We shake when cold because the brain is trying to warm the body up fast by making muscles contract and relax rapidly, which generates heat and helps protect core temperature.

Body’s thermostat kicks in

When you get cold, a part of the brain called the hypothalamus senses even small drops in your internal temperature and treats it as a threat to vital organs like the heart and brain. To pull your temperature back toward normal, it automatically triggers a series of responses, including narrowing skin blood vessels and, if that is not enough, starting shivering.

How shivering makes heat

Shivering is basically your skeletal muscles doing tiny, rapid “workouts” without you choosing to move. Those contractions burn energy and release heat, which can significantly boost your metabolism and help keep your core warmer than your surroundings.

Why it feels so intense

Because so many muscles are firing at once, you may feel your arms, legs, and even jaw trembling, which is why teeth can chatter when you are very cold. This process uses a lot of energy, which is why long periods of shivering are exhausting and can be a warning that you are getting dangerously cold, not just a little chilled.

Not only from cold

A similar shaking can happen with fevers, when the brain temporarily “raises the set point” of your internal thermostat so your body aims for a higher temperature to fight infection. Until your body heats up to that new set point, you can feel cold and shiver even though your actual temperature is already high.

Information gathered from public forums or data available on the internet and portrayed here.