Sleep Paralysis

and

Associated Hypnagogic and Hypnopompic Experiences

Sleep Paralysis

Nightmare (Detail)  Henry Fuseli

Sleep paralysis, or more properly, sleep paralysis with hypnagogic and hypnopompic hallucinations have been singled out as a particularly likely source of beliefs concerning not only alien abductions, but all manner of beliefs in alternative realities and otherworldly creatures. Sleep paralysis is a condition in which someone, most often lying in a supine position, about to drop off to sleep, or just upon waking from sleep realizes that s/he is unable to move, or speak, or cry out. This may last a few seconds or several moments, occasionally longer. People frequently report feeling a "presence" that is often described as malevolent, threatening, or evil. An intense sense of dread and terror is very common. The presence is likely to be vaguely felt or sensed just out of sight but thought to be watching or monitoring, often with intense interest, sometimes standing by, or sitting on, the bed. On some occasions the presence may attack, strangling and exerting crushing pressure on the chest. People also report auditory, visual, proprioceptive, and tactile hallucinations, as well as floating sensations and out-of-body experiences (Hufford, 1982). These various sensory experiences have been referred to collectively as hypnagogic and hypnopompic experiences (HHEs). People frequently try, unsuccessfully, to cry out. After seconds or minutes one feels suddenly released from the paralysis, but may be left with a lingering anxiety. Extreme effort to move may even produce phantom movements in which there is proprioceptive feedback of movement that conflicts with visual disconfirmation of any movement of the limb. People may also report severe pain in the limbs when trying to move them. Several recent surveys including our own suggest that between 25-30% of the population reports that they have experienced at least a mild form of sleep paralysis at least once and about 20-30% of these have had the experience on several occasions. A few people may have very elaborate experiences almost nightly (or many times in a night) for years. Aside from many of the very disturbing features of the experience itself (described in succeeding sections) the phenomenon is quite benign. It was thought in the past that it was a significant part of the so-called "narcoleptic tetrad", but recent surveys of non-clinical populations, such as ours, suggest that the prevalence may be as high among the general population as among diagnosed narcoleptics. For a summary of SP Characteristics see Table 1.

Sleep paralysis most often has an adolescent onset.  Earlier research had generally studied student populations leaving open the possibility that this might have been an artefact of the fact that only young people were surveyed.  In several surveys with older samples, (Mean age of approximately 30) we have corroborated a very clear tendency for people at all ages to report an adolescent onset for their episodes. Several large samples have produced consistent means of 17 years of age, with a sharp increase after 10 and an even sharper decline from 17 to the mid-twenties. The results do suggest, however, that sleep paralysis episodes can begin at virtually any age, although it is rare for this to happen after 30. 

Hallucinations and Sleep Paralysis

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). A 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 experient. A 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 experient. The "object" of the hallucination" is taken to exist independently of the will of the experient. 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 a 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.  A hallucination proper may be said to have occurred if the sensation seems quite authentic even if the experient 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-hallucinations (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, there 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 hallucinations

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, kinaesthetic, 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).

Sleep Paralysis as an Anomalous REM State

REM and Dreaming: A major distinction of sleep states, for close to a half century, has been accepted between REM and NREM sleep (Aserinsky & Kleitman, 1953; Jouvet, 1967). REM periods are characterized by desynchronized cortical characterized by low-voltage fast EEG patterns with synchronized hippocampal activity characterized by slow (4-8 Hz) theta activity (e.g., Culebras, 1994). It is also widely accepted that dreaming is more common and more vivid during REM than during NREM sleep (Dement & Kleitman, 1957). In addition to the characteristic desynchronized cortical low-voltage fast EEG activity, there are numerous physiological, behavioral, and sensory features associated with REM such as muscle atonia, gating of sensory input, rapid eye and middle ear movements, as well as heart rate and respiration changes (Carskadon & Dement, 1989; Symons, 1993). 

Within REM periods a distinction is sometimes made between a background tonic state (TREM) and bursts of phasic REM (PREM) every 16-120 seconds and lasting from 2-9 seconds (Aserinsky, 1971, Molinari & Foulkes, 1969). Specifically, PREM is characterized by bursts of rapid eye and middle ear movements and characteristic cortical and hippocampal EEG patterns. PREM is associated with, and may be preceded by, ponto-geniculo-occipital EEG waves (PGO spikes in animal preparations) originating in the bilateral, dorsolateral pons and projecting rostrally through the lateral geniculate nucleus and other thalamic nuclei (Hobson, Alexander, Frederickson, 1969). It has been conjectured that the most vivid dreams, or most vivid events within dreams, are associated with PREM (Molinari and Foulkes, 1969). 

REM and SP: SP has also been associated with REM states, particularly with sleep-onset and sleep-offset REM (SOREM) (Nan'no, Hishikawa, & Koida, 1970). In both REM dreams and SP hallucinations a general atonia is maintained during REM by marked and sustained hyperpolarization of the motoneurons (Chase & Morales, 1989). One likely function of the general atonia is the prevention of the physical enactment of the motor components of dreaming. There are at least two major traditional hypotheses concerning the connection between neurophysiological events and visual imagery in dreams. The visual imagery of dreaming may arise either from the direct stimulation of visual areas of the cortex during the PGO spike, in which case the rapid eye movements may reflect attempts to scan the images (Ladd, 1892; Roffwarg, Dement, & Muzio, 1962), or conversely, the mages may be produced by the oculomotor impulses in response to direct stimulation from the gigantocellular pontine reticular field (Hobson & McCarley, 1977; McCarley & Hobson, 1979). 

