Put simply, we shiver to stop our body temperature falling. The muscles contract, creating heat, almost as if you were exercising while standing still.
You can burn up to 400 calories an hour shivering in some extremely cold environments – the equivalent of jogging for 45 minutes. The hypothalamus, an area of the brain described as the “thermostat of the body”, kicks muscles into action after signals from the skin warn it of lowering temperatures.
But why we shiver when we have a raised temperature is widely debated. Viral or bacterial infections – usually the source of the fever – cause the body to produce substances called pyrogens, which send signals to the hypothalamus to raise the core body temperature. Dr Graham Archard, of the Royal College of General Practitioners, says, “Shivering occurs before the fever and helps to raise the body’s temperature. It’s not a symptom of fever.”
Research suggests that the body uses high temperatures to kill the bacteria or virus causing the illness – many cannot survive above 40ºC, close to the minimum fever temperature of 38.3ºC.