MadSci Network: Neuroscience |
Hi William This is a really interesting phenomenon. When you fall asleep, the brain turns off almost all control of the body's movement. This is so that you cannot hurt or otherwise endanger yourself while you are unconscious. This is especially true while you dream - the brains of sleepwalkers, particularly people who sleepwalk a lot, seem to have trouble switching off the body during dreaming. Dream sleep is often called REM sleep, short for rapid eye movement because peoples' eyes actually move very fast during this sleep-stage (you can see this happening if you look at someone in REM sleep, even though their eyes are shut). This kind of experience seems mostly to happen to people when they are in the REM state. Of course, when you sleep, the body's automatic processes carry on - things like breathing and maintaining a heartbeat. While you dream, though, the muscles in the body that surround your chest (called the thoracic muscles) are paralysed. This means that during dream sleep you aren't able to breathe very hard because the brain has told the thoracic muscles (which would normally help you to breathe) to stop working. But sometimes, when you are asleep, you try and take a deep breath - just the same way as you would when awake (1). You can't, though, because the thoracic muscles and some of the muscles around the lungs are switched off. This can have the effect of making the person feel a pressure on their chest or back, stopping them from breathing. Sometimes this feeling can make people panic, which only makes the effect worse. Often the experience relates to what is happening in the dream (so that for example someone might dream they are underwater and unable to breathe). Children and young people tend to report sleep disturbances of this kind more regularly but in adults it is often associated with neuropsychological disturbances of other kinds such as schizophrenia (2). People quite often feel quite awake (although they are actually asleep) when they experience this pressing sensation. What is very interesting is that lots of different cultures report people who experienced this same sensation. People from all these different countries often report the sensation of someone sitting on them or bent over them and trying to strangle them. In most cultures there are similar legends of some evil spirit or "incubus" who tries to strangle or suffocate people while they sleep. Some researchers (3) have even suggested that this type of experience, of being helpless and unable to move, might contribute to the experience of alien abduction. It has been shown (4) that people who (while asleep) experience difficulty in breathing like this often have a sense that there is someone else in the room, and sometimes experience a floating sensation or feel as though they are being pulled through the air. These factors together might account for the phenomenon of the "alien abduction experience" which is perhaps just a fashionable modern interpretation of the experience which people used to blame on an evil suffocating spirit. Below are some references that might help you - 1, 3 and 4 are from scientific journals and 2 is an entire book about the different aspects of sleep. I hope this helped! Chris :) (1) Hishikawa, Y and Shimizu, T (1995). Physiology of REM sleep, cataplexy and sleep paralysis. In S Fahn, M Hallett, HO Luders and CD Marsden (Eds). Advances in Neurology (vol 67, pp 245 - 271). Philadelphia: Lippincott- Raven. (2) Spielman, A and Herrera, C (1991). Sleep disorders. In Ellman, SJ and Antrobus, JS (Eds). The mind in sleep: Psychology and psychophysiology (2nd Ed.) Wiley series on personality processes (pp 25-80). New York: John Wiley and Sons. (3) Clark, SE and Loftus, EF (1996). The construction of space alien abduction memories. Psychological Inquiry vol 7(2) pp140-143. (4) Cheyne, JA, Rueffer SD and Newby-Clark IR (1999). Hypnagogic and hypnopompic hallucinations during sleep paralysis: neurological and cultural construction of the night-mare. Consciousness and Cognition vol 8, pp 319-337.
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