How does your body keep itself at about 37 °C, whatever the weather?

Your body holds its temperature steady with a thermostat in your brain. A small region called the hypothalamus collects reports from temperature-sensing nerve cells all over the body and reacts whenever the reading drifts. Too cold, and it turns up heat production and narrows the blood vessels in your skin so less warmth escapes; if that isn't enough, your muscles start shivering, tiny shakes that burn energy and make heat. Too hot, and it does the opposite: it widens the skin's blood vessels to dump heat and lets sweat do the rest. Biologists call this balancing act thermoregulation.

Sweat works by evaporating, and on a hot day it's your only real escape route. When the air is warmer than your skin, it actually heats you, and evaporation is the one way left to lose heat. That's why humid weather feels so heavy: damp air slows evaporation, so sweat drips instead of cooling you.

And the famous 37 °C? It was set by the German physician Carl Wunderlich in an 1868 book. Newer studies put the average a little lower, at 36.4 °C, and your own temperature rises and falls over the day, lowest in the middle of the night.