Our research has led us to conclude that hallucination is probably not too strong a term for the experiences associated with sleep paralysis. We take our definition of hallucination from Slade & Bentall (1988). An hallucination is an experience of perception in the absence of an appropriate stimulus, but which has the impact of a conventional perception and is not under the control of the experiencer. An hallucination has the quality of being a sensation related to external event rather than merely a product of the imagination. It does not seem to be merely an idea. It has the quality of objectivity, that is, something beyond the willing and wishing of the experiencer. The "object" of the hallucination" is taken to exist independently of the will of the experiencer. The experience is, in principle, a publicly available phenomenon. The hallucinator should also believe that any appropriately situated person should be able to confirm these experiences. These qualities of sensation, objectivity, existence, and independence, are among the defining qualities of hallucinations (Aggernaes, 1972).
There are probably several degrees of an hallucinatory experience,
as distinct from illusions and normal or conventional sensations.
A "full-blown" hallucination
seems like a real experience and is believed to be a real experience. One might
say the individual is both hallucinating and is deluded by the hallucination
in to accepting it as a real experience. An hallucination proper may
be said to have occurred if the sensation seems quite authentic even if the
experiencer judges the experience to be, for some reason, suspect. It seems
real but there is also something counterfeit about the experience. A pseudo-hallucination
also has this counterfeit quality but it also lacks the fullness of a conventional
sensation. It has an ethereal, "as-if" quality, lacking the richness of a true
sensation. An illusion is simply a misinterpretation of a conventional stimulus.
Sleep paralysis related experiences appear to range from what might be best termed fleeting illusions to true hallucinations. The distinction between illusions and hallucinations is one of long standing harking back at least to Esquirol (1832). Most of the experiences associated with sleep paralysis appear to be hallucinations and quasi-halluciantions (e.g., Slade & Bentall, 1988). People experiencing HHEs are sometimes convinced of their reality but are often able to take a more critical sense, at the same time. This appears to be more common after people have read something about SP and HHEs and come to believe that the experiences are of a hallucinatory nature. Often however, their is no loss of intensity or vividness of the experience. The knowledge or belief that the experience is illusory reduces, for some, the terror of the experiences but appears to have relatively little impact on the apparent reality of the experiences. The quasi-hallucinatory HHEs, though frequently vivid, often have an ethereal and insubstantial quality. These quasi-hallucinations probably best describe the large majority of sleep paralysis and range from vaguely disturbing to extremely terrifying. They usually also motivate a least some search for meaning. The HHEs of sleep paralysis would include misinterpretations of shadows and indistinct objects in a dark room. Finally there may be some people who experience full-blown hallucinations during sleep paralysis in which they not only have vivid and complex imaginative experiences but are also convinced that these experiences have objective external sources. Such people are unlikely to describe their experience as one of sleep paralysis but perhaps as one of demon possession or alien abduction.
Frequency of Associated hllucinations
Depending on the nature of the question and the populations surveyed between 20 - 40 % of people report having had such an experience. For about a third of these people (Figure 1) that is about the extent of the experience and other than a momentary concern about being paralyzed many of these people do not appear to give the matter much thought. It is entirely possibly that almost everyone has experienced such a state but has scarcely noticed and soon forgotten the experience. Another two-thirds of those experiencing sleep paralysis, however, have associated experiences sometimes referred to as hypnagogic and hypnopompic hallucinations. These hallucinations may be tactile, kinesthetic, visual, or auditory. The most common of these experiences is the "sensed" presence accompanied by fear. Individuals vary considerably in the extent to which they report such symptoms. A rather small proportion (>5%) report all the associated components (Figure 1).