Feeling intensely cold and shivering under blankets while running a fever is a common experience. This sensation, known as “the chills,” is not a sign that you are actually cold. Instead, it results from a temporary, uncomfortable mismatch between your body’s internal thermostat setting and your current core temperature. This protective physiological response is a coordinated effort by the immune system to create an environment less favorable to invading pathogens.
Setting the Normal Temperature Baseline
The body maintains a stable internal environment through thermoregulation, a process known as homeostasis. Temperature is regulated by the hypothalamus, a small area of the brain that acts as the body’s central thermostat. This control center constantly monitors the temperature of the blood and compares it to a set point, typically around 37°C (98.6°F).
When the core temperature deviates slightly from this baseline, the hypothalamus initiates adjustments to maintain equilibrium. If the temperature rises, the body increases blood flow to the skin and activates sweat glands to promote cooling. Conversely, if the temperature drops, the body conserves heat by reducing blood flow to the skin and increasing metabolic heat production. This system ensures the internal temperature remains within the optimal range necessary for bodily functions.
How Illness Raises the Body’s Temperature Target
A fever begins when the immune system detects an invading threat, such as bacteria or a virus. In response, immune cells release signaling molecules known as endogenous pyrogens, which are small proteins like cytokines. These pyrogens travel through the bloodstream to the hypothalamus, where they trigger the production of prostaglandin E2 (PGE2).
The action of PGE2 effectively “resets” the body’s internal thermostat to a higher temperature, perhaps from 37°C to 39°C (102°F). This elevated set point is a defense strategy, designed because a higher temperature inhibits pathogen reproduction and enhances immune cell activity. Once the set point is raised, the body perceives its current temperature as too cold, even if it is still 37°C.
The Role of Chills and Shivering in Heating the Body
Chills and the feeling of coldness result directly from the mismatch between the new, high set point and the actual core temperature. The body interprets this gap as hypothermia, initiating mechanisms to rapidly generate and conserve heat.
One of the first actions is peripheral vasoconstriction, where blood vessels near the skin surface narrow. This reduces blood flow to the extremities, minimizing heat loss and causing the skin to feel cool. Simultaneously, the body initiates shivering, involving rapid, involuntary contractions of the skeletal muscles. This muscular activity is extremely effective at generating metabolic heat, helping the body reach the artificially high target temperature.
What Happens When the Fever Breaks
The resolution phase, when the fever “breaks,” occurs after the immune system suppresses the infection or when fever-reducing medication is taken. When the threat subsides, pyrogen production decreases, and the hypothalamus resets the temperature set point back to the normal baseline of around 37°C.
At this point, the body’s temperature is suddenly too high relative to the new, lower target, leading to the sensation of being too hot. To shed this excess heat, the body activates its cooling mechanisms. Vasodilation occurs, causing blood vessels near the skin to widen, which flushes warm blood closer to the surface to release heat. This is followed by profuse sweating, where the evaporation of moisture from the skin provides an efficient cooling effect. This switch signals that the immune defense has been successful and the body is returning to normal thermal regulation.