Dream

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https://en.wikipedia.org/wiki/Hypnopompic

...The objective difference between the subjective experiences of dreams and hypnopompic hallucinations emerges from a close look at our sleep cycle and its attendant brain activity: there are essentially two types of sleep, R.E.M. sleep, which is indeed categorised by "rapid eye movement" and N.R.E.M., which stands for "Non-Rapid Eye Movement". In R.E.M. sleep, our brains are extremely active. In particular, during this stage, both our brain-stem, which is the home of our most fundamental physical drives, and the parts of the cortex related to our most complex logical-cognitive functions experience highly intense electrical activity. As this perhaps suggests, R.E.M. is where the majority of dreaming occurs—it has been all but confirmed at this point that dreams originate in the brain-stem, a spark from there driving our more logical mind to contemplate and consolidate, through dreaming, memories that relate to fulfilling our fundamental drives. Conversely, there is almost no electrical activity during N.R.E.M. sleep. It will thus be no surprise that N.R.E.M. is what is referred to in layman’s terms as "deep sleep", which is characterised by the complete quieting of the mind, to include dreamlessness and by muscle "atonia", which is to say paralysis, a complete motor disconnect. A last critical point in this regard is that R.E.M. sleep cycles are book-ended by N.R.E.M. stages, which is to say that when we fall asleep and wake up, we first fall into and last arise from, N.R.E.M. sleep.[3]

It is precisely at this last point, though, that a quirk in brain chemistry, can cause hypnopompic hallucinations: occasionally during deep N.R.E.M., "transient patterns of neural activation in brainstem structures resemble[ing] micro-wake "fragments" can occur".[3] As the origin of the fragments and their name implies, they have a twofold effect: first, just as in R.E.M. sleep, these brain-stem fragments essentially activate the dream mechanism. Second, they do catalyse a near-waking state. However, this spark is often not powerful enough to jar us completely out of deep sleep, and so only our mind fully awakens, leaving our body trapped in the atonia of deep sleep. Another reason why hypnopompic hallucinations are often such horrible experiences, is because micro-wake fragments appear to be related to serotonin and dopamine deficits—these deficits predispose us to negative mental states, which likely causes the hallucinations to resemble our worst dreams.[3]

Cultural manifestations

Fuseli's 1781 painting The Nightmare is thought to be a depiction of a hypnopompic hallucination
These mental experiences are indeed often deeply damaging: across cultures, the experience of hypnopompic hallucinations are strongly related to "visitations of spirits, demons or other grotesque creatures belonging to traditional folklore".[1] 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" from the name of which figure we derive the word "nightmare", and which appears to have roots in ancient Germanic superstitions.

Similarly, subjects belonging to Yoruban-African diasporas report feeling as though they are being "ridden" by the evil manifestations of their versions of the African pantheon (ridden is the vernacular for possession by the gods, who are often referred to as "divine horsemen"). Interestingly in this vein, some members of the Yoruba diaspora appear to conflate the cultural interpretation of the experience, referring to "being ridden by the witch". Perhaps most chilling of all, though, are the Japanese interpretations of the experience, which are often grouped under the heading of kanashibari, which translates loosely to "buried alive and bound in metal".[1]

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https://en.wikipedia.org/wiki/Lucid_dream

A lucid dream is a type of dream where the dreamer becomes aware that they are dreaming. During a lucid dream, the dreamer may gain some amount of control over the dream characters, narrative, and environment; however, this is not actually necessary for a dream to be described as lucid.[1][2][3][4] Lucid dreaming has been studied and reported for many years. Prominent figures from ancient to modern times have been fascinated by lucid dreams and have sought ways to better understand their causes and purpose. Many different theories have emerged as a result of scientific research on the subject and have even been shown in pop culture. Further developments in psychological research have pointed to ways in which this form of dreaming may be utilized as a form of sleep therapy. There are two means of initiating a lucid dream:

Dream induced lucid dream (DILD) - Something within the dream triggers the dreamer to understand that they are dreaming.
Wake induced lucid dream (WILD) - The dreamer moves from waking to dreaming with no loss of awareness.

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https://en.wikipedia.org/wiki/Oneirology