MadSci Network: Neuroscience
Query:

Re: What is the explanation for feeling pressure prior to falling asleep?

Date: Sun Mar 18 20:11:05 2001
Posted By: Chris Atherton, Grad student
Area of science: Neuroscience
ID: 977186727.Ns
Message:

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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