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Human heat regulation

Over thousands of years we have evolved to a state where the most comfortable outside temperature (for a clothed human being) is 21°C. We need to be surrounded by air that is cooler than our body temperature by just the right amount so that heat flows away from our bodies at the same rate we generate it. We can work and play without being either cold or sweating profusely to maintain our core body temperature at 37°C. This temperature can only be maintained when we provide our bodies with enough energy (food and water) so that the heart can pump our energy-laden blood to our extremities and back.

In a cold environment, we need to counter-attack the cold by ingesting more food and water, but that is clearly not enough. We also need suitable clothing to retain the heat produced by a moving body. A resting body produces a mere 100 watts of heat, whereas an intense workout can produce ten times as much.

Of course, there comes a time when any living creature needs to rest or sleep. And when the cold gets a grip on a cooling human body, that body starts shivering as a reaction (shivering increases the amount of heat produced four or five times). At the same time, less blood will be delivered to both the extremities and to vital organs such as the heart and the brain.