What Is Sleep Paralysis? Awake, Unable to Move, and Something in the Room

During rapid eye movement sleep, the brainstem actively paralyses the skeletal muscles, a state called REM atonia, produced by inhibitory signals that shut down motor neurons. Its purpose is straightforward: dreaming involves motor commands, and without the paralysis a sleeper would act them out, which is exactly what happens in REM sleep behaviour disorder. Sleep paralysis is a desynchronisation, in which consciousness returns before the atonia has switched off, so a person is awake, aware and paralysed, with the diaphragm and eye muscles the exceptions since they are not affected. Breathing continues automatically and feels restricted because the chest wall muscles are not under voluntary control, which is the source of the sensation of weight and of suffocation. The episode ends on its own within a couple of minutes, and it can usually be broken by concentrating on moving a finger or a toe, which recruits enough motor activity to end the state.

The accompanying experiences are dream imagery intruding into waking awareness, and researchers classify them into three consistent groups:

• •Intruder hallucinations, a sensed presence in the room, frequently malevolent, sometimes seen as a shadowy figure and heard as footsteps, breathing or voices
• •Incubus hallucinations, pressure on the chest, difficulty breathing and sometimes a sensation of being held down or attacked
• •Vestibular-motor hallucinations, including floating, flying, falling and out-of-body sensations, which are the one group frequently reported as pleasant
• •The emotional tone is dominated by fear, which is thought to follow from the amygdala being active in REM while the person is awake enough to search for a cause
• •The content varies with culture and expectation, which is why the figure is a witch in one tradition, a demon in another and an alien abduction in a third

Around eight percent of the general population report at least one episode, with higher rates in students and in psychiatric populations, and recurrent episodes in a smaller group. The strongest predictors are sleep disruption and irregularity: sleep deprivation, shift work, jet lag, irregular schedules and sleeping on the back all raise the probability substantially. Stress and anxiety are associated, as is post-traumatic stress disorder. There is a genetic component, with twin studies suggesting moderate heritability. It is one of the four core symptoms of narcolepsy, alongside excessive daytime sleepiness, cataplexy and hypnagogic hallucinations, which is why repeated episodes combined with severe daytime sleepiness warrant investigation, though the great majority of people who experience it have no sleep disorder at all. The practical management is entirely about sleep hygiene, since regularising the schedule, ensuring adequate duration and avoiding sleeping supine reduces frequency in most people who take it seriously.

The consistency of the reported content across unconnected societies is striking, and the explanations differ completely. The English word nightmare originally meant exactly this, with the mare being a supernatural being that sat on a sleeper's chest, which is the image in Fuseli's painting of 1781. Newfoundland tradition calls it the old hag. Japanese tradition has kanashibari, meaning bound in metal. In several Caribbean and West African traditions it is a witch riding the sleeper. Turkish tradition has a jinn pressing the chest. Cambodian refugees reported it in terms of a spirit visit, and rates among them were extremely high, which researchers connected to trauma. Egyptian accounts involve a jinn attack and the belief that it can kill. The consistency of the phenomenology across these, with the pressure, the presence and the paralysis recurring everywhere, is strong evidence that the experience is neurological and the interpretation is cultural. It has also been proposed as an explanation for a proportion of reports of demonic visitation, of incubus and succubus accounts, and of modern alien abduction narratives, which match the standard features closely.

Sleep paralysis is a mismatch in which consciousness returns while the muscle paralysis of REM sleep persists, leaving a person awake, aware and unable to move for up to a couple of minutes, with breathing feeling restricted because the chest muscles are not under voluntary control. Accompanying hallucinations fall into intruder, chest pressure and floating categories, with content shaped by cultural expectation. It affects around eight percent of people, is driven mainly by disrupted and irregular sleep, and the same experience is documented worldwide under entirely different explanations.