[pct-l] Why sleeping nude in a sleeping bag makes us feel warm.

[Ronald Moak <ronm@fallingwater.com>]

Blisterfree writes >>
The theory of sleeping nude in the bag has a long history, but that deosn't 
make it a good theory. :)  We sleep warm when we have the right amount of 
insulation around us, when that insulation traps a sufficient amount of 
warmth produced by the body. The less insulation, the less warmth. The 
greater, the warmer. So it follows that the hiker who sleeps nude or nearly 
nude must rely almost entirely on the bag for warmth. <<

Well Blisterfree, your logic is both right and wrong at the same time. In
general more insulation does equal more warmth. That's why bags with colder
ratings have more insulation. The flaw in your logic is that it assumes that
"being warm" and "feeling warm" are the same. 

While there is a relation between the two, it is certainly not one to one.
The classic example of this is when you take off your clothes and crawl in
to bed at night. If you're sleeping in nylon or silk sheets, for those first
few seconds before the material to warms up, you're feeling damn cold.
However, certainly in no danger of going into hypothermia. 

The primary purpose of a sleeping bag is to keep our core temperature in our
normal operating range. The problem is we don't perceive cold in our core.
As illustrated with the sheets example, we perceive cold primarily with our
skin. As a result is quite possible to spend a night feeling miserably cold
and in the morning our core temperature is just fine. 

So the trick of a sleeping system is not in just staying warm (keeping the
core warm) but to also feeling warm. Since these are different phenomena,
they need to be treated differently. In order to feel warm we need to know
how the skin perceives cold and eliminate it. When sleeping in a sleeping
bag there are a number of things that effect how we perceive cold. However,
the two primary ones we have the greatest control over are, moisture and
blood flow.

When we hike, we build up moisture in our clothes. Taking those clothes to
bed with us in order to dry them out, may be find when the temperatures
relatively warm. However, when the temperature begins to dip and our body
wants to expend it's energy keeping our core warm, the added moisture on our
skin can have a significant effect. That is because in order to drive the
moisture from the clothes it must be evaporated. As we all know evaporation
causes us to feel cooler. 

The second factor in feeling cold is lack of blood flow to the skin. Those
relatively tight clothes we wear for hiking are fine in that environment.
Those Coolmax,  MTS, or Capilene shirts do a good job for people on the
trail. Once inside that sleeping bag it's different matter. When we sleep we
frequently twist and turn. This causes our clothes to bind on our skin like
a tourniquet. It doesn't take much pressure on the capillary vessels in our
skin to reduce blood flow. Without the blood that brings heat from our core,
we quickly begin to feel chilled.

So as you can see, wearing the wrong clothes to be bed can have a
significant effect on your comfort level. My advice is if you wish to sleep
warm, remove any wet clothes. If you do want some extra insulation, I'd wear
a loose fitting fleece jacket next to my skin. Something that is less likely
to bind as I move. 

Ron "Fallingwater" Moak
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