I.e. nights where it’s too hot to be completely under the blanket but too cold to be without a blanket but having a limb covered keeps your whole body warm.
In: Biology
Your body produces heat when it metabolizes food and does stuff that keeps you alive. On average it is around 100 W. The heat spreads in your body both by conduction and by pumping blood around. The blood flow can be regulated so if you get too cold less blood flows to your limbs and the core stays warmer, if you overheat more blood flows to the skin so it gets warmer and loses heat faster.
Heat leaves your body from the surface, and the rate depends on the temperature difference between the skin and what is beside it. What is beside is typically air but it can alos be the ground or anything else. The rate heat is transferred alos depends on what is between you and it, that will be clothes, blankets, etc.
It is the total heat flow that matters, if a blanket covers a limb it will reduce the rate heat is lost from that limb because the heat is distributed around your body just adding some insulation on one part can be enough so all of your body gets warm enough.
The rate you lose heat depends on the surface area, and your limbs are a surprisingly large part of it. A bit simplified is your head, left arm, and right arm 9% of the body area each. Your right leg and your left leg are around 18% each and your torso is around 18% on the front and 18% on the back the remaining 1% is the genital area. The exact percentage will depend on the person but it it a good way to get an idea of the area
So if you cover an arm with a blanket you have to reduce the rate heat is lost from about 9% of your body. It it was a leg it is 18% So it is not an insignificant amount of heat loss reduction. If that reduction is enough for your body to remain at a comfortable temperature that is enough
Think of clothes, blankets, etc, not as something that keeps you warm but something that stops the heat you produce yourself from escaping.
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