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Introduction
The phenomenon of sleep paralysis has attracted increasing attention in the scientific community only in recent years, even though the occurrence of what may be described as sleep paralysis has been documented as early as Hellenistic times. At a fundamental level, the term "Isolated Sleep Paralysis" (ISP) is used to differentiate between an abnormal generalized muscle atonia and the usual generalized muscle atonia which is characteristic of REM sleep (dream state), as well as to distinguish such a condition from sleep paralysis occurring as an ancillary symptom of narcolepsy (which exhibits a combination of several symptoms). ISP is a common condition with a prevalence of 5-62% [1] depending on geographic location, but most affected people have a single episode or infrequent episodes during their life and the duration of an episode is generally less than a minute. In contradistinction to ISP, Recurrent Isolated Sleep Paralysis (RISP) is a rarer variant of sleep paralysis characterized by frequent episodes or complex of sequential episodes of generally longer duration, and in particular by the range and intensity of the perceptual phenomena occurring during episodes. RISP is found to be familial in most cases [1], [20]. RISP is a complex phenomenon that, at present, cannot be explained by any appropriate and comprehensive model that would integrate low-level neurophysiological processes, high-level neurocognitive functions and possibly higher-level processes. Indeed, at a higher level, to our knowledge there has been until now no research that has thoroughly addressed the content and significance of the perceptual phenomena induced by RISP. Most psychological studies have rather focused on the symptomology of RISP (e.g., circumstances of occurrence, type of sensory experience) [20], on the emotional reactions of the affected persons to their episodes [20] or on their psychological profile [4], [21]. As researchers who experience RISP, in this paper we attempt to summarize information on RISP gathered from various sources and the possible connections between RISP and other poorly understood conditions at different levels. Most importantly, we address some "hard questions" relating to the range and sense of reality of the perceptual phenomena inherent to RISP, and offer some conjectures as to the causes of such phenomena that include paranormal components such as Out-of-Body Experiences (OBE) [15,16].
The paper is organized as follows: in section 2, we present the profile of a typical RISP episode (or a complex of sequential episodes). This profile has been constructed by use of a sample of 250 direct or indirect respondents to a message posted initially by one of the authors (JCT) on the sleep web site of the University of California in Los Angeles [19], and is based as well on the authors' personal experiences. This initial description of RISP from a phenomenological viewpoint will help us, in the following sections, to use a bottom-up approach in describing: the neurophysiological aspects of RISP and its possible connections with other conditions in section 3, the neurocognitive aspects of RISP invoking hypnagogic/hypnopompic hallucinations and lucid dreaming in section 4, and the possibility of the manifestation of paranormal phenomena (OBE) in section 5. In section 6 we present the preliminary results of a statistical analysis of the RISP experiences of the above-mentioned sample population. This analysis takes into account the emotional reactions of the respondents as well as their beliefs as to the cause of their experiences in the context of a more general socio-historical study of RISP. In section 7, we briefly describe possible strategies that could be used to cope with RISP, involving both "conventional" medical treatments and a "psychological" approach. We conclude the paper in section 8 with a summary of important issues that should be addressed in future research on RISP.
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