A fever represents an elevation in the body’s core temperature, often indicating the immune system is responding to an infection. It might seem counterintuitive, but many individuals experiencing a fever also report feeling a distinct sensation of coldness or chills. This sensation occurs because the body actively works to reach a new, higher internal temperature. This article explores the biological mechanisms behind this paradoxical cold sensation.
The Body’s Temperature Set Point
The human body maintains its internal temperature within a narrow range, a process called thermoregulation, controlled by the hypothalamus in the brain. This area functions like a sophisticated thermostat, setting a target temperature.
When an infection occurs, immune cells release pyrogens. These pyrogens signal the hypothalamus, raising the temperature set point.
For instance, if the normal set point is 98.6°F (37°C), pyrogens might instruct the hypothalamus to reset it to 102°F (38.9°C). While the body’s actual temperature remains normal, the hypothalamus perceives it as too low compared to the new, elevated set point. This discrepancy triggers the feeling of cold, prompting the body to generate and conserve heat to reach the new temperature.
Physiological Responses to a Raised Set Point
To bridge the gap between the current body temperature and the new, higher set point, the body employs several physiological mechanisms that contribute to the sensation of cold. One prominent response is shivering, which involves rapid, involuntary contractions of skeletal muscles. These muscle movements generate heat through increased metabolic activity, but the rapid, jerky motions can also create a perception of coldness as the body vigorously works to warm itself.
Vasoconstriction is another mechanism, where blood vessels in the skin and extremities narrow. This redirects warm blood away from the body’s surface towards core organs, reducing heat loss. This reduction in blood flow to the skin often makes peripheral areas feel cold and may cause the skin to appear pale.
Simultaneously, piloerection, or goosebumps, occurs as tiny muscles contract, causing hairs to stand upright. This attempts to trap a layer of insulating air close to the skin, contributing to the sensation of chill.
When the Fever Breaks
Once the immune system successfully combats the infection or pyrogens are cleared from the system, the hypothalamus resets the body’s temperature set point back to its normal range. At this stage, the body’s actual temperature is still elevated, but the brain perceives this higher temperature as too warm.
To cool down and return to the normal set point, the body initiates heat-dissipating mechanisms. The primary cooling response involves vasodilation, where blood vessels near the skin’s surface widen, increasing blood flow. This allows excess heat to radiate away, often making the skin feel warm or flushed.
Simultaneously, sweating begins, as the evaporation of sweat provides an efficient way to release heat. As the body lowers its temperature, the sensation of coldness subsides, replaced by a feeling of relief or warmth.