I’m sure that just about everyone has experienced being too hot while trying to sleep and resolved the issue by uncovering an arm or leg. How is this so effective? I’d estimate that my arm makes up (Warning: WAG) 5% of the mass (and maybe surface area) of my body. How does cooling off such a small portion allow me to get back to sleep?
I guess the margins of feeling too hot are pretty slim. So it maybe doesn’t take too much to tip that feeling of comfort one way or t’other.
A hat or gloves may be the difference in cold weather, a cool leg or arm similarly when we overheat.
Once your arm or leg is exposed the heat it has immediately begins transferring into the colder air. Once the limb is cooler than the rest of your body heat will transfer from your warm body into that limb.
You can cool off doing similar heat transferring methods. Drink some ice cold liquids and your body heat will begin moving into that liquid in your stomach. Stick your feet in a cold lake on a hot summer day and your body heat will start transferring to your feet.
You’re exposing parts of your body—hands and feet—that have a lot of blood vessels close to the surface. That cooled blood then circulates through the rest of your body.
I frequently heard about this principle while I was growing up in Texas in the 1960s; air conditioning wasn’t quite as common then. In the summer there would be PSAs on television explaining how you could cool yourself by putting your arm into a sink full of cool water.
Here’s a Reddit thread asking the same question:
5% of the mass, but 10% of the surface area.
I have scrawny arms and a big stomach. It’s probably closer to 5%.
Another factor that occurred to me this morning when I exposed my arm, it was a little sweaty so evaporation was probably accelerated.
Yes, and an arm probably makes up an even bigger percentage of the surface blood vessels, which is where the heat transfer takes place.
Other parts of your body may make up a bigger part of the body mass, but not as much surface blood circulation, have less effect on heat transfer. Like your belly and your butt. Large areas of body fat there (way too large, in my case), which have little blood circulastion.
No need to guess about surface area. Medicine figured it out a long time ago.
If you change positions like that, maybe you release all that warmed up air that’s being insulated by the blankets. This would introduce cooler air all over your body. This is only a temporary effect, but it only has to last until you fall back asleep
Uncovering my feet always helped me feel cooler.