Expert: Dana Krempels, Ph.D. - 8/20/2008

Question
Dear Dr. Krempels, Infants and senior citizens aside, what stops people from falling out of bed more freqently, if they are sleeping in a warm room without retricting bedclothes ? Thank you for your time, Mike Cohen Raanana Israel

Answer
Dear Mike,

The short answer is that during sleep, the neurotransmitter activity of the body changes so that muscular movement is not possible.  Here's a brief explanation from our old pal, Wikipedia:

You can find a more technical explanation here for the condition, medically termed REM atonia:

http://www.encyclopedia.com/doc/1O87-REMatonia.html

When things go wrong with normal REM atonia, the sleeper may actually move around and physically act out dreams, a condition called REM sleep disorder.  

REM sleep disorder should not be confused with sleepwalking/somnabulism, which is something that usually happens during SWS (slow wave sleep), or "deep sleep."  Atonia also occurs in normal SWS.

This is an area of active research, so you can find lots of interesting articles in medical journals with the right keywords, such as REM atonia, narcolepsy, somnambulism, etc.  Most of what we know about normal function, as usual, comes from studies of abnormalities.

Hope that helps.

Dana