Ask A Scientist Physics Archive

name         Julie S.
status       student
age          16

Question -   Why is the bottom of my pillow always cooler than the top?
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I suspect it is only warmer on the top when your head has been resting on
it.

If this is not true, please ask again.  If the top were warmer without
something adding energy to the top, you could produce power and would have
invented a perpetual motion machine!  In which case, we should form a
company and get rich!

Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Quick answer to your pillow question.  You are on the top side of your 
pillow generating all that nice warm heat.  Your pillow is usually filled 
with some sort of fiber-fill or if you're lucky enough, down.  These are 
great insulators.  Which mean heat has a hard time getting from one side 
to the other.  So the bottom is always cooler than the top.  That is nice 
to know in the summer when you can flip the pillow over for a refreshing 
change.  It is also nice to know that a down coat works the same way; 
keeps all that body heat inside the coat next to you.
Happy Physics!

Martha Croll
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Julie,

     That is a pretty broad generalization.  Here are some questions you 
 might want to consider.

Is the bottom of your pillow always cooler than the top?  How do you 
determine that?  Is it by touch or by an actual temperature 
measurement?  Touch is pretty subjective and can be influenced by a lot of 
things, including the texture of the material you are in contact with 
(i.e., if you slide your hand between the pillow and the sheets the 
texture of the sheets may influence the perceived temperature of the 
pillow).  Scientific observations should, when possible, be quantitative 
and be performed with instrumentation.  This is not always possible (e.g., 
behavioral studies of chimpanzees in the wild) but the limitations of the 
techniques should always be considered when drawing conclusions.

If the bottom of your pillow is NOT always cooler than the top, under what 
conditions is it?  Does the history have anything to do with it?  Has the 
fact that you have been sleeping with your head on top of the pillow 
raised the temperature on the top or was the house furnace turned down at 
night and you check the temperature differential shortly after the room 
temperature was raised several degrees.  Do you sleep on a futon on the floor?

Keep asking questions!

Greg Bradburn
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