REM is thought to be generated in the lateral portions of the nucleus reticularis pontis oralis (RPO) immediately ventral to the locus ceruleus in the pontine reticular formation. The neurotransmitters in this region have not been clearly determined, but are neither cholinergic nor monoaminergic. The RPO receives projections from cholinergic regions in the laterodorsal tegmental nucleus (LDT) and the pedunculopontine tegmental nucleus (PPT) as well as from ventromedial portions of the medulla. The RPO, LDT, and PPT are collectively thought to be part of the REM-on neural population (Steriade & McCarley, 1990). These populations are hypothesized to interact with REM-off noradrenergic neurons in the locus ceruleus and seratonergic neurons in the raphe system. These latter populations are most active during waking and least active during REM. Interactions between the REM-on and REM-off populations are thought to control REM onset and offset (Steriade & McCarley, 1990). 
SP may reflect an anomaly of the functioning of the monoaminergic systems and/or their inhibition of the REM-on cholinergic system. Experimental and clinical dissociations have been demonstrated among major components of REM:  namely, PGO activity, atonia, and EEG desynchronization (Hishikawa & Shimizu, 1994). Hishikawa & Shimizu speculate that SP may be produced by hyperactivation of the Sleep-on populations or, they deem more likely, hypoactivation of the Sleep-off populations. That SP may be alleviated by serotonin and adrenergic reuptake inhibitors is taken to be consistent with this hypothesis. Also involved may be suprapontine systems involving the reticular system, including the hippocampus and amygdala. 

REM SP with HHEs differs from REM dreams in that during SP there is little or no blocking of exteroceptive stimulation and there is no loss of waking consciousness. SP with HHEs differs from dream experience in that the sensory cortex may be receiving both externally and internally generated information. The peculiarity of the HHEs in SP may, in part, be a result of the brain's attempts to integrate endogenous cortical arousal originating in the pons with normal sensory input. A similar peculiarity may exist for motor pattern arousal during SP. McCarley and Hobson argue that, during dream generation by internal stimulation of motor programs, we interpret the activity of the pattern generators and their corollary discharge as movement. The lack of peripheral feedback, though not normally necessary for effective control, may contribute to a sense of unreality to the apparent movement and hence to the "bizarreness" of dreams. Pontine activation of motor patterns during SP appears to be less common in SP than in dreams, if subjective reports of illusory movement are to be taken as evidence. Volitional attempts at movement during SP are common, however, and the absence of feedback is most often, though not always, experienced as paralysis rather than illusory movement. Thus it appears that, during SP, the frontal cortex is more sensitive to the absence of feedback than during dreaming. When motor programs are spontaneously activated during SP these might be extremely resistant to coherent interpretation am may be experienced as very unusual bodily states. In concluding sections we will relate more specifically the phenomenology of various HHEs to the underlying neurophysiology.  

A Note on Sleep Paralysis and Narcolepsy

Sleep paralysis is often thought to be associated with narcolepsy. Narcolepsy is a condition in which people are overcome with irresistible sleep attacks that occur unpredictably and at any time of day. These sleep attacks may occur while engaged in conversation, working at a desk, during meals, or even driving. These involuntary naps last for about 10 to 20 minutes after which the person will wake up feeling quite refreshed. The other major, and perhaps most distinguishing, feature of the condition is cataplexy, a sudden loss of muscle tone during excitement or arousal, such as laughter. The loss of muscle tone may be partial or complete. In the latter case, the person may simply collapse. Although individuals remain conscious during a cataplectic episode, they may experience hallucinations during prolonged attacks and subsequently fall asleep. Sleep-onset REM, sleep paralysis, and hypnagogic hallucinations are often discussed as 'associated features' of narcolepsy. Not all people with narcolepsy experience sleep paralysis, however, and, more significantly, not all people who experience sleep paralysis have narcolepsy. Indeed, this should be quite obvious when one realizes that the prevalence of sleep paralysis is 20% to 40%, whereas that of narcolepsy is 0.03% to 0.16% (ICSD-90). Moreover, estimates of the prevalence of sleep paralysis among people with narcolepsy are quite variable and are in the range of 40% to 60% (Hishikawa, 1976). Among our own sample of we have found only 1.6% of SP experients report that they have been diagnosed with narcolepsy. Although this is larger than the percentage in the general population is obviously a very small proportion of people who also experience SP. In a recent report involving narcoleptic dogs, the authors argue that 'cataplexy and a dysfunction in the maintenance of and transition between vigilance states are central to narcolepsy (Nishino, et al., 2001, p. 445). After reviewing literature that suggests that sleep onset REM is associated with several other disorders and, as I have noted above, occurs in a substantial minority of the general population, the authors find little support for “the current notion that narcolepsy is a disorder of REM sleep generation” (p. 445).

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