Why Do We Shiver when We Get Cold?

Humans, other mammals, and birds are all considered endotherms, meaning that their body temperature is not dependent on environmental conditions, but instead stays relatively constant. For humans, 98.6 °F (37.0 °C) is the norm.

The hypothalamus gland, in the primitive base of the brain, is in charge of regulating body temperature. If the skin starts to get too hot or too cold, receptors in the skin send signals to the brain. Then the hypothalamus sets in motion a series responses designed to warm the body. This regulation of body temperature is all accomplished automatically, without our conscious effort.

In hot weather, we perspire. Perspiration cools, by coating the body in liquid that carries heat away as it evaporates. In cold weather, if body temperature begins to drop, we start to shiver.

How Shivering Helps Regulate Body Temperature

When the hypothalamus detects that body temperature is getting too low, it sends signals that contract the skin, giving us goose-bumps that make the body's hair stand up. This pilomotor response helps the body retain heat. It is a little more useful in feathered birds and furry mammals, but humans are still hard-wired to do it anyway.

The hypothalamus also prompts the skeletal muscles to quickly contract and expand. That's what shivering is. These small muscle movements create heat that helps warm the body.

Why Do People Shiver After Surgery?

Anesthesia that is used to render a person unconscious during surgery can impair the body's ability to regulate temperature. After surgery, if the core body temperature has dropped, the patient may shiver for a short period of time, until body temperature returns to normal.

Why Do People Shiver When Feverish?

Fever is one of the body's responses to infection. Invading microbes can inadvertently trigger the hypothalamus to crank up the heat. And although fever can make a person feel miserable, a slightly higher body temperature impairs microbes and helps the immune system work more effectively to eliminate infection.

A feverish person may get the chills because the hypothalamus has purposely reset the body's thermostat to a higher "normal" temperature. To maintain this higher norm, the body may begin shivering.

Shivering uses up a lot of energy; energy that a sick person with a depressed appetite and busy immune system can't afford to waste. So, whatever the reason behind the shiver, this is the body's way to telling you to throw on another layer of clothes or get under a blanket.

Sources

Bauman, R. (2003) Microbiology. Pearson Benjamin Cummings.

Livescience, Why Do We Shiver When Cold?

Starr, C. & Taggart, R. (1992) Biology: The Unity and Diversity of Life. Wadsworth Publishing.

Wisegeek, Why Do We Shiver?