"[1] The word hypnagogia is sometimes used in a restricted sense to refer to the onset of sleep, and contrasted with hypnopompia, Frederic Myers's term for waking up.
[8] 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.
[9] Among the more commonly reported,[10][11] 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.
Descriptions of exceptionally vivid and elaborate hypnagogic visuals can be found in the work of Marie-Jean-Léon, Marquis d'Hervey de Saint Denys.
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.
Like the visuals, hypnagogic sounds vary in intensity from faint impressions to loud noises, like knocking and crashes and bangs (exploding head syndrome).
Proprioceptive effects may be noticed, with numbness and changes in perceived body size and proportions,[15] feelings of floating or bobbing as if their bed were a boat, and out-of-body experiences.
[16] 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.
Hypnagogia may involve a "loosening of ego boundaries ... openness, sensitivity, internalization-subjectification of the physical and mental environment (empathy) and diffuse-absorbed attention.
[20] 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.
[22] 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.
[27] 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.
[29][30] Davis et al. found short flashes of dreamlike imagery at the onset of sleep to correlate with drop-offs in alpha EEG activity.
[10] Germaine and Nielsen found spontaneous hypnagogic imagery to occur mainly during Hori sleep onset stages 4 (EEG flattening) and 5 (theta ripples).
[37] 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.
… It is an undoubted fact, that although our senses of touch and sight be for the time dead, yet our sleeping thoughts, and the visionary scenes that pass before us, will be influenced and materially influenced, by the mere silent presence of some external object; which may not have been near us when we closed our eyes: and of whose vicinity we have had no waking consciousness.”[38][39]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.
[12] [43] 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".