Hypnagogic hallucination: A vivid dreamlike hallucination that occurs as one is falling asleep. The opposite of an hypnopompic hallucination which is a vivid dreamlike hallucination that occurs as one is waking up.
Transition to and from sleep may be attended by a wide variety of sensory experiences. These can occur in any modality, individually or combined, and range from the vague and barely perceptible to vivid hallucinations.
Among the more commonly reported, and more thoroughly researched, sensory features of hypnagogia are phosphenes which can manifest as seemingly random speckles, lines or geometrical patterns, including form constants, or as figurative (representational) images. They may be monochromatic or richly coloured, still or moving, flat or three-dimensional (offering an impression of perspective). Individual images are typically fleeting and given to very rapid changes. They are said to differ from dreams proper in that hypnagogic imagery is usually static and lacking in narrative content, although others understand the state rather as a gradual transition from hypnagogia to fragmentary dreams, i.e. from simple ‘eigenlicht’ to whole imagined scenes. Hypnagogia can be induced with a Dreamachine, which uses a pulsing frequency of light close to alpha waves to create this effect. Descriptions of exceptionally vivid and elaborate hypnagogic visuals can be found in the work of Marie-Jean-Léon, Marquis d'Hervey de Saint Denys.