Sometimes the word hypnagogia is used in a restricted sense to refer to the onset of sleep, and contrasted with hypnopompia, Fredierick Myers’ term for waking up. However, hypnagogia is also regularly employed in a more general sense that covers both falling asleep and waking up. Other terms for hypnagogia, in one or both senses, that have been proposed include presomnal or anthypnic sensations, visions of half-sleep, oneirogogic images and phantasmata, the borderland of sleep, ‘half-dream state, pre-dream condition, sleep onset dreams, and wakefulness-sleep transition’ state (WST).
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 colored, still or moving, flat or three-dimensional (offering an impression of perspective). Imagery representing movement through tunnels of light is also reported. 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. Hypnagogia can be induced with a Dreamachine, which uses a pulsing frequency of light close to alpha waves to create this effect.
Hypnagogic imagery is often auditory or has an auditory component. Like the visuals, hypnagogic sounds vary in intensity from faint impressions to loud noises, such as crashes and bangs (exploding head syndrome). People may imagine their own name called or a doorbell ringing. Snatches of imagined speech are common. While typically nonsensical and fragmented, these speech events can occasionally strike the individual as apt comments on – or summations of – their thoughts at the time. They often contain word play, neologisms and made-up names. Hypnagogic speech may manifest as the subject’s own inner voice, or as the voices of others: familiar people or strangers. More rarely, poetry or music is heard.