Partial consciousness can be the reason why a person hallucinates. Your brain causes you see or hear something that does not exist in reality. Do not confuse this effect with dreaming. Hypnagogia is a different thing. Most of those who have experienced this weird sensation say that this usually occurs to them when they are falling asleep or the moment when they are waking up. Some may compare this phenomenon to the effect that people experience when they take drugs. Nevertheless, you do not have to be a drug addict to suffer from hypnagogia.