Hypnagogia

Hypnagogia, also referred to as "hypnagogic hallucinations", is the experience of the transitional state from to : the hypnagogic state of, during the  (for the transitional state from sleep to wakefulness see ). Mental phenomena that may occur during this "threshold consciousness" phase include s, lucid thought, ing, and. The latter two phenomena are themselves separate sleep conditions that are sometimes experienced during the hypnagogic state.

Definitions
The word hypnagogia is sometimes used in a restricted to refer to the onset of sleep, and contrasted with , '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 has questioned the need for separate terms. Indeed, it is not always possible in practice to assign a particular episode of any given to one or the other, given that the same kinds of  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.

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

Sights
Among the more commonly reported, and more thoroughly researched, sensory features of hypnagogia are which can manifest as seemingly random speckles, lines or geometrical patterns, including, or as  (representational). They may be or richly colored, still or moving, flat or three-dimensional (offering an impression of ). 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  to whole scenes. Descriptions of exceptionally vivid and elaborate hypnagogic visuals can be found in the work of.

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, a tendency dubbed the. This effect has even been observed in cs who otherwise have no of the original activity. When the activity involves moving objects, as in the video game , 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, 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. 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 players report the phenomenon of seeing the chess board and pieces during this state. New employees working stressful and demanding jobs often report feeling the experience of performing work-related tasks in this period before sleep.

Sounds
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. People may imagine their own name called, crumpling bags, white noise, or a doorbell ringing. Snatches of imagined are common. While typically and fragmented, these speech events can occasionally strike the individual as apt comments on—or summations of—their s at the time. They often contain, s and made-up names. Hypnagogic speech may manifest as the subject's own "", or as the voices of others: familiar people or strangers. More rarely, poetry or music is heard.

Other sensations
, and  sensations in hypnagogia have all been reported, as well as  sensations (including those kinds classed as  or ). Sometimes there is ; many people report seeing a flash of light or some other visual image in response to a real sound. effects may be noticed, with numbness and changes in perceived body size and proportions, feelings of floating or bobbing as if their bed were a boat, and s. Perhaps the most common experience of this kind is the falling sensation, and associated, encountered by many people, at least occasionally, while drifting off to sleep.

Cognitive and affective phenomena
processes on the edge of sleep tend to differ radically from those of ordinary wakefulness. For example, something that you agreed with in a state of hypnagogia may seem completely ridiculous to you in an awake state. Hypnagogia may involve a "loosening of boundaries ... openness, sensitivity, internalization-subjectification of the physical and mental environment  and diffuse-absorbed ." Hypnagogic, in comparison with that of normal, alert wakefulness, is characterized by heightened , illogic and a fluid association of s. Subjects are more receptive in the hypnagogic state to from an experimenter than at other times, and readily incorporate external  into hypnagogic trains of thought and subsequent dreams. This receptivity has a parallel;  readings show elevated responsiveness to sound around the onset of sleep.

described a process he called autosymbolism, whereby hypnagogic hallucinations seem to represent, without or censorship, whatever one is thinking at the time, turning abstract s into a concrete, which may be perceived as an apt and succinct representation thereof.

The hypnagogic state can provide into a, the best-known example being ’s realization that the structure of  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, , , , , and  — have credited hypnagogia and related states with enhancing their creativity. A 2001 study by Harvard psychologist 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. But this is a selective forgetfulness, affecting the memory system, which is responsible for  or, rather than the  memory system, responsible for. It has been suggested that hypnagogia and help in the consolidation of semantic memory, but the evidence for this has been disputed. For example, suppression of REM sleep due to s and s to the has not been found to produce detrimental effects on cognition.

Hypnagogic phenomena may be interpreted as, , , and inspiration ( or ), depending on the experiencers' beliefs and those of their culture.

Physiology
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, but may also occur with pre-sleep s. Davis et al. found short flashes of dreamlike imagery at the onset of sleep to correlate with drop-offs in alpha activity. Hori et al. regard sleep onset hypnagogia as a state distinct from both wakefulness and sleep with unique, al and characteristics, while Germaine et al. have demonstrated a resemblance between the EEG  of spontaneously occurring hypnagogic images, on the one hand, and those of both  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 (stages 1–3), suppressed waves of less than 20μV (stage 4),  ripples (stage 5), proportions of  waves (stages 6–7), and presence of  (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 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.

pattern changes have also been noted in the hypnagogic state, in addition to a lowered rate of activity.

Daydreaming and waking reveries
(short episodes of immediate sleep onset) may intrude into wakefulness at any time in the wakefulness-sleep cycle, due to and other conditions, resulting in impaired cognition and even amnesia.

In his book, , 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.

History
Early references to hypnagogia are to be found in the writings of, , , , and. brought a renewed interest in the subjective experience of the edges of sleep. In more recent centuries, many authors have referred to the state;, 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, , and , 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 (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 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  playing an important role in the emerging multidisciplinary study of. 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. The term hypnagogic was originally coined by to name the state of consciousness during the. Hypnopompic was coined by soon afterwards to denote the onset of. 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,, Richardson and Mavromatis.

Research
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 (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 "" state – produced naturally the most when we are dreaming – characterized by relaxation and theta activity.

Another method is to induce a state said to be subjectively similar to sleep onset in a setting, a form of. 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."