устраняются. Йога-нидра была использована, чтобы помочь справиться с посттравматическим стрессовым расстройством солдатам после войны.
Йога-нидра, в значении осознанного глубокого сна, в Мандукья-упанишаде называется «праджня».

Научная оценка
Первые экспериментальные данные о йога-нидре были получены в Menninger Foundation (США) в 1971 г. Под руководством доктора Элмера Грина, исследователи использовали энцефалограф для записи мозговой активности индийского йога Swami Rama, когда он расслаблялся при помощи практики йога-нидры.