by Hiro Takahashi
A person may wake up and find himself unable to move or speak as if he is frozen. He also may hear footsteps, see a ghost-like creature, or feel someone sitting on his chest. Throughout the history, people considered this phenomenon as work done by evil spirits. However, the modern science can explain the terrifying event as a Sleep Paralysis.
In order to understand how a body becomes paralyzed while the person is awake, it is necessary to understand sleep cycles. In a mammalian sleep, the brain activity undergoes two different states called non-REM (NREM) sleep and REM sleep, which differ very much from wakefulness. NREM and REM sleep alternate cyclically through the night; in human, about 80 minutes of NREM sleep starts a night of sleep, about 10 minutes of REM sleep follows, and this 90 minute cycle is repeated about 3 to 6 times during the night. During NREM sleep, a body produces few movement, but the body has capability of tossing about in bed and producing some other motor events, such as sleepwalking and sleep talking. The cardiac-muscle contraction and breathing occur at a uniform rate, and the eyes move slowly. During REM sleep, on the other hand, heart rate, respiration rate, and blood pressure vary. The eyes move rapidly because most dreaming takes place in this period, and the sleeper probably "look" at the moving objects in a dream.
The brain's control over muscles during REM sleep points out that in this period, a body is normally in the state of total paralysis, called a "nonreciprocal flaccid paralysis". Probably to prevent a person from "acting out" a dream, the brain sends signals to inhibit any muscle contractions. Although some peripheral muscles, such as the muscles of the fingers and face, still twitch, the large skeletal muscles become relaxed, or "paralyzed" as a result. Some evidence supports that the motor paralysis of REM sleep protect against the acting out of one's dreams. A patient who suffers from rare syndrome called REM Sleep Behavior Disorder lacks the normal nonreciprocal flaccid paralysis, and he acts out violent dreams during REM sleep, often with injurious consequences.
While the modern neuroscience can describe the state of Sleep Paralysis as some errors of the neural transmission in the brain during REM sleep, a person who has seen or heard ghost-like figures/voices may easily believe that evil spirits fully controlled his entire body. However, the images or noises, which the victim believes that he has seen or heard, are most likely hallucinations; and hallucinations, too, can result from the brain activity. In the 1960's, the Canadian neurologist W. Penfield introduced that electrical stimulation of the temporal lobe can cause the auditory hallucinations in the wake state. The buzzing or ringing sounds in the ears and other auditory hallucinations are closely associated with the activity of the auditory cortex and involves the temporal lobe. During the early period of sleep paralysis, the activity of the temporal lobe increases significantly, sometimes inducing hallucinatory sense. Similarly, the visual cortex generates internal visual stimuli, causing the victim to "see" terrifying figures during the paralysis.
How an episode of Sleep Paralysis induces visual or auditory hallucinations is still not clear, but it seems to have a significant relationship with anxiety. For anxiety is a neurocognitive event closely related to both psychological and physical processes, the extreme anxiety or panic may cause the release of several different signal molecules that trigger all kinds of physical events. A person experiencing Sleep Paralysis feels mortal fear or extreme panic, and hence, the brain generates and releases internal visual or auditory stimuli, producing hallucinations.
Also, hallucinations during Sleep Paralysis may happen, for one keeps dreaming even after some parts of his brain wakes up directly from REM sleep. Since the nervous and endocrine systems continue to release the neural inhibitors which sustain the paralysis, it may be possible that those systems keep releasing the neural activators that stimulate dreaming. Thus, a person continues to "see" the images and "hear" the noises produced in the dream that he has just had in REM sleep from which he has awaken.