Shadow people are the experience of perceiving patches of shadow in one's peripheral or direct line of sight to appear and behave as living, autonomous beings.    Due to the unique behaviour of these hallucinations, they can be considered a distinct subtype of autonomous entities.
Shadow people usually appear initially as fleeting images in a person's peripheral vision. However, at higher levels of intensity, shadow people may appear in full view. This allows the user to directly look at one in their central line of sight. At higher levels of intensity, it becomes possible to look away from and back to a shadow person without a change in the presence or appearance of the hallucination.
The bodies of shadow people are usually perceived as being comprised of blackness that has a sense of depth and few facial or body features. The blackness of their bodies often seem almost opaque, as if one is looking into a "black hole" in humanoid form. They may also appear in the shape of animals, uniform blobs, disembodied body parts, or a myriad of other indescribable shapes. They sometimes appear to have faces, eyes, or mouths and are able to move or change shape. The movement exhibited can be normal human movement or it can be faster, slower, or choppier than a normal person's gait. It is also possible for multiple shadow people to occupy one's field of vision simultaneously while acting autonomously from one another, sometimes even interacting with each other.
It is worth noting that the style and general behaviour of a shadow person are often largely dependent on the emotional state of the person experiencing it. For example, a person who is emotionally stable and generally happy will be more prone to experiencing neutral, interesting, or friendly shadow people. In contrast, however, a person who is emotionally unstable and generally unhappy will be more prone to experiencing sinister and fear-inducing shadow people.
Shadow people are often accompanied by other coinciding effects, such as delirium, paranoia, anxiety, and feelings of impending doom. They are most commonly induced under the influence of heavy dosages of deliriant compounds, such as DPH, datura, and benzydamine. However, they can also occur under the influence of stimulant psychosis, sleep deprivation, and during sleep paralysis. 
Although it is uncommon and not an intrinsic part of this hallucinatory effect, shadow people can be accompanied by other sensory components alongside of the person's visual perception of them. This usually only occurs during very intense states of sleep deprivation, delirium, or psychosis. For example, shadow people can potentially have an accompanying "voice", despite the lack of a visible mouth structure. This auditory communication follows an identical levelling system of progressively more detailed and coherent speech similar to a generic autonomous entity. Shadow people may converse with the person experiencing them or they may converse amongst each other, sometimes talking about the person going through the experience.
Alongside accompanying auditory hallucinations, shadow people may also present tactile and gustatory hallucinations. This is even rarer than their potential auditory effects and typically only occurs in particularly intense and advanced hallucinatory states. Their tactile effects can be indistinguishable from a real human touch and may vary in temperature. They can also include physical actions, such as pulling on clothing, hair, or one's skin.
Silhouetted and darkened human figures are some of the simplest and most common hallucinations for the human brain to manifest during altered states. This may be why shadow people have been referenced throughout popular culture and time as 'demons' or 'omens',  'ghosts',  or even 'inter-dimensional time travelers".  This wide recognition of shadow people, combined with their representation in common culture and horror films, may further contribute to the prevalence of these external hallucinations.
The more an image is spread throughout a culture, the more likely it is that a person will experience that image in a state such as sleep deprivation, delirium, or stimulant psychosis. The fear instilled by society and the negative connotation portrayed by mainstream culture may also influence the anxiety and feelings of impending doom that people commonly report when seeing these hallucinations. This has been investigated in relation to the common appearance of shadow people in sleep deprivation experiences. 
A popular example of shadow people within mainstream media includes the 20th-century show "The Twilight Zone". This is specifically within the appropriately titled episode "The Shadow Man"  , where a shadow person lives under the main character's bed. The episode was broadcast nationwide and still remains available, further exposing people to shadow people through second-hand experiences and Hollywood-style manufactured images.
Due to the cultural influence and the perception that shadow people may, in fact, be "ghosts", people who experience this external hallucination may attribute it to the paranormal or other irrational causes rather than accepting that it is a natural effect of abnormal brain chemistry levels that may stem from a wide variety of mental states. The de-stigmatization of the shadow person experience and rational discussion of their true origin may grant many sufferers relief from the associated stigma of paranoia and mental illness.
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- "4 Types of Shadow People" | https://pararational.com/types-of-shadow-people/
- "Types of Shadow People" Blog | http://www.angelsghosts.com/types_of_shadow_people_shadow_ghosts
- Nonscientific Shadow Person Blog | http://www.shadowpeople.org/
- IMDB "Shadow Man" | http://www.imdb.com/title/tt0734758/
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