Why do we shiver?

Shivering is involuntary muscle contraction generating heat, triggered when the body detects that core temperature is falling below its set point.

The mechanism. Temperature sensors in the skin and in the core report to the hypothalamus, which acts as the body's thermostat. When core temperature drops, the hypothalamus initiates a coordinated response:

Vasoconstriction — blood vessels near the skin narrow, reducing blood flow to the surface and therefore heat loss. This is why hands and feet go cold and pale first; the body is sacrificing the extremities to protect the core.

Piloerection — hairs stand up, producing goosebumps. In furred mammals this traps an insulating air layer. In humans it is vestigial and achieves essentially nothing.

Shivering — rapid, rhythmic contraction of opposing muscle groups. Because the muscles work against each other, no useful movement is produced, and nearly all the energy becomes heat. Shivering can increase heat production several-fold above resting rate.

Why it starts in the jaw and shoulders is that these muscle groups are recruited early, which is why teeth chatter.

Non-shivering thermogenesis is a parallel mechanism, particularly important in infants, using brown adipose tissue — fat specialised to burn energy directly as heat rather than storing it. Adults retain some, and interest in it has grown.

Why you shiver with a fever. This is the same machinery running for a different reason. During infection the hypothalamus raises its set point, so your normal temperature is now interpreted as too cold — and the body shivers to reach the new target. The chills at a fever's onset are the mechanism working correctly.

When shivering is a warning. In hypothermia, shivering typically stops as the condition worsens, because the body can no longer sustain it. A cold person who has stopped shivering and is confused or drowsy needs emergency help.