But rarely can we count on an ideal forecast as this, our bodies adapt. In the heat, we sweat and through the process of evaporation we are cooled off. In the cold, blood vessels near the surface of the skin squeeze tight to increase the flow of blood to our core. That blood transfer between skin and core has several effects. The brain, the heart, and other important organs stay toasty, albeit at the expense of the now blood-restricted hands and feet. The lack of surface blood flow greatly decreases the insulating properties of the skin, the subcutaneous layer of fat just underneath it, and nearby muscles. And, then starts the shivering. Those shaking muscles generate extra heat.
But we're not as thermostatically sensitive above the neck as we are below it. Blood vessels in the head don't have the luxury of constricting like they do in the skin. All that blood vessel constriction are mechanisms topreserve life. Since preserving life is the main point, the body can't restrict blood flow to the brain as we need it to functioning to think. This thinking and problem solving is facilitated by a healthy supply of warm oxygen rich blood. There's little subcutaneous fat for insulation. As a result, even if the rest of your body is nicely wrapped up, if your head is uncovered you'll lose lots of body heat — potentially up to 50% of it — in certain cold-weather conditions. What's more, an uncovered head can trigger blood vessel constriction in the other parts of the body, so it can make your hands and feet feel cold even if you are wearing mittens and warm socks and shoes.
The solution, of course, is a hat and, if it's really cold and you want to really stay warm, maybe one of those face-covering balaclavas. Wool is a good insulating fabric because it traps air, but not if it gets wet. Some of the warmest have some protection against the wind around the ears but allow moisture to evaporate through the crown.
For all these reasons we now include the Polar Vortex Beanie with ever Youth Adventure Sleep System.