Why does your body shiver when temperatures drop?

When you're cold, your body temperature starts to fall below its ideal 98.6°F (37°C). Your brain's hypothalamus detects this drop and triggers an automatic response to generate heat quickly. Shivering is essentially rapid, involuntary muscle contractions happening in quick succession. These tiny muscle movements produce heat as a byproduct, the same way any physical activity warms you up, just in a fast, repetitive cycle. The process is purely mechanical. Muscles burn energy and release heat as they contract, so shivering is your body essentially running its own internal furnace. It's a short-term survival mechanism, though it's inefficient and tiring. If you warm up or put on layers, the hypothalamus signals the shivering to stop.