A fever, an elevated body temperature, commonly signals the body’s response to illness. Despite the internal heat, many people experience a sensation of coldness or chills during a fever. This paradoxical feeling often leaves individuals wondering why their body, though hot, makes them shiver and feel chilled. Understanding this phenomenon requires insight into the body’s temperature regulation system and how it adapts during an infection.
The Body’s Temperature Control System
The human body maintains a stable internal temperature, around 37°C (98.6°F), a process known as thermoregulation. This precise control is orchestrated by the hypothalamus, a small region in the brain that acts as the body’s thermostat. The hypothalamus constantly monitors blood temperature and receives signals from temperature sensors throughout the body.
When body temperature deviates from its set point, the hypothalamus triggers mechanisms to either generate or release heat. To cool down, the body increases blood flow to the skin and initiates sweating, allowing heat to dissipate through evaporation. To warm up, it can reduce blood flow to the skin through vasoconstriction and induce shivering, which generates heat through muscle activity. These responses ensure the body’s internal environment remains optimal for cellular function.
How Fever Tricks Your Body Into Feeling Cold
During a fever, the body’s thermostat, the hypothalamus, is reset to a higher temperature set point. This reset is triggered by substances called pyrogens, released by pathogens (like bacteria or viruses) or immune cells fighting infection. With this new, elevated set point, the body perceives its current, normal temperature as too low.
To reach this higher temperature, the body activates its heat-generating and heat-conserving mechanisms. A primary response is vasoconstriction, where blood vessels near the skin’s surface narrow. This reduces blood flow to the skin, minimizing heat loss and shunting warm blood towards the core, which can make the skin feel cool despite rising internal temperature. The body also initiates shivering, involuntary muscle contractions that generate internal heat, helping to raise the body’s core temperature towards the new set point. The sensation of feeling cold and chills are the body’s attempts to warm itself to this elevated target.
The Resolution of a Fever
As the body fights off the infection, or if fever-reducing medications are taken, the concentration of pyrogens decreases. This allows the hypothalamus to reset its temperature set point to normal. At this point, the body’s elevated temperature is now perceived as too high relative to the normalized set point.
To shed excess heat, the body activates its cooling mechanisms. Vasodilation occurs, causing blood vessels near the skin to widen, increasing blood flow to the surface. This increased circulation allows heat to radiate away from the body, often leading to flushed skin. Sweating intensifies, and the evaporation of sweat from the skin’s surface provides an effective cooling effect. This is why individuals experience profuse sweating as a fever “breaks” and their body temperature returns to normal.