People in this state may have experiences that resemble hallucinations, but are not. The person usually knows that it is not real, but very often nothing can be done against it. Most of these pseudohallucinations are visual experiences.
The writings of Aristotle already talk about hypnagogia. In more recent centuries, many authors have referred to the state. Edgar Allan Poe, for example, wrote of the "fancies" he experienced "only when I am on the brink of sleep, with the consciousness that I am so".
With electroencephalography (EEG) in the mid 20th century, detecting such a state has become easier.
- Mavromatis, Andreas (1987). Hypnagogia: the unique state of consciousness between wakefulness and sleep. London: Routledge and Kegan Paul. p. 4. ISBN 0-7102-0282-2.
- Leroy, E.B. 1933. Les visions du demi-sommeil. Paris: Alcan.
- Tart, Charles T. 1969. Altered states of consciousness: a book of readings. New York: Wiley. ISBN 0-471-84560-4