Sounds terrifying, doesn’t it? But this is no horror story and for an increasing number of people, the above experience is a familiar occurrence, while others only experience it once or twice in their entire lifetime. These nightmarish symptoms are typical of a sleep disorder called sleep paralysis.Sleep paralysis is described as the most terrifying of all sleep disorders. Once considered very rare, recent studies have revealed that almost half of all people have suffered some form of sleep paralysis at least once during their lives.According to current studies, 17 to 20 per cent of people report having some form of sleep paralysis during their lives, although many leading specialists in the area believe the figure to be much higher. According to David J Hufford, Professor Emeritus of Medical Humanities in the Penn State College of Medicine and lifelong researcher of subjects such as sleep paralysis, a large amount of cases of sleep paralysis remain unreported. There are even people who had experienced sleep paralysis and believed themselves to be mentally ill, prior to hearing about the condition.Clinically, this disorder is characterised by a brief period of being unable to perform voluntary movements which occurs at the onset of sleep or when awakening, either during the night or in the morning. In a nutshell, sleep paralysis is summarised as the sufferer being awake but unable to move for a period of time, lasting anywhere from a few seconds to between 15 and 20 minutes.This is the basic definition, although in over 80 per cent of cases, it is accompanied by an acute sense of danger as well as terrifying hallucinatory experiences (likened to waking dreams) that include any or all of the following: feeling of pressure on the chest, feeling phantom hands on the throat, the sound of shuffling or dragging footsteps, noises and bright lights, phantom voices, visions of humanoid forms, sensation of floating and out-of-body experiences. The sound of footsteps and feeling of pressure on the chest are the most common experiences reported by sufferers of this frightening condition.The phenomenon has been known to man for centuries. History is littered with reports of sleep paralysis and the hallucinations that often accompany it. In Eastern culture, descriptions of the condition date back as far as 400BC. The famous Greek physician Galen attempted to explain these ‘nightmares’ logically, believing them to be caused by gastric disturbances.Some scholars believe that sleep paralysis accounted for quite a number of centuries-old claims of nocturnal demonic activity and witch attack, as until about three centuries ago, sleep paralysis was widely explained as the work of evil spirits. Legends have arisen in every culture as a means of explaining why a person awakes unable to move, with a weight pressing down on his chest.In western culture, incubi sitting on the chests of their victims apparently caused these episodes. In Old English, the name for these beings was ‘mare’, while the Icelandic term is ‘mara’ that translates as ‘The Crusher’.In South Asia, sleep paralysis is considered to be an encounter with evil djinns and demons. In Pakistan and Iran, this evil entity is known as a ‘bakhtak’, while in eastern culture the demon is the evil female spirit called ‘Lilitu’ who attacks men in their sleep. The ‘Old Hag’ is a female demon in British folklore that sat on the chests of those sleeping on their backs, sometimes choking her victims.Several scientists have proposed that there is a direct link between sleep paralysis and the increasingly huge number of alleged alien abductions reported every year. This is especially evident in the USA, where the 1992 survey known as Roper Poll claimed that over four million Americans were victims of alien abduction. When scrutinised closely, many of the cases of reported alien abductions have common characteristics that sound all too familiar. Many testimonies report flashing lights, strange sounds, figures moving around the victim, sensations of paralysis and levitation and intense pressure on his or her chest.Richard McNally, a professor of psychology at Harvard University who has studied alleged alien abductees for the past decade, is convinced that these people were actually victims of sleep paralysis, whose hallucinations had been generated by their exposure to popular alien culture.Due to the advent of the electrooculogram (EOG) in the 1950s, we have, after thousands of years of blaming it on demons incubi and other evil spirits, finally discovered the real scientific basis behind this strange phenomenon.The disorder is produced by a physiological mechanism in the brain. Human sleep consists of two states of brain activity; non-REM sleep and REM (rapid eye movement) sleep which alternate in cycles.During non-REM sleep, the body can toss and turn in bed, as well as engage in activities such as sleeptalking and sleepwalking. But during REM, sleep atonia occurs – the brain locks down the muscles in the body to prevent the body from acting out its dreams and hurting itself during the dream stage. Physiologically, sleep paralysis is closely related to REM atonia, the paralysis that occurs as a natural part of REM sleep.Sleep paralysis strikes when a person is moving into or out of REM sleep and results when the nervous and endocrine systems erringly continue to send out signals for movement inhibition, even when the person has woken directly from REM sleep. When this happens, the person wakes up unable to move for a short period of time until the brain realises it is awake and stops inhibiting movement. Thus, the individual experiences sleep paralysis, as demonstrated in a series of experiments by Kzuhiko Fukuda of Fukushima University.But what is the scientific basis behind the hallucinations associated with sleep paralysis? According to scientists, these too are a result of brain activity. Activity in the temporal lobe induces auditory hallucinations, while the visual cortex generates visual stimuli, which manifests in the person’s mind as a malevolent presence watching them.Researchers Adie and Wilson discovered that many sleep paralysis sufferers also suffer from narcolepsy, a sleep disorder characterized by sudden and uncontrollable episodes of deep sleep during the day. Sleep paralysis may also occur in those who do not suffer from narcoleptic fits, a condition called ‘isolated recurrent sleep paralysis’. Sleep paralysis occurs either at sleep onset (predormital form) or when waking up (postdormital form). Isolated sleep paralysis usually occurs when waking up in 64 per cent of patients, while in narcolepsy-related and familial sleep paralysis, the episodes usually happen at sleep onset.Detailed research on sleep patterns, as well as the anthropological work of researchers such as Robert Ness and David J Hufford, has given us a new understanding of what we now know to be a neurological glitch and not an evil age-old entity trying to hurt us at night.