Humans shake when cold because the body is trying to generate extra heat and protect its core temperature. This automatic shaking is called shivering and is controlled by the brain.
How shivering works
When the body’s core temperature starts to drop below its ideal level (around 37°C), temperature sensors in the skin and inside the body send signals to a region in the brain called the hypothalamus, which acts like a thermostat. The hypothalamus responds by triggering rapid, involuntary contractions and relaxations of skeletal muscles, which is what feels like shaking.
Why shaking warms you up
Every time muscles contract, they use energy molecules and release a small amount of heat as a byproduct. When many muscles contract and relax quickly during shivering, this “extra” heat adds up and helps raise or maintain core body temperature, reducing the risk of hypothermia.
Other times we shiver
The same shaking can also happen for reasons other than being cold, such as during the early stages of a fever when the body is trying to raise its temperature set point to fight infection. Strong emotions like fear or anxiety can also cause similar trembling because stress hormones activate muscle and temperature-regulation pathways in the brain.
