Sherwood in 2012 examines the studied, yet bizarre condition known as a hypnagogic (HG) or hypnopompic episode (HG). Not too well known among the public, these near-sleep episodes are a cross between waking and dreaming states. As such, people are prone to hallucinations when HP/HG events occur. These episodes can be frightening, as most people are unaware that they are hallucinating and can mistake an episode for a paranormal encounter.
Sherwood conducted a study to examine the type of hallucinatory content among people who have had HP/HG episodes. Sherwood found hallucinations such as falling, visual imagery, and a sense of a presence as the most common features of these episodes (> 70%). Auditory and touch sensations occurred with greater than 50% of the HP/HG participants.
The author further divided these experiences into themes, although there are far too many to list here. The summary of these themes shows that hallucinations can be both relatively simple or very complex. Furthermore, some events were frightening or negative in their themes. Thus, for many years, HG/HP episodes have been a common explanation for sleep-related paranormal experiences.
Sherwood, S. J. (2012). A web survey of the content, sensory modalities, and interpretation of hypnogogic and hypnopompic episodes. Journal of Parapsychology, 76, 27-56.