Sleep paralysis.

Sleep paralysis - sounds mysterious, terrible, terrifying - and this is how it is perceived by people who experienced it.

Scenes straight from the horror - immobilization of the body, inability to say words, feeling fear and pressure on the chest, a sense of someone's presence in the room. All of these phenomenon seem unbelievable, and it turns out that at least 40% of society once in their lifetime had contact with them.

Source: Oil painting by Johann Heinrich Füssli, "The Nightmare". https://pl.khanacademy.org/humanities/becoming-modern/romanticism/romanticism-in-england/a/henry-fuseli-the-nightmare

Sleep paralysis is a phenomenon that occurs mostly when you fall asleep or when you waking up. In a simplified way, the body is already (or still) sleeping, while the mind is awake.

As a result of this dissociation, the experiencing person is fully aware, but cannot move. In addition, hallucinations may occur, because in reality the brain is still working. Visions that we experiencing are related to the fact that our brain still works in the REM phase during the awaking, which is characterized by increased brain activity and heart contractions as well as the production of dreams.

The whole experience is accompanied by a strong sense of fear and anxiety, which in the most likely way is caused by stress and shock, caused by quick awakening. Our brain identifies these factors as something bad and inappropriate that's why images we are experiencing usually represent something bad and disturbing. These hallucinations may be tactile, motor, visual or auditory. Most often, however, there is a feeling of someone's presence combined with fear.

Source: https://www.newgrounds.com/art/view/purpledemontoons/oney-s-sleep-paralysis-sleepycast-illustrated

Hypnagogic and hypnopompic experiences, although often colorful, give the impression of foggy and unreal events. These are most likely to accompany most cases of sleep paralysis. You can also include incorrect interpretations of shadows and fuzzy objects in the dark bedroom into HHE (these are delusions). Some people also experience fully developed hallucinations, during which they do not only experience colorful and complex sensations from different senses, but are also convinced that these experiences come from real, external sources. These people usually recognize their sensations, not as a result of sleep paralysis, but more often as an effect of "devil possession or abduction by aliens."

The dream consists of two significantly different states: REM sleep and NREM sleep.

Source: http://www.thegood.co/my-dream-predictions/

During REM sleep we observe desynchronization of cortical activity with a characteristic low-voltage record of high-frequency EEG activity(which is responsible for changes in electrical potential on the surface of the skin, derived from the activity of neurons in the cerebral cortex), as well as synchronized hippocampal activity characterized by slow oscillations of theta (4-8 Hz) waves. It is also believed that dreams occur more often and are more colorful during REM as compared to NREM.

In addition, the REM phase has many additional sensory, behavioral and physiological features, such as skeletal muscle atony, rapid eye movements, as well as changes in heart and breath rate.

Source: http://www.vitahealth.com.my/healthtopics/sleepdiary/stages.aspx?act=HT

Hallucinations accompanying sleep paralysis can be explained on the basis of the theory of activation-synthesis on the formation of dreams (Hobson and McCarley 1977). This theory implies that REM sleep is initiated as a result of REM-off inhibitory activity in retroconversion with REM-on cells. These mechanisms acting in the brainstem result in the inhibition of motor output and sensory input, so the cerebral cortex begins to be stimulated mainly by signals generated internally. These signals, generated without major rules, by accident, reach the bark, which tries to interpret them somehow and "adds" meaning of them, which results in dreams.

The intense fear that accompanies episodes of sleep paralysis, especially in people who encounter it for the first time, causes that this phenomenon is considered - to put it mildly - a very unpleasant experience. Therefore, it is important to realize that this state is a transitional state, the presence of someone who wants to hurt us is only a misinterpretation of stimulation by our brain, and the visual and auditory sensations are mostly hallucinations.

Mainly this phenomenon is caused by factors such as:

excess stress
narcolepsy
irregular sleep hours
lack of enough sleep
alcohol and psychoactive substances
sleeping on the back
some specific medicines

Nevertheless, the phenomenon of sleep paralysis played not a small culture-forming role. This is evidenced by numerous myths and legends from around the world, telling about creatures appearing at night, known as the Roman Incubus, German Nachtmahr or Newfoundland Old Hag.