Relationships between sleep paralysis and sleep quality: current insights

Abstract

Sleep paralysis is the unusual experience of waking up in the night without the ability to move. Currently little is known about the experience, despite the fact that the vast majority of episodes are associated with extreme fear and in a minority of cases can lead to clinically significant levels of distress. The aim of this work was to review the existing literature pertaining to the relationship sleep paralysis has to sleep more generally, measured both with subjective questionnaires and objective laboratory recordings. In terms of subjective sleep variables, worse sleep quality has been found in multiple studies to be associated with increased odds of sleep paralysis occurrence. In addition, insomnia symptoms (but not a diagnosed insomnia disorder) have also been found to predict sleep paralysis. Associations between sleep paralysis and other unusual and/or threatening sleep experiences such as nightmares, exploding head syndrome, and lucid dreaming have been reported. In terms of objective measurements, the limited literature to date shows sleep paralysis to be a "mixed" state of consciousness, combining elements of rapid eye movement sleep with elements of wakefulness. Future research needs to focus on longitudinal designs to disentangle the direction of effects and more typically employ a broader assessment of sleep paralysis that better captures associated features such as hallucinations, fear, and distress.

Keywords: hypnagogic; hypnopompic; incubus; parasomnia.

Figure 1

Physiology associated with SP. Notes: (A) Hypnograms of five participants reporting SP. Each arrow indicates the awakening points when the participant reported SP. Note the close association between SP and REM sleep. Taken from the study by Takeuchi et al. (B) PSG recording during an SP episode. EEG shows REM activity mixed with abundant alpha. Both rapid and slow eye movements can be seen in the EOG channels. Muscle activity (measured by the EMG) remains flat throughout until the participant fully awakens at the end of the recording. Taken from the study by Takeuchi et al. (C) Spectral EEG data showing (clockwise from top-left) eyes closed waking, REM sleep, SP, and “reconstructed” SP by statistical combination of waking and REM sleep activity. Taken from the study by Terzaghi et al. (D) The AIM state-space model. SP is characterized by high activation, mixed internal/external input, and mixed neuromodulation. Abbreviations: AIM, activation-input-modulation; FFT, fast Fourier transform; LD, lucid dreaming; NREM, nonrapid eye movement sleep; REM, rapid eye movement sleep; SP, sleep paralysis.