The terms "hypnagogic imagery" and "hypnopompic imagery" were introduced by Maury (1848) and Myers (1903), respectively, and refer to imagery of varying sensory modalities that is experienced in the borderline states just as one is falling asleep or awakening from sleep, respectively.
Between wakefulness and sleep: Hypnagogic imagery. British Journal of Psychology, 45, 266-276.
A self-observational study of spontaneous hypnagogic imagery using the upright napping procedure.
Additionally, as they begin falling asleep, many people also experience what is called hypnagogic imagery
which is generally short fleeting dreams.
Recall of hypnagogic imagery has been found to peak around the middle of standard Stage 1 sleep when the EEG mainly consists of theta activity (Hori et al., 1994).
Laboratory studies have also found that hypnagogic imagery and sleep paralysis can occur during sleep-onset REM periods (SOREMPs) and that isolated sleep paralysis is characterised by abundant alpha activity (Takeuchi, Miyasita, Inugami, Sasaki, & Fukuda, 1994; Takeuchi, Miyasita, Sasaki, Inugami, & Fukuda, 1992).
Studies have shown that as one moves through the sleep-onset period, the amount of visual hypnagogic imagery tends to increase (Hori et al., 1994), it becomes more dreamlike (Foulkes & Vogel, 1965; Stickgold & Hobson, 1994), and the image quality, vividness, luminosity, and intensity of colour also increase (Mavromatis, 1987; Nielsen, 1992).
To use an analogy, dreaming resembles a lecture illustrated by slides which form part of it; hypnagogic imagery is more like a display of slides meant to illustrate some other lecture.
Generally speaking, hypnagogic imagery seems to be more common than hypnopompic imagery.