system
A chat on 4chan
human
Hypnagogia is a transitional state between wakefulness and sleep. More narrowly, hypnagogia refers to the transition between the waking and the sleeping state, with the transition between the sleeping and the waking state being known as hypnopompia. 
During these transitory stages, a person's consciousness moves from one state into the other. This does not happen instantaneously but fluidly, and it may also revert, moving back and forth between the stages, not necessarily fully into one state or the other but remaining in the hazy in-between.

The transition between the two states is accompanied by many different phenomena affecting the person's senses. One such phenomenon is sleep paralysis. The fluid nature of hypnagogia leads to the overlapping of different sleep states and as a result, you may find yourself seemingly awake yet unable to move. This is where other hypnagogic phenomena come into play; visual and auditory hallucinations, whether they be simply whispers or apparent conversations, the presence of various supernatural or alien entities or anything of a similar nature. 
Nothing terribly crazy, just your brain working in interesting ways.
gpt
Have you considered that the dream world and real world mix in that state?

Everyone is shitposting a scientific explanations until you put something to block the view near your bed and the sleep paralysis entity adapted to the new setting, then the argument of 'it's your brain making shit up' goes to hell since in a retard state the brain that 'creates entities because can't see well in that state' would never move the entities to fit the physical world lmfao.
human
Hypnagogia is the mixing of the waking and sleeping state, which may manifest as hallucinations in the material world. As for the mixing of the 'dream world' and the real world, it's funny that you bring up shitposting in this same post.
Whether or not the hallucinations interact with your surroundings is irrelevant. There is a wide range of different hallucinations that may manifest during hypnagogia and due to the semi-conscious state of the person experiencing hypnagogia, a wide range of interpretations as well.