Hypnopompia

Finally, a unique characteristic of hypnopompic hallucinations is that as opposed to dreams, wherein they rarely understand that they are in fact asleep, here sleepers do indeed have "the clear subjective awareness of being awake" yet are frequently mentally and physically trapped in the experience.

In particular, during this stage, both the brain-stem, which is the home of the most fundamental physical drives, and the parts of the cortex related to the most complex logical-cognitive functions experience highly intense electrical activity.

[4] It is precisely at this last point, though, that can cause hypnopompic hallucinations: occasionally during deep N.R.E.M., "transient patterns of neural activation in brainstem structures [resembling] micro-wake "fragments" can occur".

[2] Thus, in the Anglosphere, hypnopompic experiences often entail the sense that an "Old Hag" or some similar "nocturnal spirit" is sitting on the sleeper's chest, inducing both paralysis and an increasing, suffocating inability to move.

Anthropologists have discovered references dating back to the High Middle Ages of similar figures in Anglo-Saxon and Anglo-Norman traditions, most prominently the "mæra", the source of the word "nightmare", and which appears to have roots in ancient Germanic superstitions.

Japanese interpretations of the experience are often grouped under the heading of 金縛り kanashibari, a term which literally means "bound in gold or metal" and derives from the name of an esoteric Buddhist technique for paralyzing enemies.

[2] Owing to similarities between hypnopompic hallucinations and those experienced by people with dementia, Parkinson's and schizophrenia, significant progress is being made on understanding the neurobiological basis of this experience.